Optimization of Piezoelectric Performance in Lead-Free BNT–5BT Ceramics by Cu/Zr Co- Doping: Structure–Property Correlations

Optimization of Piezoelectric Performance in Lead-Free BNT–5BT Ceramics by Cu/Zr Co- Doping: Structure–Property Correlations | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Optimization of Piezoelectric Performance in Lead-Free BNT–5BT Ceramics by Cu/Zr Co- Doping: Structure–Property Correlations Mauro Difeo, Javier Camargo, Miriam Castro, Leandro Ramajo This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9223133/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Lead-free 0.95Bi 0.5 Na 0.5 TiO 3 –0.05BaTiO 3 (BNT–5BT) ceramics were Cu/Zr co-doped to improve the electromechanical properties relevant to low-power energy harvesting. Based on the optimized single-dopant contents (1 mol% CuO and 2 mol% Zr), the co-doped composition 0.95(Bi 0.5 Na 0.5 )(Ti 0.98 Zr 0.02 )O 3 –0.05Ba(Ti 0.98 Zr 0.02 )O 3 –0.01CuO (BNT–5BT–2Zr–1Cu) was synthesized by solid-state reaction with mechanochemical activation. X-ray diffraction and Raman spectroscopy confirmed the formation of a single perovskite phase and an increased tetragonal contribution compared to the single-doped counterparts, consistent with morphotropic phase boundary (MPB) stabilization. Co-doping increased the ferroelectric–relaxor crossover temperature (T F–R = 195°C) while reducing the room-temperature permittivity, thereby enhancing the piezoelectric voltage response. The co-doped ceramic exhibited ρ = 5.74 g·cm -3 , d 33 = 130 pC·N -1 , and g 33 = 10.71×10 − 3 V·m·N -1 , outperforming the undoped and single-doped reference compositions. These results indicate that Cu/Zr co-doping, combining Cu-assisted densification and defect equilibration with Zr-induced structural tuning, is an effective strategy to enhance the voltage response of lead-free BNT–BT ceramics for low-power energy-harvesting applications. lead-free piezoceramics co-doping Cu Zr energy harvesting BNT–BT Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction The global shift toward environmentally sustainable materials has intensified research on lead-free piezoelectric ceramics as replacements for Pb(Zr,Ti)O 3 (PZT), which, despite its outstanding electromechanical performance, raises significant environmental and health concerns because of lead toxicity [ 1 , 2 ]. Among various alternatives, Bi 0.5 Na 0.5 TiO 3 (BNT)-based systems have attracted particular attention due to their strong ferroelectricity, high Curie temperature (~ 320°C), and compositional tunability [ 3 – 5 ]. The solid solution 0.95Bi 0.5 Na 0.5 TiO₃–0.05BaTiO₃ (BNT–5BT) is located near a morphotropic phase boundary (MPB) between rhombohedral and tetragonal symmetries, where enhanced piezoelectric and dielectric responses can arise from phase coexistence and field-induced polarization rotation/extension mechanisms [ 6 , 7 ]. These features make BNT–BT ceramics promising candidates for actuation, sensing, and low-power energy-harvesting applications, in which a high piezoelectric voltage coefficient (g 33 ​), favored by combining a reasonably high d 33 with suppressed permittivity, is required [ 8 – 10 ]. Nevertheless, undoped BNT–BT exhibits limited poling efficiency and moderate voltage sensitivity, mainly due to its relatively high permittivity, which motivates the exploration of chemical modifications to tailor its defect chemistry and structural response. In contrast to PZT ceramics, where functional properties can be effectively tailored through low-level aliovalent doping, commonly classified as softening or hardening, previous studies have shown that in BNT–BT ceramics, doping with a single A- or B-site element, or even co-doping at both sites, often fails to achieve the desired properties [ 11 , 12 ]. In PZT systems, donor dopants (e.g., Nb⁵⁺ substituting for Zr⁴⁺/Ti⁴⁺) typically enhance dielectric and piezoelectric responses but increase dielectric and mechanical losses [ 13 ], whereas acceptor dopants (e.g., Fe³⁺) reduce these responses while improving the mechanical quality factor and lowering losses [ 14 ]. These contrasting effects are generally attributed to defect–domain wall interactions: oxygen vacancies generated by acceptor doping pin domain walls, while donor doping facilitates their motion. However, in BNT-based ceramics, the piezoelectric response deviates from this conventional hardening–softening paradigm [ 11 ]. For instance, Fe acceptor doping has been reported to enhance the d₃₃, whereas Nb donor doping reduces it [ 15 ]. This unconventional behavior has been mainly related to the stabilization or disruption of long-range ferroelectric order, indicating that the effect of doping in BNT-based systems depends strongly on the preservation of ferroelectric correlations. Among the dopants explored in BNT-based ceramics, Cu and Zr are particularly attractive because they can influence different aspects of the structure–microstructure–property relationship; Cu 2+ can act as a sintering aid, promoting densification at lower temperatures and improving microstructural homogeneity, commonly through transient liquid-phase formation, while also modifying local defect equilibria and dielectric losses [ 16 , 17 ]. In contrast, Zr 4+ substitution for Ti 4+ at the perovskite B-site expands the lattice and can modify the rhombohedral/tetragonal phase balance, often increasing the tetragonal contribution near the MPB and thereby improving the electromechanical response [ 18 ]. Recent studies have shown that moderate Zr incorporation can reduce the coercive field while preserving relatively high remanent polarization, leading to enhanced d 33 values in BNT–BT ceramics [ 19 , 20 ]. Although both dopants have been individually investigated, their combined effect in BNT–5BT has not yet been reported, to the best of our knowledge. Their distinct roles suggest a potentially synergistic effect: Cu may promote densification and microstructural uniformity, whereas Zr may tune the B-site lattice and the phase balance near the MPB, thereby modifying the dielectric response and voltage-related electromechanical parameters. Therefore, Cu/Zr co-doping may provide an effective route to simultaneously tailor microstructure, symmetry, and permittivity while preserving piezoelectric activity. Based on earlier single-dopant screening, where 1 mol% CuO (BNT–5BT–1Cu) maximized densification and piezoelectric response [ 20 ], and 2 mol% Zr (BNT–5BT–2Zr) delivered the best voltage coefficient [ 4 ], this work examines four reference compositions: undoped BNT–5BT, the single-doped BNT–5BT–1Cu and BNT–5BT–2Zr, and the co-doped BNT–5BT–2Zr–1Cu samples. Zirconium was introduced via a substitutional approach to favor incorporation into the perovskite lattice, while CuO was added in excess to promote Cu-assisted densification (often associated with transient liquid-phase effects) during sintering. The aim of this work is to determine how Cu/Zr co-doping modifies (i) average symmetry and local structure (through X-ray diffraction and Raman spectroscopy), (ii) the ferroelectric–relaxor transition window (T F−R ​ and T max ​), and (iii) the electromechanical and dielectric response (d 33 , ε r ​, and g 33 =d 33 /(ε 0 ε r )), thereby establishing structure–property correlations relevant to lead-free piezoelectric ceramics for low-power energy harvesting. 2. Experimental Procedure Ceramic powders were prepared by the solid-state reaction method using high-purity Bi₂O₃, Na₂CO₃, BaCO₃, TiO₂, ZrO₂, and CuO precursors. Four benchmark compositions were studied: the undoped 0.95Bi 0.5 Na 0.5 TiO 3 -0.05BaTiO 3 (BNT–5BT), Cu-doped 0.95Bi 0.5 Na 0.5 TiO 3 -0.05BaTiO 3 -yCuO (BNT-5BT-1Cu; y = 1 mol%); and Zr-doped 0.95Bi 0.5 Na 0.5 Ti 1-x Zr x O 3 -0.05BaTi 1-x Zr x O 3 (BNT-5BT-2Zr; x = 2 mol%) and co-doped 0.95Bi 0.5 Na 0.5 Ti 1-x Zr x O 3 –0.05BaTi 1-x Zr x O 3 -yCuO (BNT-5BT-2Zr-1Cu; x = 2 mol% and y = 1 mol%) sample, which combines both dopants at their optimized single-dopant contents [ 20 ]. The stoichiometric powders were mixed in isopropanol with zirconia balls and mechanically activated in a planetary ball mill (500 rpm, 3 h). After drying, the mixtures were calcined at 850°C for 2 h to ensure perovskite phase formation. The calcined powders were milled and uniaxially pressed into disk-shaped pellets (Ø = 10 mm, t = 1 mm) at a pressure of 200 MPa. According to previous studies [ 20 ], Cu-doped samples (BNT–5BT–1Cu and BNT–5BT–2Zr–1Cu) were sintered at 1050°C for 2 h in air, whereas Cu-free samples (BNT–5BT and BNT–5BT–2Zr) were sintered at 1150°C for 2 h in air. All compositions were prepared following the same solid-state processing route and characterization protocol; the only difference was the sintering temperature selected for Cu-containing versus Cu-free ceramics. Structural analyses were conducted by X-ray diffraction (PANalytical X’Pert PRO, CuKα radiation, 2θ = 20–80°) and Raman spectroscopy (Renishaw inVia, 514 nm excitation). Microstructural features were observed using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) after thermal etching at 100°C below the corresponding sintering temperature, for 15 min. The apparent density was determined by the Archimedes method. Dielectric properties were measured using an LCR meter (HP 4284A) in the frequency range of 1 kHz to 1 MHz and over the temperature range 25 to 500°C. Ferroelectric hysteresis (P–E) loops were recorded using a modified Sawyer–Tower circuit at 50 Hz under fields up to 7 kV·mm -1 at room temperature. The piezoelectric constant (d 33 ) was obtained with a quasi-static piezometer (YE2730A, Sinocera) after poling at 2.5 kV·mm -1 in silicone oil at 150°C for 30 min. The piezoelectric voltage coefficient was calculated from g 33 = d 33 /(ε 0 ε r ), where ε 0 is the vacuum permittivity, and ε r is the real part of the dielectric permittivity measured at room temperature at 1 kHz. 3. Results and Discussion 3.1. Structural Analysis Figure 1 a shows the room-temperature X-ray diffraction (XRD) patterns of BNT–5BT and the Cu/Zr-modified compositions. All diffractograms were indexed on the basis of a perovskite structure, and no secondary phases were observed within the instrumental resolution, suggesting that the dopants did not produce detectable crystalline segregates. Expanded views of the {111} diffraction peaks (39.5°- 40.5° in 2θ) are presented in Fig. 1 b. The peak splitting reveals the characteristic peak asymmetry/splitting associated with the morphotropic phase boundary (MPB) of BNT–BT-based systems [ 21 ], i.e., the coexistence of rhombohedral-like (R3c) and tetragonal-like (P4mm) contributions at room temperature. To evaluate the evolution of this coexistence, the experimental profiles were fitted using three Gaussian components (two associated with the rhombohedral-like contribution and one with the tetragonal-like contribution). Although this approach provides only a semi-quantitative estimate, it enables a consistent comparison across the full compositional series. The diffraction patterns of the two end-member compositions (undoped BNT–5BT and co-doped BNT–5BT–2Zr–1Cu) were refined by the Rietveld method using a two-phase model (R3c + P4mm) (Table 1 ). A full Rietveld quantification for the single-doped ceramics was not pursued here, and their comparative discussion relies on the same peak-deconvolution protocol applied to all samples, complemented by Raman signatures of local symmetry. The refined weight fractions corroborate the deconvolution trend: BNT–5BT contains 67.77 ± 2.20 wt% R3c and 32.23 ± 1.97 wt% P4mm, whereas BNT–5BT–2Zr–1Cu exhibits 42.67 ± 0.01 wt% R3c and 57.33 ± 0.02 wt% P4mm. These results support the conclusion that co-doping shifts the room-temperature phase balance toward a tetragonal-rich perovskite state. The refined unit-cell volumes further support effective Zr incorporation at the B-site. For the rhombohedral phase, the unit-cell volume increases from 353.39 ų to 358.13 ų upon Zr incorporation. When normalized per ABO₃ unit, V/Z increases from ~ 58.90 ų to ~ 59.69 ų, consistent with lattice expansion due to substitution of Ti⁴⁺ (0.605 Å) by larger Zr⁴⁺ (0.72 Å). A slight volume increase is also observed for the tetragonal phase. These results are consistent with Zr incorporation into the perovskite lattice rather than with the formation of detectable secondary phases. Table 1 Rietveld refinement results for the two bounding compositions, BNT–5BT and BNT–5BT–2Zr–1Cu, using a two-phase model (R3c + P4mm): refined phase weight fractions, unit-cell volumes (V), Z (number of formula units per unit cell), and refinement quality indicators (R Bragg and R f ). These refinements are included as an independent check of the phase-balance trend inferred from peak deconvolution; the comparative discussion across the full series is based on the same deconvolution protocol applied to all samples. Uncertainties correspond to the refinement output. Sample Phase Fraction (%) Unit-cell volume (A˚ 3 ) Z V/Z ų per ABO 3 R Bragg R f BNT-5BT R3c 67.77 ± 2.20 353.39 ± 0.045 6 58.90 ± 0.01 23.3 19.9 P4mm 32.23 ± 1.97 59.71 ± 0.027 1 59.71 ± 0.03 23.9 21.2 BNT-5BT-2Zr-1Cu R3c 42.67 ± 0.01 358.13 ± 0.023 6 59.69 ± 0.01 27.9 21.7 P4mm 57.33 ± 0.02 59.75 ± 0.021 1 59.75 ± 0.02 31.9 19.7 From a functional point of view, shifting the MPB balance toward a tetragonal-rich state is relevant because it modifies the polarization landscape and the available switching/rotation pathways under field or stress. In BNT-BT systems, enhanced electromechanical response is frequently associated with optimized rhombohedral–tetragonal (R–T) coexistence and the ease of polarization reorientation near the MPB [ 22 ]. Here, the Rietveld results provide a quantitative structural basis to interpret the improved performance of the Cu/Zr co-doped ceramic, as the stabilized tetragonal contribution and the associated lattice modifications are expected to facilitate more efficient domain processes during poling and operation. Figure 2 shows the room-temperature Raman spectra of BNT–5BT, BNT-5BT-2Zr, BNT-5BT-1Cu, and the co-doped BNT-5BT-2Zr-1Cu, together with their Lorentzian deconvolution. The spectral features are consistent with previous reports and indicate a progressive stabilization of tetragonal-like character across the series. In particular, the band near 310 cm⁻¹ (Ti–O vibrations in distorted TiO 6 octahedra), shifts systematically from 310.2 cm⁻¹ (undoped) → 310.6 cm⁻¹ (2Zr ) → 312.7 cm⁻¹ (1Cu) → 318.6 cm⁻¹ (2Zr–1Cu), and its relative area increases in the co-doped sample, indicating enhanced octahedral anisotropy and tetragonality [ 21 , 23 ]. Moreover, the doublet in the 520–600 cm⁻¹ region, assigned to Ti–O–Ti stretching in edge-shared octahedra, becomes more clearly resolved upon co-doping, with an increased separation between the ∼520 and ∼592 cm⁻¹ components, which is commonly associated with enhanced tetragonal splitting [ 24 , 25 ]. These trends are consistent with (i) Zr 4+ substitution at the B-site (larger ionic radius than Ti 4+ ), introducing local strain that favors tetragonal distortions, and (ii) Cu 2+ acting as a sintering/defect-chemistry modifier that homogenizes the lattice and reduces random fields, which may contribute to a clearer spectroscopic differentiation of the tetragonal features [ 26 ]. Altogether, the Raman results, together with the XRD peak analysis, indicate a more defined tetragonal contribution in the co-doped ceramic, which is consistent with its improved electromechanical response. 3.2. Microstructural Evolution Figure 3 shows FE-SEM micrographs and grain-size distributions for all compositions. All samples exhibit dense microstructures with well-defined grain boundaries, consistent with the absence of detectable secondary phases in the XRD and Raman analyses. Undoped BNT–5BT shows equiaxed grains with a relatively broad size distribution (1.09 ± 0.89 µm) and minor residual porosity. Zr doping promotes grain growth and improves microstructural homogeneity, increasing the average grain size to 1.37 ± 0.92 µm. This behavior is consistent with the enhanced diffusional processes and structural stabilization reported for Zr-modified BNT–BT systems [ 17 , 18 ]. In contrast, Cu-only doping yields a finer and more homogeneous microstructure (0.95 ± 0.46 µm), primarily due to the lower sintering temperature enabled by the addition of CuO. The transient liquid-phase sintering effect associated with CuO enhances densification while limiting excessive grain growth [ 27 , 28 ]. By contrast, Cu/Zr co-doping produces the largest average grain size (1.54 ± 1.20 µm), despite a sintering temperature approximately 100°C lower than that of undoped BNT–5BT. This behavior suggests a combined effect of Cu-assisted viscous-phase diffusion, which enhances densification kinetics, and Zr incorporation, which may favor grain growth and structural stabilization [ 29 , 30 ]. No macroscopic secondary phases are evident in the FE-SEM micrographs within the inspected areas, consistent with the XRD and Raman analyses. However, minor dopant-rich regions, nanoscale segregations, or residual transient liquid phases below the resolution and detection limits of SEM/EDS cannot be ruled out. The distinct microstructural evolution, characterized by grain refinement in the Cu-doped ceramic and controlled grain growth in the Cu/Zr co-doped composition, is consistent with the observed differences in dielectric and piezoelectric behavior. 3.3. Dielectric and Ferroelectric Behavior Figure 4 a-b shows the temperature dependence of the relative permittivity (ε r ) and dielectric loss (tanδ) at 1 kHz. All compositions exhibit a broad permittivity maximum characteristic of diffuse phase transitions in chemically complex perovskites near morphotropic phase boundaries. For undoped BNT–5BT, the ferroelectric–relaxor crossover temperature (T F–R ) occurs at 168°C, while T max is located at 264°C. Upon Cu/Zr co-doping, T F–R shifts upward to 195°C, whereas T max decreases to 232°C, narrowing the T F–R -T max interval [ 31 , 32 ]. A clear frequency dependence is observed not only in the vicinity of T F−R but also around T max (Fig. 4 c-d), indicating the presence of highly dynamic polar nanoregions (PNRs) and thermally activated reconfiguration processes [ 33 ]. The significant frequency dispersion of T max is a well-known hallmark of classical relaxor behavior [ 34 ], confirming that at these elevated temperatures, the system evolves toward a highly disordered ergodic relaxor state. Within a phenomenological framework [ 35 ], this enhanced relaxor behavior can be rationalized by the introduction of local random fields due to compositional disorder. Specifically, Zr 4+ substitution at the B-site generates local strain fields because of ionic size mismatch, whereas Cu-related defect equilibria may modify the charge-compensation mechanisms, possibly promoting the formation of defect dipoles such as \(\:{Cu}_{Ti}^{"}-{V}_{O}^{\bullet\:\bullet\:}\) . These combined effects generate spatial fluctuations in the local free-energy landscape, effectively disrupting the long-range ferroelectric order, broadening the dielectric anomaly, and stabilizing the highly dispersive relaxor state at high temperatures. To assess the diffuseness of the permittivity maximum, the modified Curie–Weiss approach was applied around T max using the following equation [ 32 ]: $$\:\frac{1}{{\epsilon\:}_{r}}-\frac{1}{{\epsilon\:}_{max}}=\frac{{\left(T-{T}_{max}\right)}^{\gamma\:}}{C´}$$ 1 where \(\:{\epsilon\:}_{max}\) is the maximum dielectric permittivity, \(\:{\varvec{T}}_{\varvec{m}\varvec{a}\varvec{x}}\) is the temperature of maximum dielectric permittivity, γ is the degree of diffuseness (values closer to 2 indicate stronger relaxor-like behavior, whereas values closer to 1 indicate more conventional ferroelectric behavior), and C´ is the modified Curie-Weiss constant. All samples exhibited diffuseness exponents γ approaching 2 (see Table 2 ). In the present case, γ should be interpreted as a descriptor of peak broadening arising from compositional heterogeneity and structural disorder, rather than as direct evidence of a canonical relaxor regime. The upward shift of T F–R in the co-doped composition indicates improved thermal stability of the non-ergodic ferroelectric state. Structurally, this aligns with the tetragonal-enriched MPB configuration identified by XRD/Rietveld analysis. The increased tetragonal fraction may deepen the free-energy minimum associated with long-range polarization, thus delaying the onset of dynamic disorder upon heating. This stabilization is functionally relevant. In BNT–BT systems, the proximity to T F–R strongly influences domain-wall mobility and the balance between reversible and irreversible polarization components. By shifting T F–R upward while simultaneously reducing ε r , Cu/Zr co-doping modifies the dielectric stiffness and polarization dynamics, enhancing the piezoelectric voltage coefficient (see Section 3.4 ). Thus, the dielectric behavior provides a useful link between structural phase balance and electromechanical performance. Table 2 Ferroelectric–relaxor crossover temperature (T F–R ), maximum permittivity temperature (T max ), maximum relative permittivity (ε max ), and diffuseness exponent (γ) obtained from the modified Curie–Weiss analysis performed in the vicinity of T max for BNT–5BT and Cu/Zr-modified compositions. Values were extracted from ε r (T) data (1 kHz) using the linear fit of ln(1/ε r−1 /ε max ) versus ln(T−T max ). Sample T F−R (ºC) T max (ºC) ε max γ BNT-5BT 168 264 6183 1.92 BNT-5BT-1Cu 160 278 5652 1.83 BNT-5BT-2Zr 157 253 5017 1.99 BNT-5BT-2Zr-1Cu 195 232 5008 1.92 Figure 5 shows the polarization–electric field (P–E) loops of BNT–5BT and doped BNT–5BT ceramics. At room temperature, all compositions display well-developed ferroelectric-like hysteresis loops with non-negligible remanent polarization and coercive field, confirming that they remain in a non-ergodic ferroelectric state under the explored conditions. Quantitatively, Cu/Zr co-doping leads to a slight increase in the coercive field, from 41.8 to 43.0 kV·cm -1 , together with a decrease in remanent polarization (P r ) from 25.0 to 21.6 µC·cm -2 . For comparison, Cu-only doping results in an even lower P r (~ 20.1 µC·cm -2 ) [ 20 ], whereas Zr-only doping yields a higher P r (30.1 µC·cm -2 ) and a lower E c (39.5 kV·cm -1 ) [ 4 ]. These trends are consistent with changes in the polar state and in the phase balance near the MPB, including relaxor-like contributions and variations in the relative extent of reversible and irreversible switching, rather than providing direct evidence for a specific defect-domain pinning mechanism. Therefore, the detailed role of defect chemistry and domain/PNR interactions in controlling P r and E c requires further investigation. To further clarify this behavior, the ferroelectric-like hysteresis observed at room temperature must be reconciled with the pronounced relaxor character indicated by the diffuseness parameter γ. This apparent discrepancy arises from the different temperature regimes in which these properties are evaluated. The γ parameter is obtained from dielectric data collected well above T max (e.g., > 240°C), and therefore reflects the intrinsic structural disorder and the ergodic relaxor (ER) character of the high-temperature state. In contrast, the broad hysteresis loops measured at room temperature indicate that the unpoled material remains in a non-ergodic relaxor (NER) state, or at least in a state with strong ferroelectric correlations. Under a sufficiently strong electric field, this initial configuration can be driven into a field-induced long-range ferroelectric state, giving rise to the observed wide P–E loops. As the temperature approaches T max , thermal activation progressively stabilizes the ergodic relaxor state and reduces the persistence of the field-induced ferroelectric order. Consequently, the P–E loops are expected to become progressively slimmer with increasing temperature, in agreement with the high relaxor character quantified by γ. From an energy-harvesting perspective, however, the hysteresis loop shape can only provide a qualitative indication of switching-related losses, since the loop area represents dissipated energy per cycle. A rigorous comparison would require direct quantification of this dissipated energy under identical electric-field amplitude and frequency. Therefore, in the present work, the harvesting performance is discussed primarily in terms of voltage sensitivity and figures of merit derived from d 33 and ε r (Section 3.4 ), particularly through the voltage coefficient g 33 =d 33 /(ε 0 ε r ) [ 16 ], rather than through indirect inferences based solely on hysteresis features [ 36 ]. At room temperature, all compositions exhibit ferroelectric-like P–E loops with non-negligible P r and E c , indicating that the materials remain outside a fully ergodic relaxor state within the explored field range. From an energy-harvesting standpoint, however, hysteresis itself is not a direct performance metric, since the loop area is associated with energy dissipation. For this reason, the harvesting relevance discussed in Section 3.4 is based mainly on voltage sensitivity and dielectric–piezoelectric figures of merit, rather than on presumed reductions in hysteretic losses. 3.4. Piezoelectric Response and Energy Generation Performance From a functional standpoint, the larger mean grain size of the co-doped ceramic (1.54 ± 1.21 µm) compared with undoped BNT-5BT (1.09 ± 0.83 µm) is relevant because grain boundaries constrain domain-wall motion and can reduce the fraction of switchable polarization during poling. In this context, the observed relationship between remanent/saturation polarization and grain size can be qualitatively interpreted using the domain-kinetics framework proposed by Orihara et al., often discussed in terms of an Avrami-type expression [ 37 ] for the fraction of reoriented ferroelectric domains f: 𝑓 = 𝑓 0 [1 − exp(-𝐺 𝑎 𝑑 3 ⁄𝑘𝑇)] (2) where G a represents the anisotropy energy density, k is the Boltzmann constant, T is the absolute temperature, and d is the grain size. This relation implies that larger grains increase f , i.e., a larger proportion of domains can reorient under an applied field, which generally favors higher switchable polarization and improved ferroelectric response. In the present work, this framework is used as a qualitative basis to interpret the effect of grain size on polarization behavior. In addition, all doped samples exhibited relative densities higher than the 95% (see Table 3 ), comparable to the Zr-only composition and higher than the undoped ceramic. Therefore, the improved ferroelectric behavior of BNT-5BT-2Zr-1Cu can be consistently interpreted as the combined outcome of (i) a tetragonal-enriched MPB state (Section 3.1 ), (ii) microstructural coarsening that facilitates domain reorientation, and (iii) a moderately reduced dielectric permittivity. Finally, the piezoelectric charge and voltage coefficients (d₃₃ and g₃₃) reflect the combined influence of structural symmetry and dielectric response. Table 3 summarizes the key piezoelectric and dielectric parameters. The higher g₃₃ and harvesting figure of merit (FoM) values arise mainly from the lower dielectric permittivity (ε r = 1372) of the co-doped samples, since g₃₃=d₃₃/(ε₀ε r ) and FoM = d₃₃ 2 /ε r . Table 3 Density (ρ), relative density (%, considering 6.01 g/cm 3 as the theoretical density), relative permittivity (ε r ), and charge (d 33 ), and voltage (g 33 ) piezoelectric coefficients (at room temperature), and the figure of merit (FoM) for harvesting of the sintered samples. Sample ρ (g/cm 3 ) Relative density (%) d 33 (pC/N) ε r g 33 (10 -3 Vm/N) FoM (pC²/N²) BNT-5BT 5.70 94.84 110 1560 7.98 7.76 BNT-5BT-2Zr-1Cu 5.74 95.51 130 1372 10.71 12.32 BNT-5BT-1Cu 5.82 96.84 130 1443 10.19 11.70 BNT-5BT-2Zr 5.74 95.51 135 1503 10.54 12.12 The improvement in g 33 highlights the effectiveness of co-doping in balancing polarization strength and dielectric softness. The doped compositions exhibit a significant enhancement in the piezoelectric voltage coefficient g₃₃, despite only moderate changes in d₃₃. Furthermore, the co-doped composition shows a marked improvement in the energy-harvesting figure of merit (d₃₃ 2 /ε r ), reaching almost twice the value of the undoped ceramic, which indicates an improved potential for electromechanical energy conversion. This behavior is mainly attributed to the reduced dielectric permittivity combined with a comparatively stable coercive field, thereby favoring a large reversible polarization component under mechanical loading. The resulting increase in g₃₃, together with the stable room-temperature ferroelectric response, suggests that co-doped BNT–BT ceramics are suitable for energy-harvesting applications, under low-power mechanical excitation. Therefore, under the room-temperature conditions explored here, co-doping improves the material’s voltage response and harvesting-relevant metrics, whereas the effect of the T F-R –T max interval should be considered case-dependent. Furthermore, the incorporation of CuO enables a reduction in the sintering temperature of about 100°C, representing an additional benefit of the co-doped compositions. 4. Conclusions Cu/Zr co-doping in BNT–5BT is shown to be an effective strategy for enhancing voltage-related electromechanical performance through combined structural tuning and microstructural control. XRD and Raman analyses indicate stabilization of a tetragonal-enriched morphotropic phase boundary (MPB) state without secondary phases. Rietveld refinement and lattice expansion support effective Zr⁴⁺ incorporation at the B-site, while Cu addition facilitates densification and enables a sintering temperature reduction of ~ 100°C. The resulting ceramics exhibit dense and homogeneous microstructures with controlled grain growth. Dielectric measurements reveal an upward shift of the ferroelectric–relaxor crossover temperature (T F–R ≈ 195°C) and a narrowed T F–R –T max interval. Although the permittivity maximum is diffuse and the crossover region shows frequency dependence, the presence of well-developed ferroelectric hysteresis loops indicates that the system remains predominantly non-ergodic at room temperature. The broad dielectric response is therefore attributed to compositional heterogeneity and local free-energy fluctuations. Functionally, the co-doped composition combines moderate d₃₃ (~ 130 pC·N⁻¹) with reduced dielectric permittivity (εr ≈ 1372), yielding an enhanced voltage coefficient (g₃₃ ≈ 10.71 × 10⁻³ V·m·N⁻¹) and improved harvesting figure of merit (d₃₃²/ε r ). This enhancement originates from an optimized balance between polarization magnitude, dielectric stiffness, and phase-boundary stabilization, rather than from hysteresis suppression. Compared with single-doped counterparts, the co-doped composition combines (i) a higher T F–R , (ii) suppressed ε r , and (iii) stable ferroelectric behavior at room temperature, and this interplay enhances g 33 = d 33 /(ε 0 ε r ) and benefits low-power electromechanical transduction. Overall, the results demonstrate that compositional co-design (Cu primarily promoting densification and defect equilibration, and Zr contributing to structural tuning) is an effective route to improve voltage sensitivity in lead-free BNT–BT ceramics. In order to translate these materials-level gains into operational harvesting performance, application-level validation under realistic excitation spectra and cycling/aging conditions, together with temperature-dependent electromechanical characterization, remains a relevant next step. Declarations Competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution M.D. performed the experimental work, prepared the figures, and wrote the manuscript text. M.C. and L.R. were responsible for the conceptualization and supervision of the study. J.C. contributed to data curation. All authors reviewed the manuscript. Acknowledgements The authors would like to thank the National University of Mar del Plata (Argentina) Project (15/G689 - ING693/23), CONICET PIP 11220200102487CO and the National Agency for Scientific and Technological Promotion (ANPCyT) PICT Start up 2021 for providing financial support. Data Availability All data generated or analyzed during this study are included in this published article and no additional source data are required. References Y. Saito, H. Takao, T. Toshihiko, N. Toshihiko, Lead-free piezoceramics. Nature. 432 , 84–87 (2004). https://doi.org/10.1038/nature03028 T. Takenaka, K. Maruyama, K. Sakata, (Bi 1/2 Na 1/2 )TiO 3 -BaTiO 3 System for Lead-Free Piezoelectric Ceramics. Jpn J. Appl. Phys. 30 , 2236–2239 (1991). https://doi.org/10.1143/JJAP.30.2236 F. Rubio-Marcos, J.J. Romero, D.A. Ochoa, J.E. García, R. Perez, J.F. Fernandez, Effects of Poling Process on KNN-Modified Piezoceramic Properties. J. Am. Ceram. Soc. 93 , 318–321 (2010). https://doi.org/10.1111/j.1551-2916.2009.03404.x M. Difeo, M. Castro, L. Ramajo, Effect of Zr Doping on BNT–5BT Lead-Free Ceramics: Substitutional and Excess Incorporation Analysis. Micro. 5 , 35 (2025). https://doi.org/10.3390/micro5030035 M.K. Sharma, P. Singh, D. Panda, P. Sharma, Tuning structural and functional properties of Bi 0.5 Na 0.5 TiO 3 -Ba 0.7 Ca 0.3 TiO 3 ceramics through compositional engineering near the morphotropic phase boundary. Ceram. Int. 51 , 54081–54087 (2025). https://doi.org/10.1016/j.ceramint.2025.09.150 S. Subramanian, S. R, S. Subramanian, A. Karuppanan, Charge correlation of ferroelectric and piezoelectric properties of (1 – x)(Na 0.5 Bi 0.5 )TiO 3 –xBaTiO 3 lead-free ceramic solid solution. J. Mater. Sci.: Mater. Electron. 28 (2017). https://doi.org/10.1007/s10854-017-6753-5 Y.-J. Dai, J. Pan, X. Zhang, Composition Range of Morphotropic Phase Boundary and Electrical Properties of NBT-BT System. Key Eng. Mater. 336–338 (2007). 206–209. https://doi.org/10.4028/www.scientific.net/KEM.336-338.206 N. Sezer, M. Koç, A comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Nano Energy. 80 , 105567 (2021). https://doi.org/10.1016/j.nanoen.2020.105567 M. Ozgul, A. Kucuk, B 2 O 3 doping in 0.94(Bi 0.5 Na 0.5 )TiO 3 –0.06BaTiO 3 lead-free piezoelectric ceramics. Ceram. Int. 42 , 19119–19123 (2016). https://doi.org/10.1016/j.ceramint.2016.09.073 N.B. Mahmood, E.K. Al-Shakarchi, Dielectric properties of BNT-xBT prepared by hydrothermal process. J. Adv. Dielect. 07 , 1750019 (2017). https://doi.org/10.1142/S2010135X17500199 X. Cheng, L.K. Venkataraman, Y. Li, Simultaneous enhancement of piezoelectric constant and thermal stability in lead-free Fe-doped 0.94(Na 1/2 Bi 1/2 )TiO 3 -0.06BaTiO 3 ceramics. J. Alloys Compd. 891 , 161880 (2022). https://doi.org/10.1016/j.jallcom.2021.161880 G. Liang, Y. Zhang, J. Zhu, Q. Zhang, B. Peng, Tailoring and improving the strong-electric-field electrical properties of the BNT-BT ferroelectric ceramics by a functional-group-doping. Ceram. Int. 47 , 6584–6590 (2021). https://doi.org/10.1016/j.ceramint.2020.10.247 M. Pereira, A.G. Peixoto, M.J.M. Gomes, Effect of Nb doping on the microstructural and electrical properties of the PZT ceramics. J. Eur. Ceram. Soc. 21 , 1353–1356 (2001). https://doi.org/10.1016/S0955-2219(01)00017-6 M.M.S. Pojucan, M.C.C. Santos, F.R. Pereira, M.A.S. Pinheiro, M.C. Andrade, Piezoelectric properties of pure and (Nb 5+ + Fe 3+ ) doped PZT ceramics. Ceram. Int. 36 , 1851–1855 (2010). https://doi.org/10.1016/j.ceramint.2010.03.025 Y. Hu, H. Zhang, Z. Wang, J. Shen, J. Zhou, W. Chen, Ferroelectric-order-dominated doping effects in Fe, Nb co-doped Bi 1/2 (Na 0.8 K 0.2 ) 1/2 TiO 3 piezoelectric ceramics. Ceram. Int. (2026). https://doi.org/10.1016/j.ceramint.2026.02.120 P. Jaita, S. Manotham, G. Rujijanagul, Influence of Al 2 O 3 nanoparticle doping on depolarization temperature, and electrical and energy harvesting properties of lead-free 0.94(Bi 0.5 Na 0.5 )TiO 3 –0.06BaTiO 3 ceramics. RSC Adv. 10 , 32078–32087 (2020). https://doi.org/10.1039/D0RA04866F B.K. Barick, R.N.P. Choudhary, D.K. Pradhan, Dielectric and impedance spectroscopy of zirconium modified (Na 0.5 Bi 0.5 )TiO 3 ceramics. Ceram. Int. 39 , 5695–5704 (2013). https://doi.org/10.1016/j.ceramint.2012.12.087 A. Watcharapasorn, S. Jiansirisomboon, T. Tunkasiri, Microstructure and mechanical properties of zirconium-doped bismuth sodium titanate ceramics. Chiang Mai J. Sci. 33 , 169–173 (2006) T.R. Armstrong, L.E. Morgens, A.K. Maurice, R.C. Buchanan, Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics. J. Am. Ceram. Soc. 72 , 605–611 (1989). https://doi.org/10.1111/j.1151-2916.1989.tb06182.x M. Difeo, L. Ramajo, M. Castro, Influence of CuO addition on dielectric and piezoelectric properties of (Bi 0.5 Na0.5)TiO 3 –BaTiO 3 lead-free piezoceramics. J. Adv. Dielect. 11 , 2140004 (2021). https://doi.org/10.1142/S2010135X2140004X D. Rout, K.-S. Moon, V.S. Rao, S.-J.L. Kang, Study of the morphotropic phase boundary in the lead-free Na 1/2 Bi 1/2 TiO 3 -BaTiO 3 system by Raman spectroscopy. J. Ceram. Soc. Japan. 117 , 797–800 (2009). https://doi.org/10.2109/jcersj2.117.797 J. Yao, N. Monsegue, M. Murayama, W. Leng, W.T. Reynolds, Q. Zhang, H. Luo, J. Li, W. Ge, D. Viehland, Role of coexisting tetragonal regions in the rhombohedral phase of Na 0.5 Bi 0.5 TiO 3 -xat.%BaTiO 3 crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary. Appl. Phys. Lett. 100 , 012901 (2012). https://doi.org/10.1063/1.3673832 B. Wylie-van Eerd, D. Damjanovic, N. Klein, N. Setter, J. Trodahl, Structural complexity of (Na 0.5 Bi 0.5 )TiO 3 -BaTiO 3 as revealed by Raman spectroscopy. Phys. Rev. B 82 , 104112 (2010). https://doi.org/10.1103/PhysRevB.82.104112 A. Prado, F. Rubio-Marcos, L. Ramajo, M.S. Castro, Dielectric and ferroelectric properties evolution of (1 – x)(Bi 0.5 Na 0.5 TiO 3 )–xK 0.5 Na 0.5 NbO 3 piezoceramics. Bull. Mater. Sci. 43 , 282 (2020). https://doi.org/10.1007/s12034-020-02246-9 A.A. Bokov, Z.-G. Ye, Recent Progress in Relaxor Ferroelectrics with Perovskite Structure. Progress in Advanced Dielectrics , World Scientific, Singapore, 2019: 105–164. https://doi.org/10.1142/9789811210433_0003 W.-C. Lee, C.-Y. Huang, L.-K. Tsao, Y.-C. Wu, Chemical composition and tolerance factor at the morphotropic phase boundary in (Bi 0.5 Na 0.5 )TiO 3 -based piezoelectric ceramics. J. Eur. Ceram. Soc. 29 , 1443–1448 (2009). https://doi.org/10.1016/j.jeurceramsoc.2008.08.028 W. Jo, J.-B. Ollagnier, J.-L. Park, E.-M. Anton, O.-J. Kwon, C. Park, H.-H. Seo, J.-S. Lee, E. Erdem, R.-A. Eichel, J. Rödel, CuO as a sintering additive for (Bi 1/2 Na 1/2 )TiO 3 –(K 0.5 Na 0.5 )NbO 3 lead-free piezoceramics. J. Eur. Ceram. Soc. 31 , 2107–2117 (2011). https://doi.org/10.1016/j.jeurceramsoc.2011.05.008 Z.-H. Zhao, R.-F. Ge, Y. Dai, Large electro-strain signal of the BNT–BT–KNN lead-free piezoelectric ceramics with CuO doping. J. Adv. Dielect. 09 , 1950022 (2019). https://doi.org/10.1142/S2010135X1950022X H. Vu, D. Nguyen, J.G. Fisher, W.-H. Moon, S. Bae, H.-G. Park, B.-G. Park, CuO-based sintering aids for low temperature sintering of BaFe 12 O 19 ceramics. J. Asian. Ceam. Soc. 1 , 170–177 (2013). https://doi.org/10.1016/j.jascer.2013.05.002 M. Reda, S.I. El-Dek, M.M. Arman, Improvement of ferroelectric properties via Zr doping in barium titanate nanoparticles. J. Mater. Sci.: Mater. Electron. 33 , 16753–16776 (2022). https://doi.org/10.1007/s10854-022-08541-x W. Kleemann, The relaxor enigma — charge disorder and random fields in ferroelectrics. J. Mater. Sci. 41 , 129–136 (2006). https://doi.org/10.1007/s10853-005-5954-0 D. Viehland, S.J. Jang, L.E. Cross, M. Wuttig, Deviation from Curie-Weiss behavior in relaxor ferroelectrics. Phys. Rev. B 46 , 8003–8006 (1992). https://doi.org/10.1103/PhysRevB.46.8003 J. Glaum, Y. Heo, M. Acosta, P. Sharma, J. Seidel, M. Hinterstein, Revealing the role of local stress on the depolarization of BNT-BT-based relaxors. Phys. Rev. Mater. 3 , 054406 (2019). https://doi.org/10.1103/PhysRevMaterials.3.054406 M. Kar, L.K. Pradhan, Relaxor Ferroelectric Oxides: Concept to Applications, in: D.R. Sahu (Ed.), Multifunctional Ferroelectric Materials, IntechOpen, London, 2021. https://doi.org/10.5772/intechopen.96185 K.P. Kelley, A.N. Morozovska, E.A. Eliseev, Y. Liu, S.S. Fields, S.T. Jaszewski, T. Mimura, S. Calderon, E.C. Dickey, J.F. Ihlefeld, S.V. Kalinin, Ferroelectricity in hafnia controlled via surface electrochemical state. Nat. Mater. 22 , 1144–1151 (2023). https://doi.org/10.1038/s41563-023-01619-9 Q. Li, J. Wang, Y. Ma, L. Ma, G. Dong, H. Fan, Enhanced energy-storage performance and dielectric characterization of 0.94Bi 0.5 Na 0.5 TiO 3 –0.06BaTiO 3 modified by CaZrO 3 . J. Alloys Compd. 663 , 701–707 (2016). https://doi.org/10.1016/j.jallcom.2015.12.194 H. Orihara, S. Hashimoto, Y. Ishibashi, A Theory of D-E Hysteresis Loop Based on the Avrami Model. J. Phys. Soc. Jpn. 63 , 1031–1035 (1994). https://doi.org/10.1143/JPSJ.63.1031 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9223133","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":612525675,"identity":"643cf4bd-44f1-4e8a-bfed-35dd2f25673e","order_by":0,"name":"Mauro Difeo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+0lEQVRIiWNgGAWjYNACNgl+BgbmAx8SGBgYG4B8CWK0SDYwsCXOIEULA1ALj+EMBmK06LafMfvwocxCgp+952PDA4ZtstsZmA/e5mGwy8elxexMjvHMGeckJCR7zm5sSGC4bbyzgS3Zmoch2bIBl5YDOcbMvG0SdQY3crc/AGpJ3HCAx0yah4HZAKct59+AtUjY38h52ADRwv8NqKUet5YbEFskDCRyGKFaeNiAWg7j0fKsmBHkF4kzxwwbEgyAfmlmM7acY3Acj8OSNzN8KKuT4G9vftj4o+K27Hb25oc33lRU49SCBoDqDJihDOIBSYpHwSgYBaNgRAAA9TZT5UUlZR0AAAAASUVORK5CYII=","orcid":"","institution":"Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), CONICET – UNMdP","correspondingAuthor":true,"prefix":"","firstName":"Mauro","middleName":"","lastName":"Difeo","suffix":""},{"id":612525678,"identity":"66fdb761-68cb-459a-9901-bf2a0f64f362","order_by":1,"name":"Javier Camargo","email":"","orcid":"","institution":"Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), CONICET – UNMdP","correspondingAuthor":false,"prefix":"","firstName":"Javier","middleName":"","lastName":"Camargo","suffix":""},{"id":612525682,"identity":"1cfa0fca-b80e-45fb-ba37-a459d6039268","order_by":2,"name":"Miriam Castro","email":"","orcid":"","institution":"Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), CONICET – UNMdP","correspondingAuthor":false,"prefix":"","firstName":"Miriam","middleName":"","lastName":"Castro","suffix":""},{"id":612525686,"identity":"7abff22d-30ed-4cc4-9c9f-0c116b628494","order_by":3,"name":"Leandro Ramajo","email":"","orcid":"","institution":"Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), CONICET – UNMdP","correspondingAuthor":false,"prefix":"","firstName":"Leandro","middleName":"","lastName":"Ramajo","suffix":""}],"badges":[],"createdAt":"2026-03-25 12:38:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9223133/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9223133/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107359885,"identity":"bd7818ef-60fc-4190-8ebb-8f84d7ca0724","added_by":"auto","created_at":"2026-04-20 17:55:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":30131,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) \u0026nbsp;\u003c/strong\u003eRoom-temperature X-ray diffraction (XRD) patterns of BNT–5BT and Cu/Zr-modified BNT–5BT ceramics (labels indicate representative perovskite reflections). No secondary phases are detected within the instrumental resolution. \u003cstrong\u003e(b) \u003c/strong\u003eEnlarged views of the pseudocubic {111} region (2θ = 39.5°–40.5°), highlighting MPB-related peak asymmetry/splitting associated with rhombohedral-like and tetragonal-like contributions. The experimental profiles (thick line) were peak-profile fitted with three Gaussian components, assigned to P4mm- and R3c-type reflections (color components), to provide a model-dependent, semi-quantitative indicator of the relative rhombohedral-like/tetragonal-like contributions. A whole-pattern Rietveld refinement using a two-phase model (R3c + P4mm) for the two bounding compositions is included as an independent check of the phase-balance trend (\u003cstrong\u003eTable 1).\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/fb27e8de95d110406b4a1ec2.png"},{"id":107359886,"identity":"56a91d6b-4415-4691-bd83-9f440b81c2e7","added_by":"auto","created_at":"2026-04-20 17:55:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":21805,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of the BNT-5BT, BNT-5BT-2Zr-1Cu, BNT-5BT-1Cu, and BNT-5BT-2Zr sintered samples.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/e9e6195e89c05ec58c1e5bbf.png"},{"id":107359889,"identity":"be298c6b-09de-4f43-9720-5a1c94d36e95","added_by":"auto","created_at":"2026-04-20 17:55:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":478021,"visible":true,"origin":"","legend":"\u003cp\u003eFE-SEM images and the corresponding grain size distributions of the BNT-5BT, BNT-5BT-2Zr-1Cu, BNT-5BT-1Cu, and BNT-5BT-2Zr sintered samples.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/a37242b2ea1913cb7d946a2b.png"},{"id":107359887,"identity":"500af73a-ea6b-45ef-9a5d-2913e7890a7a","added_by":"auto","created_at":"2026-04-20 17:55:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":86654,"visible":true,"origin":"","legend":"\u003cp\u003eDependence of the real permittivity \u003cstrong\u003e(a)\u003c/strong\u003e, dielectric loss \u003cstrong\u003e(b)\u003c/strong\u003e on temperature (at 1 kHz) of the BNT-5BT, BNT-5BT-2Zr-1Cu, BNT-5BT-1Cu, and BNT-5BT-2Zr sintered samples; and \u003cstrong\u003e(c)\u003c/strong\u003e dependence of the real permittivity, and dielectric loss \u003cstrong\u003e(d)\u003c/strong\u003e on frequency and temperature of the co-doped samples (BNT-5BT-2Zr-1Cu).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/e006addd6b2d42bde68bda42.png"},{"id":107488563,"identity":"3378edb8-31ec-4c73-975d-2ae1478116ac","added_by":"auto","created_at":"2026-04-22 02:45:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":24560,"visible":true,"origin":"","legend":"\u003cp\u003ePolarization–electric field (P–E) hysteresis loops of BNT–5BT, BNT–5BT–2Zr–1Cu, BNT–5BT–1Cu, and BNT–5BT–2Zr ceramics measured at room temperature.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/8cc2e9adcd40556f1b775e70.png"},{"id":107489822,"identity":"80ce0045-32a1-4f4d-844f-2f42cec9a32a","added_by":"auto","created_at":"2026-04-22 02:49:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1074276,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9223133/v1/2f74b061-dbc4-4b42-b9fb-93f654cceee2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Optimization of Piezoelectric Performance in Lead-Free BNT–5BT Ceramics by Cu/Zr Co- Doping: Structure–Property Correlations","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe global shift toward environmentally sustainable materials has intensified research on lead-free piezoelectric ceramics as replacements for Pb(Zr,Ti)O\u003csub\u003e3\u003c/sub\u003e (PZT), which, despite its outstanding electromechanical performance, raises significant environmental and health concerns because of lead toxicity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Among various alternatives, Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e (BNT)-based systems have attracted particular attention due to their strong ferroelectricity, high Curie temperature (~\u0026thinsp;320\u0026deg;C), and compositional tunability [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The solid solution 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO₃\u0026ndash;0.05BaTiO₃ (BNT\u0026ndash;5BT) is located near a morphotropic phase boundary (MPB) between rhombohedral and tetragonal symmetries, where enhanced piezoelectric and dielectric responses can arise from phase coexistence and field-induced polarization rotation/extension mechanisms [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. These features make BNT\u0026ndash;BT ceramics promising candidates for actuation, sensing, and low-power energy-harvesting applications, in which a high piezoelectric voltage coefficient (g\u003csub\u003e33\u003c/sub\u003e​), favored by combining a reasonably high d\u003csub\u003e33\u003c/sub\u003e with suppressed permittivity, is required [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNevertheless, undoped BNT\u0026ndash;BT exhibits limited poling efficiency and moderate voltage sensitivity, mainly due to its relatively high permittivity, which motivates the exploration of chemical modifications to tailor its defect chemistry and structural response. In contrast to PZT ceramics, where functional properties can be effectively tailored through low-level aliovalent doping, commonly classified as softening or hardening, previous studies have shown that in BNT\u0026ndash;BT ceramics, doping with a single A- or B-site element, or even co-doping at both sites, often fails to achieve the desired properties [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In PZT systems, donor dopants (e.g., Nb⁵⁺ substituting for Zr⁴⁺/Ti⁴⁺) typically enhance dielectric and piezoelectric responses but increase dielectric and mechanical losses [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], whereas acceptor dopants (e.g., Fe\u0026sup3;⁺) reduce these responses while improving the mechanical quality factor and lowering losses [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These contrasting effects are generally attributed to defect\u0026ndash;domain wall interactions: oxygen vacancies generated by acceptor doping pin domain walls, while donor doping facilitates their motion. However, in BNT-based ceramics, the piezoelectric response deviates from this conventional hardening\u0026ndash;softening paradigm [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. For instance, Fe acceptor doping has been reported to enhance the d₃₃, whereas Nb donor doping reduces it [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. This unconventional behavior has been mainly related to the stabilization or disruption of long-range ferroelectric order, indicating that the effect of doping in BNT-based systems depends strongly on the preservation of ferroelectric correlations.\u003c/p\u003e \u003cp\u003eAmong the dopants explored in BNT-based ceramics, Cu and Zr are particularly attractive because they can influence different aspects of the structure\u0026ndash;microstructure\u0026ndash;property relationship; Cu\u003csup\u003e2+\u003c/sup\u003e can act as a sintering aid, promoting densification at lower temperatures and improving microstructural homogeneity, commonly through transient liquid-phase formation, while also modifying local defect equilibria and dielectric losses [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. In contrast, Zr\u003csup\u003e4+\u003c/sup\u003e substitution for Ti\u003csup\u003e4+\u003c/sup\u003e at the perovskite B-site expands the lattice and can modify the rhombohedral/tetragonal phase balance, often increasing the tetragonal contribution near the MPB and thereby improving the electromechanical response [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Recent studies have shown that moderate Zr incorporation can reduce the coercive field while preserving relatively high remanent polarization, leading to enhanced d\u003csub\u003e33\u003c/sub\u003e values in BNT\u0026ndash;BT ceramics [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAlthough both dopants have been individually investigated, their combined effect in BNT\u0026ndash;5BT has not yet been reported, to the best of our knowledge. Their distinct roles suggest a potentially synergistic effect: Cu may promote densification and microstructural uniformity, whereas Zr may tune the B-site lattice and the phase balance near the MPB, thereby modifying the dielectric response and voltage-related electromechanical parameters. Therefore, Cu/Zr co-doping may provide an effective route to simultaneously tailor microstructure, symmetry, and permittivity while preserving piezoelectric activity.\u003c/p\u003e \u003cp\u003eBased on earlier single-dopant screening, where 1 mol% CuO (BNT\u0026ndash;5BT\u0026ndash;1Cu) maximized densification and piezoelectric response [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], and 2 mol% Zr (BNT\u0026ndash;5BT\u0026ndash;2Zr) delivered the best voltage coefficient [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], this work examines four reference compositions: undoped BNT\u0026ndash;5BT, the single-doped BNT\u0026ndash;5BT\u0026ndash;1Cu and BNT\u0026ndash;5BT\u0026ndash;2Zr, and the co-doped BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu samples. Zirconium was introduced via a substitutional approach to favor incorporation into the perovskite lattice, while CuO was added in excess to promote Cu-assisted densification (often associated with transient liquid-phase effects) during sintering. The aim of this work is to determine how Cu/Zr co-doping modifies (i) average symmetry and local structure (through X-ray diffraction and Raman spectroscopy), (ii) the ferroelectric\u0026ndash;relaxor transition window (T\u003csub\u003eF\u0026minus;R\u003c/sub\u003e ​ and T\u003csub\u003emax\u003c/sub\u003e​), and (iii) the electromechanical and dielectric response (d\u003csub\u003e33\u003c/sub\u003e, ε\u003csub\u003er\u003c/sub\u003e​, and g\u003csub\u003e33\u003c/sub\u003e=d\u003csub\u003e33\u003c/sub\u003e/(ε\u003csub\u003e0\u003c/sub\u003eε\u003csub\u003er\u003c/sub\u003e)), thereby establishing structure\u0026ndash;property correlations relevant to lead-free piezoelectric ceramics for low-power energy harvesting.\u003c/p\u003e"},{"header":"2. Experimental Procedure","content":"\u003cp\u003eCeramic powders were prepared by the solid-state reaction method using high-purity Bi₂O₃, Na₂CO₃, BaCO₃, TiO₂, ZrO₂, and CuO precursors. Four benchmark compositions were studied: the undoped 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e-0.05BaTiO\u003csub\u003e3\u003c/sub\u003e (BNT\u0026ndash;5BT), Cu-doped 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e-0.05BaTiO\u003csub\u003e3\u003c/sub\u003e-yCuO (BNT-5BT-1Cu; y\u0026thinsp;=\u0026thinsp;1 mol%); and Zr-doped 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTi\u003csub\u003e1-x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-0.05BaTi\u003csub\u003e1-x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (BNT-5BT-2Zr; x\u0026thinsp;=\u0026thinsp;2 mol%) and co-doped 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTi\u003csub\u003e1-x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.05BaTi\u003csub\u003e1-x\u003c/sub\u003eZr\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-yCuO (BNT-5BT-2Zr-1Cu; x\u0026thinsp;=\u0026thinsp;2 mol% and y\u0026thinsp;=\u0026thinsp;1 mol%) sample, which combines both dopants at their optimized single-dopant contents [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The stoichiometric powders were mixed in isopropanol with zirconia balls and mechanically activated in a planetary ball mill (500 rpm, 3 h). After drying, the mixtures were calcined at 850\u0026deg;C for 2 h to ensure perovskite phase formation. The calcined powders were milled and uniaxially pressed into disk-shaped pellets (\u0026Oslash; = 10 mm, t\u0026thinsp;=\u0026thinsp;1 mm) at a pressure of 200 MPa.\u003c/p\u003e \u003cp\u003eAccording to previous studies [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], Cu-doped samples (BNT\u0026ndash;5BT\u0026ndash;1Cu and BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu) were sintered at 1050\u0026deg;C for 2 h in air, whereas Cu-free samples (BNT\u0026ndash;5BT and BNT\u0026ndash;5BT\u0026ndash;2Zr) were sintered at 1150\u0026deg;C for 2 h in air. All compositions were prepared following the same solid-state processing route and characterization protocol; the only difference was the sintering temperature selected for Cu-containing versus Cu-free ceramics. Structural analyses were conducted by X-ray diffraction (PANalytical X\u0026rsquo;Pert PRO, CuKα radiation, 2θ\u0026thinsp;=\u0026thinsp;20\u0026ndash;80\u0026deg;) and Raman spectroscopy (Renishaw inVia, 514 nm excitation). Microstructural features were observed using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) after thermal etching at 100\u0026deg;C below the corresponding sintering temperature, for 15 min. The apparent density was determined by the Archimedes method.\u003c/p\u003e \u003cp\u003eDielectric properties were measured using an LCR meter (HP 4284A) in the frequency range of 1 kHz to 1 MHz and over the temperature range 25 to 500\u0026deg;C. Ferroelectric hysteresis (P\u0026ndash;E) loops were recorded using a modified Sawyer\u0026ndash;Tower circuit at 50 Hz under fields up to 7 kV\u0026middot;mm\u003csup\u003e-1\u003c/sup\u003e at room temperature. The piezoelectric constant (d\u003csub\u003e33\u003c/sub\u003e) was obtained with a quasi-static piezometer (YE2730A, Sinocera) after poling at 2.5 kV\u0026middot;mm\u003csup\u003e-1\u003c/sup\u003e in silicone oil at 150\u0026deg;C for 30 min. The piezoelectric voltage coefficient was calculated from g\u003csub\u003e33\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;d\u003csub\u003e33\u003c/sub\u003e/(ε\u003csub\u003e0\u003c/sub\u003eε\u003csub\u003er\u003c/sub\u003e), where ε\u003csub\u003e0\u003c/sub\u003e is the vacuum permittivity, and ε\u003csub\u003er\u003c/sub\u003e is the real part of the dielectric permittivity measured at room temperature at 1 kHz.\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Structural Analysis\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the room-temperature X-ray diffraction (XRD) patterns of BNT\u0026ndash;5BT and the Cu/Zr-modified compositions. All diffractograms were indexed on the basis of a perovskite structure, and no secondary phases were observed within the instrumental resolution, suggesting that the dopants did not produce detectable crystalline segregates.\u003c/p\u003e\n \u003cp\u003eExpanded views of the {111} diffraction peaks (39.5\u0026deg;- 40.5\u0026deg; in 2\u0026theta;) are presented in Fig. \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. The peak splitting reveals the characteristic peak asymmetry/splitting associated with the morphotropic phase boundary (MPB) of BNT\u0026ndash;BT-based systems [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], i.e., the coexistence of rhombohedral-like (R3c) and tetragonal-like (P4mm) contributions at room temperature. To evaluate the evolution of this coexistence, the experimental profiles were fitted using three Gaussian components (two associated with the rhombohedral-like contribution and one with the tetragonal-like contribution). Although this approach provides only a semi-quantitative estimate, it enables a consistent comparison across the full compositional series.\u003c/p\u003e\n \u003cp\u003eThe diffraction patterns of the two end-member compositions (undoped BNT\u0026ndash;5BT and co-doped BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu) were refined by the Rietveld method using a two-phase model (R3c\u0026thinsp;+\u0026thinsp;P4mm) (Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A full Rietveld quantification for the single-doped ceramics was not pursued here, and their comparative discussion relies on the same peak-deconvolution protocol applied to all samples, complemented by Raman signatures of local symmetry. The refined weight fractions corroborate the deconvolution trend: BNT\u0026ndash;5BT contains 67.77\u0026thinsp;\u0026plusmn;\u0026thinsp;2.20 wt% R3c and 32.23\u0026thinsp;\u0026plusmn;\u0026thinsp;1.97 wt% P4mm, whereas BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu exhibits 42.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 wt% R3c and 57.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 wt% P4mm. These results support the conclusion that co-doping shifts the room-temperature phase balance toward a tetragonal-rich perovskite state.\u003c/p\u003e\n \u003cp\u003eThe refined unit-cell volumes further support effective Zr incorporation at the B-site. For the rhombohedral phase, the unit-cell volume increases from 353.39 \u0026Aring;\u0026sup3; to 358.13 \u0026Aring;\u0026sup3; upon Zr incorporation. When normalized per ABO₃ unit, V/Z increases from ~\u0026thinsp;58.90 \u0026Aring;\u0026sup3; to ~\u0026thinsp;59.69 \u0026Aring;\u0026sup3;, consistent with lattice expansion due to substitution of Ti⁴⁺ (0.605 \u0026Aring;) by larger Zr⁴⁺ (0.72 \u0026Aring;). A slight volume increase is also observed for the tetragonal phase. These results are consistent with Zr incorporation into the perovskite lattice rather than with the formation of detectable secondary phases.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eRietveld refinement results for the two bounding compositions, BNT\u0026ndash;5BT and BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu, using a two-phase model (R3c\u0026thinsp;+\u0026thinsp;P4mm): refined phase weight fractions, unit-cell volumes (V), Z (number of formula units per unit cell), and refinement quality indicators (R\u003csub\u003eBragg\u003c/sub\u003e and R\u003csub\u003ef\u003c/sub\u003e). These refinements are included as an independent check of the phase-balance trend inferred from peak deconvolution; the comparative discussion across the full series is based on the same deconvolution protocol applied to all samples. Uncertainties correspond to the refinement output.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003ePhase\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eFraction\u003c/p\u003e\n \u003cp\u003e(%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003eUnit-cell volume (A˚\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eZ\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eV/Z\u003c/p\u003e\n \u003cp\u003e\u0026Aring;\u0026sup3; per ABO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eR\u003csub\u003eBragg\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eR\u003csub\u003ef\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\n \u003cp\u003eBNT-5BT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eR3c\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c3\"\u003e\n \u003cp\u003e67.77\u0026thinsp;\u0026plusmn;\u0026thinsp;2.20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c4\"\u003e\n \u003cp\u003e353.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.045\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c6\"\u003e\n \u003cp\u003e58.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e23.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e19.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eP4mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c3\"\u003e\n \u003cp\u003e32.23\u0026thinsp;\u0026plusmn;\u0026thinsp;1.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c4\"\u003e\n \u003cp\u003e59.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.027\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c6\"\u003e\n \u003cp\u003e59.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e23.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e21.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\n \u003cp\u003eBNT-5BT-2Zr-1Cu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eR3c\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c3\"\u003e\n \u003cp\u003e42.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c4\"\u003e\n \u003cp\u003e358.13\u0026thinsp;\u0026plusmn;\u0026thinsp;0.023\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c6\"\u003e\n \u003cp\u003e59.69\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e27.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e21.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eP4mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c3\"\u003e\n \u003cp\u003e57.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c4\"\u003e\n \u003cp\u003e59.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.021\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\"±\" colname=\"c6\"\u003e\n \u003cp\u003e59.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e31.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e19.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eFrom a functional point of view, shifting the MPB balance toward a tetragonal-rich state is relevant because it modifies the polarization landscape and the available switching/rotation pathways under field or stress. In BNT-BT systems, enhanced electromechanical response is frequently associated with optimized rhombohedral\u0026ndash;tetragonal (R\u0026ndash;T) coexistence and the ease of polarization reorientation near the MPB [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Here, the Rietveld results provide a quantitative structural basis to interpret the improved performance of the Cu/Zr co-doped ceramic, as the stabilized tetragonal contribution and the associated lattice modifications are expected to facilitate more efficient domain processes during poling and operation.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the room-temperature Raman spectra of BNT\u0026ndash;5BT, BNT-5BT-2Zr, BNT-5BT-1Cu, and the co-doped BNT-5BT-2Zr-1Cu, together with their Lorentzian deconvolution. The spectral features are consistent with previous reports and indicate a progressive stabilization of tetragonal-like character across the series. In particular, the band near 310 cm⁻\u0026sup1; (Ti\u0026ndash;O vibrations in distorted TiO\u003csub\u003e6\u003c/sub\u003e octahedra), shifts systematically from 310.2 cm⁻\u0026sup1; (undoped) \u0026rarr; 310.6 cm⁻\u0026sup1; (2Zr ) \u0026rarr; 312.7 cm⁻\u0026sup1; (1Cu) \u0026rarr; 318.6 cm⁻\u0026sup1; (2Zr\u0026ndash;1Cu), and its relative area increases in the co-doped sample, indicating enhanced octahedral anisotropy and tetragonality [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Moreover, the doublet in the 520\u0026ndash;600 cm⁻\u0026sup1; region, assigned to Ti\u0026ndash;O\u0026ndash;Ti stretching in edge-shared octahedra, becomes more clearly resolved upon co-doping, with an increased separation between the \u0026sim;520 and \u0026sim;592 cm⁻\u0026sup1; components, which is commonly associated with enhanced tetragonal splitting [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. These trends are consistent with (i) Zr\u003csup\u003e4+\u003c/sup\u003e substitution at the B-site (larger ionic radius than Ti\u003csup\u003e4+\u003c/sup\u003e), introducing local strain that favors tetragonal distortions, and (ii) Cu\u003csup\u003e2+\u003c/sup\u003e acting as a sintering/defect-chemistry modifier that homogenizes the lattice and reduces random fields, which may contribute to a clearer spectroscopic differentiation of the tetragonal features [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Altogether, the Raman results, together with the XRD peak analysis, indicate a more defined tetragonal contribution in the co-doped ceramic, which is consistent with its improved electromechanical response.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Microstructural Evolution\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows FE-SEM micrographs and grain-size distributions for all compositions. All samples exhibit dense microstructures with well-defined grain boundaries, consistent with the absence of detectable secondary phases in the XRD and Raman analyses.\u003c/p\u003e\n \u003cp\u003eUndoped BNT\u0026ndash;5BT shows equiaxed grains with a relatively broad size distribution (1.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.89 \u0026micro;m) and minor residual porosity. Zr doping promotes grain growth and improves microstructural homogeneity, increasing the average grain size to 1.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.92 \u0026micro;m. This behavior is consistent with the enhanced diffusional processes and structural stabilization reported for Zr-modified BNT\u0026ndash;BT systems [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. In contrast, Cu-only doping yields a finer and more homogeneous microstructure (0.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.46 \u0026micro;m), primarily due to the lower sintering temperature enabled by the addition of CuO. The transient liquid-phase sintering effect associated with CuO enhances densification while limiting excessive grain growth [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eBy contrast, Cu/Zr co-doping produces the largest average grain size (1.54\u0026thinsp;\u0026plusmn;\u0026thinsp;1.20 \u0026micro;m), despite a sintering temperature approximately 100\u0026deg;C lower than that of undoped BNT\u0026ndash;5BT. This behavior suggests a combined effect of Cu-assisted viscous-phase diffusion, which enhances densification kinetics, and Zr incorporation, which may favor grain growth and structural stabilization [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eNo macroscopic secondary phases are evident in the FE-SEM micrographs within the inspected areas, consistent with the XRD and Raman analyses. However, minor dopant-rich regions, nanoscale segregations, or residual transient liquid phases below the resolution and detection limits of SEM/EDS cannot be ruled out. The distinct microstructural evolution, characterized by grain refinement in the Cu-doped ceramic and controlled grain growth in the Cu/Zr co-doped composition, is consistent with the observed differences in dielectric and piezoelectric behavior.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Dielectric and Ferroelectric Behavior\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-b shows the temperature dependence of the relative permittivity (\u0026epsilon;\u003csub\u003er\u003c/sub\u003e) and dielectric loss (tan\u0026delta;) at 1 kHz. All compositions exhibit a broad permittivity maximum characteristic of diffuse phase transitions in chemically complex perovskites near morphotropic phase boundaries.\u003c/p\u003e\n \u003cp\u003eFor undoped BNT\u0026ndash;5BT, the ferroelectric\u0026ndash;relaxor crossover temperature (T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e) occurs at 168\u0026deg;C, while T\u003csub\u003emax\u003c/sub\u003e is located at 264\u0026deg;C. Upon Cu/Zr co-doping, T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e shifts upward to 195\u0026deg;C, whereas T\u003csub\u003emax\u003c/sub\u003e decreases to 232\u0026deg;C, narrowing the T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e-T\u003csub\u003emax\u003c/sub\u003e interval [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eA clear frequency dependence is observed not only in the vicinity of T\u003csub\u003eF\u0026minus;R\u003c/sub\u003e but also around T\u003csub\u003emax\u003c/sub\u003e (Fig. \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec-d), indicating the presence of highly dynamic polar nanoregions (PNRs) and thermally activated reconfiguration processes [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The significant frequency dispersion of T\u003csub\u003emax\u003c/sub\u003e is a well-known hallmark of classical relaxor behavior [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], confirming that at these elevated temperatures, the system evolves toward a highly disordered ergodic relaxor state. Within a phenomenological framework [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], this enhanced relaxor behavior can be rationalized by the introduction of local random fields due to compositional disorder. Specifically, Zr\u003csup\u003e4+\u003c/sup\u003e substitution at the B-site generates local strain fields because of ionic size mismatch, whereas Cu-related defect equilibria may modify the charge-compensation mechanisms, possibly promoting the formation of defect dipoles such as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Cu}_{Ti}^{\u0026quot;}-{V}_{O}^{\\bullet\\:\\bullet\\:}\\)\u003c/span\u003e\u003c/span\u003e. These combined effects generate spatial fluctuations in the local free-energy landscape, effectively disrupting the long-range ferroelectric order, broadening the dielectric anomaly, and stabilizing the highly dispersive relaxor state at high temperatures.\u003c/p\u003e\n \u003cp\u003eTo assess the diffuseness of the permittivity maximum, the modified Curie\u0026ndash;Weiss approach was applied around T\u003csub\u003emax\u003c/sub\u003e using the following equation [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]:\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:\\frac{1}{{\\epsilon\\:}_{r}}-\\frac{1}{{\\epsilon\\:}_{max}}=\\frac{{\\left(T-{T}_{max}\\right)}^{\\gamma\\:}}{C\u0026acute;}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\epsilon\\:}_{max}\\)\u003c/span\u003e\u003c/span\u003e is the maximum dielectric permittivity, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{T}}_{\\varvec{m}\\varvec{a}\\varvec{x}}\\)\u003c/span\u003e\u003c/span\u003e is the temperature of maximum dielectric permittivity, \u0026gamma; is the degree of diffuseness (values closer to 2 indicate stronger relaxor-like behavior, whereas values closer to 1 indicate more conventional ferroelectric behavior), and C\u0026acute; is the modified Curie-Weiss constant. All samples exhibited diffuseness exponents \u0026gamma; approaching 2 (see Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In the present case, \u0026gamma; should be interpreted as a descriptor of peak broadening arising from compositional heterogeneity and structural disorder, rather than as direct evidence of a canonical relaxor regime.\u003c/p\u003e\n \u003cp\u003eThe upward shift of T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e in the co-doped composition indicates improved thermal stability of the non-ergodic ferroelectric state. Structurally, this aligns with the tetragonal-enriched MPB configuration identified by XRD/Rietveld analysis. The increased tetragonal fraction may deepen the free-energy minimum associated with long-range polarization, thus delaying the onset of dynamic disorder upon heating.\u003c/p\u003e\n \u003cp\u003eThis stabilization is functionally relevant. In BNT\u0026ndash;BT systems, the proximity to T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e strongly influences domain-wall mobility and the balance between reversible and irreversible polarization components. By shifting T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e upward while simultaneously reducing \u0026epsilon;\u003csub\u003er\u003c/sub\u003e, Cu/Zr co-doping modifies the dielectric stiffness and polarization dynamics, enhancing the piezoelectric voltage coefficient (see Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e). Thus, the dielectric behavior provides a useful link between structural phase balance and electromechanical performance.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eFerroelectric\u0026ndash;relaxor crossover temperature (T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e), maximum permittivity temperature (T\u003csub\u003emax\u003c/sub\u003e), maximum relative permittivity (\u0026epsilon;\u003csub\u003emax\u003c/sub\u003e), and diffuseness exponent (\u0026gamma;) obtained from the modified Curie\u0026ndash;Weiss analysis performed in the vicinity of T\u003csub\u003emax\u003c/sub\u003e for BNT\u0026ndash;5BT and Cu/Zr-modified compositions. Values were extracted from \u0026epsilon;\u003csub\u003er\u003c/sub\u003e(T) data (1 kHz) using the linear fit of ln(1/\u0026epsilon;\u003csub\u003er\u0026minus;1\u003c/sub\u003e/\u0026epsilon;\u003csub\u003emax\u003c/sub\u003e) versus ln(T\u0026minus;T\u003csub\u003emax\u003c/sub\u003e).\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eT\u003csub\u003eF\u0026minus;R\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(\u0026ordm;C)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eT\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(\u0026ordm;C)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e\u0026epsilon;\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e\u0026gamma;\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e168\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e264\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e6183\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1.92\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT-1Cu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e160\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e278\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e5652\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1.83\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT-2Zr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e157\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e253\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e5017\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1.99\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eBNT-5BT-2Zr-1Cu\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003e195\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cstrong\u003e232\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e\u003cstrong\u003e5008\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e\u003cstrong\u003e1.92\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the polarization\u0026ndash;electric field (P\u0026ndash;E) loops of BNT\u0026ndash;5BT and doped BNT\u0026ndash;5BT ceramics. At room temperature, all compositions display well-developed ferroelectric-like hysteresis loops with non-negligible remanent polarization and coercive field, confirming that they remain in a non-ergodic ferroelectric state under the explored conditions. Quantitatively, Cu/Zr co-doping leads to a slight increase in the coercive field, from 41.8 to 43.0 kV\u0026middot;cm\u003csup\u003e-1\u003c/sup\u003e, together with a decrease in remanent polarization (P\u003csub\u003er\u003c/sub\u003e) from 25.0 to 21.6 \u0026micro;C\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e. For comparison, Cu-only doping results in an even lower P\u003csub\u003er\u003c/sub\u003e (~\u0026thinsp;20.1 \u0026micro;C\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], whereas Zr-only doping yields a higher P\u003csub\u003er\u003c/sub\u003e (30.1 \u0026micro;C\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e) and a lower E\u003csub\u003ec\u003c/sub\u003e (39.5 kV\u0026middot;cm\u003csup\u003e-1\u003c/sup\u003e) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eThese trends are consistent with changes in the polar state and in the phase balance near the MPB, including relaxor-like contributions and variations in the relative extent of reversible and irreversible switching, rather than providing direct evidence for a specific defect-domain pinning mechanism. Therefore, the detailed role of defect chemistry and domain/PNR interactions in controlling P\u003csub\u003er\u003c/sub\u003e and E\u003csub\u003ec\u003c/sub\u003e requires further investigation.\u003c/p\u003e\n \u003cp\u003eTo further clarify this behavior, the ferroelectric-like hysteresis observed at room temperature must be reconciled with the pronounced relaxor character indicated by the diffuseness parameter \u0026gamma;. This apparent discrepancy arises from the different temperature regimes in which these properties are evaluated. The \u0026gamma; parameter is obtained from dielectric data collected well above T\u003csub\u003emax\u003c/sub\u003e (e.g., \u0026gt; 240\u0026deg;C), and therefore reflects the intrinsic structural disorder and the ergodic relaxor (ER) character of the high-temperature state. In contrast, the broad hysteresis loops measured at room temperature indicate that the unpoled material remains in a non-ergodic relaxor (NER) state, or at least in a state with strong ferroelectric correlations. Under a sufficiently strong electric field, this initial configuration can be driven into a field-induced long-range ferroelectric state, giving rise to the observed wide P\u0026ndash;E loops. As the temperature approaches T\u003csub\u003emax\u003c/sub\u003e, thermal activation progressively stabilizes the ergodic relaxor state and reduces the persistence of the field-induced ferroelectric order. Consequently, the P\u0026ndash;E loops are expected to become progressively slimmer with increasing temperature, in agreement with the high relaxor character quantified by \u0026gamma;.\u003c/p\u003e\n \u003cp\u003eFrom an energy-harvesting perspective, however, the hysteresis loop shape can only provide a qualitative indication of switching-related losses, since the loop area represents dissipated energy per cycle. A rigorous comparison would require direct quantification of this dissipated energy under identical electric-field amplitude and frequency. Therefore, in the present work, the harvesting performance is discussed primarily in terms of voltage sensitivity and figures of merit derived from d\u003csub\u003e33\u003c/sub\u003e and \u0026epsilon;\u003csub\u003er\u003c/sub\u003e (Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e), particularly through the voltage coefficient g\u003csub\u003e33\u003c/sub\u003e=d\u003csub\u003e33\u003c/sub\u003e/(\u0026epsilon;\u003csub\u003e0\u003c/sub\u003e\u0026epsilon;\u003csub\u003er\u003c/sub\u003e) [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], rather than through indirect inferences based solely on hysteresis features [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eAt room temperature, all compositions exhibit ferroelectric-like P\u0026ndash;E loops with non-negligible P\u003csub\u003er\u003c/sub\u003e and E\u003csub\u003ec\u003c/sub\u003e, indicating that the materials remain outside a fully ergodic relaxor state within the explored field range. From an energy-harvesting standpoint, however, hysteresis itself is not a direct performance metric, since the loop area is associated with energy dissipation. For this reason, the harvesting relevance discussed in Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e is based mainly on voltage sensitivity and dielectric\u0026ndash;piezoelectric figures of merit, rather than on presumed reductions in hysteretic losses.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4. Piezoelectric Response and Energy Generation Performance\u003c/h2\u003e\n \u003cp\u003eFrom a functional standpoint, the larger mean grain size of the co-doped ceramic (1.54\u0026thinsp;\u0026plusmn;\u0026thinsp;1.21 \u0026micro;m) compared with undoped BNT-5BT (1.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.83 \u0026micro;m) is relevant because grain boundaries constrain domain-wall motion and can reduce the fraction of switchable polarization during poling. In this context, the observed relationship between remanent/saturation polarization and grain size can be qualitatively interpreted using the domain-kinetics framework proposed by Orihara et al., often discussed in terms of an Avrami-type expression [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e] for the fraction of reoriented ferroelectric domains f:\u003c/p\u003e\n \u003cp\u003e𝑓 = 𝑓\u003csub\u003e0\u003c/sub\u003e [1\u0026thinsp;\u0026minus;\u0026thinsp;exp(-𝐺\u003csub\u003e𝑎\u003c/sub\u003e𝑑\u003csup\u003e3\u003c/sup\u003e\u0026frasl;𝑘𝑇)] (2)\u003c/p\u003e\n \u003cp\u003ewhere \u003cem\u003eG\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e represents the anisotropy energy density, k is the Boltzmann constant, \u003cem\u003eT\u003c/em\u003e is the absolute temperature, and \u003cem\u003ed\u003c/em\u003e is the grain size. This relation implies that larger grains increase \u003cem\u003ef\u003c/em\u003e, i.e., a larger proportion of domains can reorient under an applied field, which generally favors higher switchable polarization and improved ferroelectric response. In the present work, this framework is used as a qualitative basis to interpret the effect of grain size on polarization behavior. In addition, all doped samples exhibited relative densities higher than the 95% (see Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), comparable to the Zr-only composition and higher than the undoped ceramic. Therefore, the improved ferroelectric behavior of BNT-5BT-2Zr-1Cu can be consistently interpreted as the combined outcome of (i) a tetragonal-enriched MPB state (Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e), (ii) microstructural coarsening that facilitates domain reorientation, and (iii) a moderately reduced dielectric permittivity.\u003c/p\u003e\n \u003cp\u003eFinally, the piezoelectric charge and voltage coefficients (d₃₃ and g₃₃) reflect the combined influence of structural symmetry and dielectric response. Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e summarizes the key piezoelectric and dielectric parameters. The higher g₃₃ and harvesting figure of merit (FoM) values arise mainly from the lower dielectric permittivity (\u0026epsilon;\u003csub\u003er\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1372) of the co-doped samples, since g₃₃=d₃₃/(\u0026epsilon;₀\u0026epsilon;\u003csub\u003e\u003cstrong\u003er\u003c/strong\u003e\u003c/sub\u003e) and FoM\u0026thinsp;=\u0026thinsp;d₃₃\u003csup\u003e2\u003c/sup\u003e/\u0026epsilon;\u003csub\u003e\u003cstrong\u003er\u003c/strong\u003e\u003c/sub\u003e.\u0026nbsp;\u003c/p\u003e\n \u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eDensity (\u0026rho;), relative density (%, considering 6.01 g/cm\u003csup\u003e3\u003c/sup\u003e as the theoretical density), relative permittivity (\u0026epsilon;\u003csub\u003er\u003c/sub\u003e), and charge (d\u003csub\u003e33\u003c/sub\u003e), and voltage (g\u003csub\u003e33\u003c/sub\u003e) piezoelectric coefficients (at room temperature), and the figure of merit (FoM) for harvesting of the sintered samples.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u0026rho;\u003c/p\u003e\n \u003cp\u003e(g/cm\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eRelative density (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003ed\u003csub\u003e33\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(pC/N)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e\u0026epsilon;\u003csub\u003er\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eg\u003csub\u003e33\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(10\u003csup\u003e-3\u003c/sup\u003eVm/N)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eFoM\u003c/p\u003e\n \u003cp\u003e(pC\u0026sup2;/N\u0026sup2;)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e5.70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e94.84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e110\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1560\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\n \u003cp\u003e7.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e7.76\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eBNT-5BT-2Zr-1Cu\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003e5.74\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cstrong\u003e95.51\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e\u003cstrong\u003e130\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e\u003cstrong\u003e1372\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\n \u003cp\u003e\u003cstrong\u003e10.71\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e\u003cstrong\u003e12.32\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT-1Cu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e5.82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e96.84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e130\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1443\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\n \u003cp\u003e10.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e11.70\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eBNT-5BT-2Zr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e5.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e95.51\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e135\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e1503\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\n \u003cp\u003e10.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\n \u003cp\u003e12.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003eThe improvement in g\u003csub\u003e33\u003c/sub\u003e highlights the effectiveness of co-doping in balancing polarization strength and dielectric softness. The doped compositions exhibit a significant enhancement in the piezoelectric voltage coefficient g₃₃, despite only moderate changes in d₃₃. Furthermore, the co-doped composition shows a marked improvement in the energy-harvesting figure of merit (d₃₃\u003csup\u003e2\u003c/sup\u003e/\u0026epsilon;\u003csub\u003er\u003c/sub\u003e), reaching almost twice the value of the undoped ceramic, which indicates an improved potential for electromechanical energy conversion. This behavior is mainly attributed to the reduced dielectric permittivity combined with a comparatively stable coercive field, thereby favoring a large reversible polarization component under mechanical loading. The resulting increase in g₃₃, together with the stable room-temperature ferroelectric response, suggests that co-doped BNT\u0026ndash;BT ceramics are suitable for energy-harvesting applications, under low-power mechanical excitation. Therefore, under the room-temperature conditions explored here, co-doping improves the material\u0026rsquo;s voltage response and harvesting-relevant metrics, whereas the effect of the T\u003csub\u003eF-R\u003c/sub\u003e\u0026ndash;T\u003csub\u003emax\u003c/sub\u003e interval should be considered case-dependent. Furthermore, the incorporation of CuO enables a reduction in the sintering temperature of about 100\u0026deg;C, representing an additional benefit of the co-doped compositions.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eCu/Zr co-doping in BNT\u0026ndash;5BT is shown to be an effective strategy for enhancing voltage-related electromechanical performance through combined structural tuning and microstructural control. XRD and Raman analyses indicate stabilization of a tetragonal-enriched morphotropic phase boundary (MPB) state without secondary phases. Rietveld refinement and lattice expansion support effective Zr⁴⁺ incorporation at the B-site, while Cu addition facilitates densification and enables a sintering temperature reduction of ~\u0026thinsp;100\u0026deg;C. The resulting ceramics exhibit dense and homogeneous microstructures with controlled grain growth.\u003c/p\u003e \u003cp\u003eDielectric measurements reveal an upward shift of the ferroelectric\u0026ndash;relaxor crossover temperature (T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e \u0026asymp; 195\u0026deg;C) and a narrowed T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e\u0026ndash;T\u003csub\u003emax\u003c/sub\u003e interval. Although the permittivity maximum is diffuse and the crossover region shows frequency dependence, the presence of well-developed ferroelectric hysteresis loops indicates that the system remains predominantly non-ergodic at room temperature. The broad dielectric response is therefore attributed to compositional heterogeneity and local free-energy fluctuations.\u003c/p\u003e \u003cp\u003eFunctionally, the co-doped composition combines moderate d₃₃ (~\u0026thinsp;130 pC\u0026middot;N⁻\u0026sup1;) with reduced dielectric permittivity (εr\u0026thinsp;\u0026asymp;\u0026thinsp;1372), yielding an enhanced voltage coefficient (g₃₃ \u0026asymp; 10.71 \u0026times; 10⁻\u0026sup3; V\u0026middot;m\u0026middot;N⁻\u0026sup1;) and improved harvesting figure of merit (d₃₃\u0026sup2;/ε\u003csub\u003er\u003c/sub\u003e). This enhancement originates from an optimized balance between polarization magnitude, dielectric stiffness, and phase-boundary stabilization, rather than from hysteresis suppression.\u003c/p\u003e \u003cp\u003eCompared with single-doped counterparts, the co-doped composition combines (i) a higher T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e, (ii) suppressed ε\u003csub\u003er\u003c/sub\u003e, and (iii) stable ferroelectric behavior at room temperature, and this interplay enhances g\u003csub\u003e33\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;d\u003csub\u003e33\u003c/sub\u003e/(ε\u003csub\u003e0\u003c/sub\u003eε\u003csub\u003er\u003c/sub\u003e) and benefits low-power electromechanical transduction. Overall, the results demonstrate that compositional co-design (Cu primarily promoting densification and defect equilibration, and Zr contributing to structural tuning) is an effective route to improve voltage sensitivity in lead-free BNT\u0026ndash;BT ceramics. In order to translate these materials-level gains into operational harvesting performance, application-level validation under realistic excitation spectra and cycling/aging conditions, together with temperature-dependent electromechanical characterization, remains a relevant next step.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interest\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.D. performed the experimental work, prepared the figures, and wrote the manuscript text. M.C. and L.R. were responsible for the conceptualization and supervision of the study. J.C. contributed to data curation. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors would like to thank the National University of Mar del Plata (Argentina) Project (15/G689 - ING693/23), CONICET PIP 11220200102487CO and the National Agency for Scientific and Technological Promotion (ANPCyT) PICT Start up 2021 for providing financial support.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data generated or analyzed during this study are included in this published article and no additional source data are required.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eY. Saito, H. Takao, T. Toshihiko, N. Toshihiko, Lead-free piezoceramics. Nature. \u003cb\u003e432\u003c/b\u003e, 84\u0026ndash;87 (2004). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/nature03028\u003c/span\u003e\u003cspan address=\"10.1038/nature03028\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT. Takenaka, K. Maruyama, K. Sakata, (Bi\u003csub\u003e1/2\u003c/sub\u003eNa\u003csub\u003e1/2\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e-BaTiO\u003csub\u003e3\u003c/sub\u003e System for Lead-Free Piezoelectric Ceramics. Jpn J. Appl. Phys. \u003cb\u003e30\u003c/b\u003e, 2236\u0026ndash;2239 (1991). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1143/JJAP.30.2236\u003c/span\u003e\u003cspan address=\"10.1143/JJAP.30.2236\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eF. Rubio-Marcos, J.J. Romero, D.A. Ochoa, J.E. Garc\u0026iacute;a, R. Perez, J.F. Fernandez, Effects of Poling Process on KNN-Modified Piezoceramic Properties. J. Am. Ceram. Soc. \u003cb\u003e93\u003c/b\u003e, 318\u0026ndash;321 (2010). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1551-2916.2009.03404.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1551-2916.2009.03404.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Difeo, M. Castro, L. Ramajo, Effect of Zr Doping on BNT\u0026ndash;5BT Lead-Free Ceramics: Substitutional and Excess Incorporation Analysis. Micro. \u003cb\u003e5\u003c/b\u003e, 35 (2025). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/micro5030035\u003c/span\u003e\u003cspan address=\"10.3390/micro5030035\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM.K. Sharma, P. Singh, D. Panda, P. Sharma, Tuning structural and functional properties of Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e-Ba\u003csub\u003e0.7\u003c/sub\u003eCa\u003csub\u003e0.3\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e ceramics through compositional engineering near the morphotropic phase boundary. Ceram. Int. \u003cb\u003e51\u003c/b\u003e, 54081\u0026ndash;54087 (2025). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2025.09.150\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2025.09.150\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eS. Subramanian, S. R, S. Subramanian, A. Karuppanan, Charge correlation of ferroelectric and piezoelectric properties of (1\u0026thinsp;\u0026ndash;\u0026thinsp;x)(Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;xBaTiO\u003csub\u003e3\u003c/sub\u003e lead-free ceramic solid solution. J. Mater. Sci.: Mater. Electron. \u003cb\u003e28\u003c/b\u003e (2017). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10854-017-6753-5\u003c/span\u003e\u003cspan address=\"10.1007/s10854-017-6753-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY.-J. Dai, J. Pan, X. Zhang, Composition Range of Morphotropic Phase Boundary and Electrical Properties of NBT-BT System. Key Eng. Mater. 336\u0026ndash;338 (2007). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e206\u0026ndash;209. https://doi.org/10.4028/www.scientific.net/KEM.336-338.206\u003c/span\u003e\u003cspan address=\"206\u0026ndash;209. 10.4028/www.scientific.net/KEM.336-338.206\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eN. Sezer, M. Ko\u0026ccedil;, A comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Nano Energy. \u003cb\u003e80\u003c/b\u003e, 105567 (2021). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.nanoen.2020.105567\u003c/span\u003e\u003cspan address=\"10.1016/j.nanoen.2020.105567\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Ozgul, A. Kucuk, B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e doping in 0.94(Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.06BaTiO\u003csub\u003e3\u003c/sub\u003e lead-free piezoelectric ceramics. Ceram. Int. \u003cb\u003e42\u003c/b\u003e, 19119\u0026ndash;19123 (2016). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2016.09.073\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2016.09.073\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eN.B. Mahmood, E.K. Al-Shakarchi, Dielectric properties of BNT-xBT prepared by hydrothermal process. J. Adv. Dielect. \u003cb\u003e07\u003c/b\u003e, 1750019 (2017). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1142/S2010135X17500199\u003c/span\u003e\u003cspan address=\"10.1142/S2010135X17500199\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eX. Cheng, L.K. Venkataraman, Y. Li, Simultaneous enhancement of piezoelectric constant and thermal stability in lead-free Fe-doped 0.94(Na\u003csub\u003e1/2\u003c/sub\u003eBi\u003csub\u003e1/2\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e-0.06BaTiO\u003csub\u003e3\u003c/sub\u003e ceramics. J. Alloys Compd. \u003cb\u003e891\u003c/b\u003e, 161880 (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jallcom.2021.161880\u003c/span\u003e\u003cspan address=\"10.1016/j.jallcom.2021.161880\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eG. Liang, Y. Zhang, J. Zhu, Q. Zhang, B. Peng, Tailoring and improving the strong-electric-field electrical properties of the BNT-BT ferroelectric ceramics by a functional-group-doping. Ceram. Int. \u003cb\u003e47\u003c/b\u003e, 6584\u0026ndash;6590 (2021). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2020.10.247\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2020.10.247\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Pereira, A.G. Peixoto, M.J.M. Gomes, Effect of Nb doping on the microstructural and electrical properties of the PZT ceramics. J. Eur. Ceram. Soc. \u003cb\u003e21\u003c/b\u003e, 1353\u0026ndash;1356 (2001). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S0955-2219(01)00017-6\u003c/span\u003e\u003cspan address=\"10.1016/S0955-2219(01)00017-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM.M.S. Pojucan, M.C.C. Santos, F.R. Pereira, M.A.S. Pinheiro, M.C. Andrade, Piezoelectric properties of pure and (Nb\u003csup\u003e5+\u003c/sup\u003e+ Fe\u003csup\u003e3+\u003c/sup\u003e) doped PZT ceramics. Ceram. Int. \u003cb\u003e36\u003c/b\u003e, 1851\u0026ndash;1855 (2010). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2010.03.025\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2010.03.025\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Hu, H. Zhang, Z. Wang, J. Shen, J. Zhou, W. Chen, Ferroelectric-order-dominated doping effects in Fe, Nb co-doped Bi\u003csub\u003e1/2\u003c/sub\u003e(Na\u003csub\u003e0.8\u003c/sub\u003eK\u003csub\u003e0.2\u003c/sub\u003e)\u003csub\u003e1/2\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e piezoelectric ceramics. Ceram. Int. (2026). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2026.02.120\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2026.02.120\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eP. Jaita, S. Manotham, G. Rujijanagul, Influence of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e nanoparticle doping on depolarization temperature, and electrical and energy harvesting properties of lead-free 0.94(Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.06BaTiO\u003csub\u003e3\u003c/sub\u003e ceramics. RSC Adv. \u003cb\u003e10\u003c/b\u003e, 32078\u0026ndash;32087 (2020). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/D0RA04866F\u003c/span\u003e\u003cspan address=\"10.1039/D0RA04866F\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB.K. Barick, R.N.P. Choudhary, D.K. Pradhan, Dielectric and impedance spectroscopy of zirconium modified (Na\u003csub\u003e0.5\u003c/sub\u003e Bi\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e ceramics. Ceram. Int. \u003cb\u003e39\u003c/b\u003e, 5695\u0026ndash;5704 (2013). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ceramint.2012.12.087\u003c/span\u003e\u003cspan address=\"10.1016/j.ceramint.2012.12.087\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. Watcharapasorn, S. Jiansirisomboon, T. Tunkasiri, Microstructure and mechanical properties of zirconium-doped bismuth sodium titanate ceramics. Chiang Mai J. Sci. \u003cb\u003e33\u003c/b\u003e, 169\u0026ndash;173 (2006)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT.R. Armstrong, L.E. Morgens, A.K. Maurice, R.C. Buchanan, Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics. J. Am. Ceram. Soc. \u003cb\u003e72\u003c/b\u003e, 605\u0026ndash;611 (1989). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1151-2916.1989.tb06182.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1151-2916.1989.tb06182.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Difeo, L. Ramajo, M. Castro, Influence of CuO addition on dielectric and piezoelectric properties of (Bi\u003csub\u003e0.5\u003c/sub\u003eNa0.5)TiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;BaTiO\u003csub\u003e3\u003c/sub\u003elead-free piezoceramics. J. Adv. Dielect. \u003cb\u003e11\u003c/b\u003e, 2140004 (2021). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1142/S2010135X2140004X\u003c/span\u003e\u003cspan address=\"10.1142/S2010135X2140004X\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Rout, K.-S. Moon, V.S. Rao, S.-J.L. Kang, Study of the morphotropic phase boundary in the lead-free Na\u003csub\u003e1/2\u003c/sub\u003eBi\u003csub\u003e1/2\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e-BaTiO\u003csub\u003e3\u003c/sub\u003e system by Raman spectroscopy. J. Ceram. Soc. Japan. \u003cb\u003e117\u003c/b\u003e, 797\u0026ndash;800 (2009). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2109/jcersj2.117.797\u003c/span\u003e\u003cspan address=\"10.2109/jcersj2.117.797\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Yao, N. Monsegue, M. Murayama, W. Leng, W.T. Reynolds, Q. Zhang, H. Luo, J. Li, W. Ge, D. Viehland, Role of coexisting tetragonal regions in the rhombohedral phase of Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e-xat.%BaTiO\u003csub\u003e3\u003c/sub\u003e crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary. Appl. Phys. Lett. \u003cb\u003e100\u003c/b\u003e, 012901 (2012). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/1.3673832\u003c/span\u003e\u003cspan address=\"10.1063/1.3673832\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB. Wylie-van Eerd, D. Damjanovic, N. Klein, N. Setter, J. Trodahl, Structural complexity of (Na\u003csub\u003e0.5\u003c/sub\u003eBi\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e-BaTiO\u003csub\u003e3\u003c/sub\u003e as revealed by Raman spectroscopy. Phys. Rev. B \u003cb\u003e82\u003c/b\u003e, 104112 (2010). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1103/PhysRevB.82.104112\u003c/span\u003e\u003cspan address=\"10.1103/PhysRevB.82.104112\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA. Prado, F. Rubio-Marcos, L. Ramajo, M.S. Castro, Dielectric and ferroelectric properties evolution of (1\u0026thinsp;\u0026ndash;\u0026thinsp;x)(Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e)\u0026ndash;xK\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eNbO\u003csub\u003e3\u003c/sub\u003e piezoceramics. Bull. Mater. Sci. \u003cb\u003e43\u003c/b\u003e, 282 (2020). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s12034-020-02246-9\u003c/span\u003e\u003cspan address=\"10.1007/s12034-020-02246-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eA.A. Bokov, Z.-G. Ye, Recent Progress in Relaxor Ferroelectrics with Perovskite Structure. \u003cem\u003eProgress in Advanced Dielectrics\u003c/em\u003e, World Scientific, Singapore, 2019: 105\u0026ndash;164. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1142/9789811210433_0003\u003c/span\u003e\u003cspan address=\"10.1142/9789811210433_0003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW.-C. Lee, C.-Y. Huang, L.-K. Tsao, Y.-C. Wu, Chemical composition and tolerance factor at the morphotropic phase boundary in (Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e-based piezoelectric ceramics. J. Eur. Ceram. Soc. \u003cb\u003e29\u003c/b\u003e, 1443\u0026ndash;1448 (2009). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jeurceramsoc.2008.08.028\u003c/span\u003e\u003cspan address=\"10.1016/j.jeurceramsoc.2008.08.028\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. Jo, J.-B. Ollagnier, J.-L. Park, E.-M. Anton, O.-J. Kwon, C. Park, H.-H. Seo, J.-S. Lee, E. Erdem, R.-A. Eichel, J. R\u0026ouml;del, CuO as a sintering additive for (Bi\u003csub\u003e1/2\u003c/sub\u003eNa\u003csub\u003e1/2\u003c/sub\u003e)TiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;(K\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003e)NbO\u003csub\u003e3\u003c/sub\u003e lead-free piezoceramics. J. Eur. Ceram. Soc. \u003cb\u003e31\u003c/b\u003e, 2107\u0026ndash;2117 (2011). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jeurceramsoc.2011.05.008\u003c/span\u003e\u003cspan address=\"10.1016/j.jeurceramsoc.2011.05.008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZ.-H. Zhao, R.-F. Ge, Y. Dai, Large electro-strain signal of the BNT\u0026ndash;BT\u0026ndash;KNN lead-free piezoelectric ceramics with CuO doping. J. Adv. Dielect. \u003cb\u003e09\u003c/b\u003e, 1950022 (2019). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1142/S2010135X1950022X\u003c/span\u003e\u003cspan address=\"10.1142/S2010135X1950022X\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Vu, D. Nguyen, J.G. Fisher, W.-H. Moon, S. Bae, H.-G. Park, B.-G. Park, CuO-based sintering aids for low temperature sintering of BaFe\u003csub\u003e12\u003c/sub\u003eO\u003csub\u003e19\u003c/sub\u003e ceramics. J. Asian. Ceam. Soc. \u003cb\u003e1\u003c/b\u003e, 170\u0026ndash;177 (2013). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jascer.2013.05.002\u003c/span\u003e\u003cspan address=\"10.1016/j.jascer.2013.05.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Reda, S.I. El-Dek, M.M. Arman, Improvement of ferroelectric properties via Zr doping in barium titanate nanoparticles. J. Mater. Sci.: Mater. Electron. \u003cb\u003e33\u003c/b\u003e, 16753\u0026ndash;16776 (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10854-022-08541-x\u003c/span\u003e\u003cspan address=\"10.1007/s10854-022-08541-x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eW. Kleemann, The relaxor enigma \u0026mdash; charge disorder and random fields in ferroelectrics. J. Mater. Sci. \u003cb\u003e41\u003c/b\u003e, 129\u0026ndash;136 (2006). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10853-005-5954-0\u003c/span\u003e\u003cspan address=\"10.1007/s10853-005-5954-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eD. Viehland, S.J. Jang, L.E. Cross, M. Wuttig, Deviation from Curie-Weiss behavior in relaxor ferroelectrics. Phys. Rev. B \u003cb\u003e46\u003c/b\u003e, 8003\u0026ndash;8006 (1992). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1103/PhysRevB.46.8003\u003c/span\u003e\u003cspan address=\"10.1103/PhysRevB.46.8003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJ. Glaum, Y. Heo, M. Acosta, P. Sharma, J. Seidel, M. Hinterstein, Revealing the role of local stress on the depolarization of BNT-BT-based relaxors. Phys. Rev. Mater. \u003cb\u003e3\u003c/b\u003e, 054406 (2019). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1103/PhysRevMaterials.3.054406\u003c/span\u003e\u003cspan address=\"10.1103/PhysRevMaterials.3.054406\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM. Kar, L.K. Pradhan, Relaxor Ferroelectric Oxides: Concept to Applications, in: D.R. Sahu (Ed.), Multifunctional Ferroelectric Materials, IntechOpen, London, 2021. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.5772/intechopen.96185\u003c/span\u003e\u003cspan address=\"10.5772/intechopen.96185\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK.P. Kelley, A.N. Morozovska, E.A. Eliseev, Y. Liu, S.S. Fields, S.T. Jaszewski, T. Mimura, S. Calderon, E.C. Dickey, J.F. Ihlefeld, S.V. Kalinin, Ferroelectricity in hafnia controlled via surface electrochemical state. Nat. Mater. \u003cb\u003e22\u003c/b\u003e, 1144\u0026ndash;1151 (2023). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41563-023-01619-9\u003c/span\u003e\u003cspan address=\"10.1038/s41563-023-01619-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQ. Li, J. Wang, Y. Ma, L. Ma, G. Dong, H. Fan, Enhanced energy-storage performance and dielectric characterization of 0.94Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.06BaTiO\u003csub\u003e3\u003c/sub\u003e modified by CaZrO\u003csub\u003e3\u003c/sub\u003e. J. Alloys Compd. \u003cb\u003e663\u003c/b\u003e, 701\u0026ndash;707 (2016). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jallcom.2015.12.194\u003c/span\u003e\u003cspan address=\"10.1016/j.jallcom.2015.12.194\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eH. Orihara, S. Hashimoto, Y. Ishibashi, A Theory of D-E Hysteresis Loop Based on the Avrami Model. J. Phys. Soc. Jpn. \u003cb\u003e63\u003c/b\u003e, 1031\u0026ndash;1035 (1994). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1143/JPSJ.63.1031\u003c/span\u003e\u003cspan address=\"10.1143/JPSJ.63.1031\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"lead-free piezoceramics, co-doping, Cu, Zr, energy harvesting, BNT–BT","lastPublishedDoi":"10.21203/rs.3.rs-9223133/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9223133/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLead-free 0.95Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003eTiO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.05BaTiO\u003csub\u003e3\u003c/sub\u003e (BNT\u0026ndash;5BT) ceramics were Cu/Zr co-doped to improve the electromechanical properties relevant to low-power energy harvesting. Based on the optimized single-dopant contents (1 mol% CuO and 2 mol% Zr), the co-doped composition 0.95(Bi\u003csub\u003e0.5\u003c/sub\u003eNa\u003csub\u003e0.5\u003c/sub\u003e)(Ti\u003csub\u003e0.98\u003c/sub\u003eZr\u003csub\u003e0.02\u003c/sub\u003e)O\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.05Ba(Ti\u003csub\u003e0.98\u003c/sub\u003eZr\u003csub\u003e0.02\u003c/sub\u003e)O\u003csub\u003e3\u003c/sub\u003e\u0026ndash;0.01CuO (BNT\u0026ndash;5BT\u0026ndash;2Zr\u0026ndash;1Cu) was synthesized by solid-state reaction with mechanochemical activation. X-ray diffraction and Raman spectroscopy confirmed the formation of a single perovskite phase and an increased tetragonal contribution compared to the single-doped counterparts, consistent with morphotropic phase boundary (MPB) stabilization. Co-doping increased the ferroelectric\u0026ndash;relaxor crossover temperature (T\u003csub\u003eF\u0026ndash;R\u003c/sub\u003e = 195\u0026deg;C) while reducing the room-temperature permittivity, thereby enhancing the piezoelectric voltage response. The co-doped ceramic exhibited ρ\u0026thinsp;=\u0026thinsp;5.74 g\u0026middot;cm\u003csup\u003e-3\u003c/sup\u003e, d\u003csub\u003e33\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;130 pC\u0026middot;N\u003csup\u003e-1\u003c/sup\u003e, and g\u003csub\u003e33\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;10.71\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e V\u0026middot;m\u0026middot;N\u003csup\u003e-1\u003c/sup\u003e, outperforming the undoped and single-doped reference compositions. These results indicate that Cu/Zr co-doping, combining Cu-assisted densification and defect equilibration with Zr-induced structural tuning, is an effective strategy to enhance the voltage response of lead-free BNT\u0026ndash;BT ceramics for low-power energy-harvesting applications.\u003c/p\u003e","manuscriptTitle":"Optimization of Piezoelectric Performance in Lead-Free BNT–5BT Ceramics by Cu/Zr Co- Doping: Structure–Property Correlations","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-20 17:54:47","doi":"10.21203/rs.3.rs-9223133/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5dcd5fd9-75f0-4f25-b717-ce58a8aaafa5","owner":[],"postedDate":"April 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-16T10:08:10+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-20 17:54:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9223133","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9223133","identity":"rs-9223133","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}