Investigation of electrical properties for Ca-modified BaBiNb5O15 compound

Investigation of electrical properties for Ca-modified BaBiNb5O15 compound | 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 Investigation of electrical properties for Ca-modified BaBiNb5O15 compound Xinye Yun, Han Wang, Xinyu Hu, Jiamiao Tuo, Shuyao Cao, Qinfu Zhao, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8539661/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 BaBiNb 5 O 15 , a tungsten bronze-structured oxygen ion conductor, shows great promise for medium-temperature solid oxide fuel cells due to its excellent electrical conductivity and thermal stability. The effects of Ca 2+ ion doping on the structural, microstructural, and electrical conductivity characteristics of the BaBiNb 5 − x Ca x O 15−δ compounds were analyzed. Measurements using AC impedance spectroscopy revealed that at 773 K, the grain conductivity of BaBi 0.95 Ca 0.05 Nb₅O 15−δ sample can reach 3.79×10 − 4 S/cm, representing an approximate 5.1-fold increase compared to the undoped BaBiNb 5 O 15 compound. Dielectric modulus analysis identified an activation energy of 0.37 eV for oxygen ion diffusion, with relaxation parameters indicating that dielectric loss peaks originate from vacancy-mediated oxide ion motion in the BaBi 0.95 Ca 0.05 Nb 5 O 15−δ sample. The enhanced ionic conductivity arises from increased mobile oxygen vacancy density, improved mobility, lower activation energy, and reduced relaxation time. These findings elucidate the mechanism of oxygen vacancy diffusion and the origin of oxygen relaxation behavior in BaBiNb 5 O 15 compound, advancing fundamental understanding of oxygen ion conductor characteristics while guiding future investigations into the electrical properties of BaBiNb 5 O 15 -based materials. BaBiNb5O15 oxygen ion conductor acceptor doping dielectric modulus spectroscopy mobility Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction The development of novel solid electrolytes has predominantly centered on oxygen-ion conductors with fluorite, perovskite, pyrochlore, and lanthanum molybdate (LaMOX) structures [ 1 , 2 ] . Fluorite-type materials demonstrate high ionic conductivity at elevated temperatures, structural robustness, tunable defect chemistry, and favorable chemical compatibility, establishing them as benchmark systems [ 3 ] . Perovskite-structured variants offer exceptional compositional flexibility [ 4 ] , where A/B-site doping enables optimization of conductivity alongside superior intermediate-temperature performance and mechanical integrity [ 5 ] . Pyrochlore-based compounds, such as Gd 2 Zr 2 O 7 compound, exhibit remarkable thermal stability, reduction resistance, and adjustable defect concentrations, sustaining efficient ion transport under extreme conditions [ 6 ] . LaMOX electrolytes combine high ionic mobility, low activation energy, and electrode compatibility in the intermediate-temperature range, rendering them promising for solid oxide fuel cells (SOFCs). These materials collectively underpin advancements in SOFCs, oxygen sensors, and solid-state ionic devices [ 7 , 8 ] . Although there are many types of oxygen ion conductors widely studied and applied in SOFCs nowadays, there are also inevitable drawbacks, such as excessively high operating temperatures, which cause electronic conductivity, the difficulty in synthesizing pure phases during phase transitions, and poor compatibility with electrode materials, all of which limit their commercial development [ 9 , 10 ] . Therefore, seeking electrolyte materials with high ionic rates and good chemical and mechanical stability is an important research direction for achieving intermediate-temperature in solid oxide fuel cells [ 11 ] . Tungsten bronze-structured dielectric ceramics represent a significant class of functional materials distinguished by their complex crystallography, pronounced dielectric relaxation behavior, and ferroelectric phase transitions. These characteristics confer superior functionality, including pyroelectricity, nonlinear optical activity, and piezoelectricity, relative to conventional oxygen-ion conductors. The structural flexibility of tungsten bronze frameworks arises from interstitial sites within oxygen octahedra, enabling multivalent cation occupancy (A/B/C sites) that facilitates tailored defect engineering [ 12 – 14 ] . This architectural versatility permits precise control over phase transition temperatures and polarization mechanisms, as evidenced in systems like Sr 1 − x Ba x Nb 2 O 6 (SBN) [ 15 ] , PbNb 2 O 6 [ 16 ] , and K 3 Li 2 Nb 2 O 15 [ 17 ] . Notably, SBN compound exhibits enhanced pyroelectric and electro-optical coefficients compared to calcium ferrite-based materials, positioning it for high-temperature piezoelectric and photonic applications. Extensive research has investigated the electrical properties of oxygen-ion conductors based on barium bismuth niobium oxide (BaBiNb 5 O 15 , BBN), which adopts a tungsten bronze-type structure. While the intrinsic electrical conductivity of stoichiometric BBN compound is relatively low, tuning non-stoichiometry through controlled bismuth deficiency enables manipulation of oxygen vacancy concentration. This strategy significantly enhances ionic conductivity by facilitating oxygen ion migration. Furthermore, BBN-based conductors exhibit inherent advantages for oxygen ion transport: the high polarizability of Bi 3+ ions reduces oxygen migration energy, while weak Bi-O bond strength and minimal lattice restraint promote ion mobility [ 18 ] . The crystal structure also accommodates abundant cation vacancies and free volume, further accelerating oxygen ion diffusion. Additionally, these materials demonstrate excellent thermal stability. Consequently, BBN-based compounds show significant promise as solid electrolytes for the intermediate-temperature oxygen ion applications. Therefore, this study focuses on BBN-based tungsten bronze structural ceramics. Owing to their unique crystal framework and ionic transport properties, these materials have emerged as promising candidates for solid electrolytes. However, the pristine BBN material exhibits critical limitations, including restricted ionic mobility, inadequate electrical stability, and an unresolved oxygen relaxation mechanism, which impede practical applications in intermediate-temperature energy storage and conversion devices. To address these challenges, acceptor doping at the Bi 3+ site represents a well-established strategy for enhancing the electrical properties of BBN compound, wherein monovalent Na + or divalent Sr 2+ substitution induces compensatory oxygen vacancies. For instance, BaBi 0.98 Sr 0.02 Nb 5 O 15−δ achieves a bulk conductivity of 1.40×10 − 4 S/cm at 773 K [ 19 ] , a 1.4-fold improvement over pristine BBN under identical conditions. Complementary B-site engineering via Ti 4+ substitution for Nb 5+ further demonstrates efficacy, with BaBiNb 0.95 Ti 0.05 O 15−δ exhibiting grain conductivity of 1.28×10 − 4 S/cm at 623 K, ~ 5 times higher than undoped BBN compound [ 20 ] . Critical gaps persist in understanding how lower-valence cation doping at Nb 5+ /Bi 3+ sites influences the structural and electrochemical stability of BBN, despite established benefits of A/B-site engineering. Herein, we rationally select Ca 2+ as a dopant candidate based on two critical considerations: (i) Ionic radius compatibility-Ca 2+ (0.134 nm, CN = 12) closely matches Bi 3+ (0.144 nm, CN = 12), minimizing lattice distortion while preserving the tungsten bronze framework integrity; (ii) Electronic structure modulation-the lower valence state of Ca 2+ relative to Bi 3+ introduces charge compensation mechanisms that potentially optimize oxygen ion mobility. This work establishes a foundation for understanding how controlled Ca 2+ doping influences defect chemistry and transport properties in the BBN-based electrolytes. 2. Experimental Section 2.1 Experimental synthesis BaBiNb 5 − x Ca x O 15−δ (x = 0, 0.05) ceramics were synthesized using a conventional solid-state reaction method. High-purity BaCO 3 , Bi 2 O 3 , Nb 2 O 5 , and CaCO 3 powders were pre-dried at 473 K for 10 h to eliminate adsorbed moisture and CO₂. Stoichiometric quantities of the dried precursors were mixed via ball-milling in anhydrous ethanol for 12 h. The resulting slurry was air-dried, then calcined at 973 K for 6 h in air. After furnace cooling, the powder was reground, repelletized with 5 wt% polyvinyl alcohol binder, and pressed into discs at around 300 MPa. Finally, pellets were sintered at 1423 K for 6 h under sacrificial powder beds of matching composition to suppress volatilization of low-melting-point elements (Bi), yielding samples for dielectric relaxation and conductivity measurements. 2.2 Structure and Property Characterization Phase composition analysis of BaBiNb 5 − x Ca x O 15−δ samples was conducted using a PANalytical X’Pert-PRO MPD diffractometer with Cu K α radiation (λ = 0.154 nm), scanning from 10° to 80° at a step size of 0.0167°. Scanning electron microscopy (SEM; Czech TESCAN MIRA LMS) characterized surface microstructures. Thermal etching was performed at 1323 K for 15 minutes. Silver electrodes were deposited on both pellet faces and annealed at 973 K for 30 min to obtain the working electrodes. Electrical conductivity measurements employed the AC impedance spectroscopy (HIOKI IM3536) across 10 Hz-1 MHz and 473–773 K. 3. Experimental Results 3.1 Structure characteristic Figure 1 (a) shows the room-temperature powder XRD patterns of BaBi 1 − x Ca x Nb 5 O 15−δ samples with x = 0 and x = 0.05. As seen in Fig. 1 a, there are no additional impurity peaks in the XRD pattern of the BaBi 0.95 Ca 0.05 Nb 5 O 15−δ (BBN-Ca0.05) sample when contrasted with the undoped BBN compound. This absence indicates that the primary perovskite structure remains unchanged upon Ca 2+ ion doping, confirming successful substitution of Ca 2+ for Bi 3+ sites and the formation of a single-phase solid solution, BaBiNb 4.95 Ca 0.05 O 15−δ . The XRD pattern of the BBN-Ca0.05 sample exhibits a diffraction peak shift to higher angles relative to the parent BBN compound (Fig. 1 b). According to the Bragg’s equation, this increase in diffraction angle indicates reduced interplanar spacing ( d ) and lattice contraction, consistent with the smaller ionic radius of Ca 2+ substituting for Bi 3+ . This contraction confirms successful calcium incorporation into the crystal lattice, forming a substitutional solid solution. Figure 2 presents the microscopic morphology characteristics and grain size statistics of the BaBi 1 − x Ca x Nb 5 O 15−δ samples. SEM analysis reveals a dense microstructure with minimal porosity and cracking in the BaBi 1 − x Ca x Nb 5 O 15−δ samples. The interconnected network of larger grains incorporates smaller grains occupying interstitial positions within the matrix. The observed short columnar morphology correlates with c-axis-preferred anisotropic growth, consistent with the characteristic crystallographic behavior of tungsten bronze-type materials. Grain size distribution was quantitatively analyzed by measuring forty randomly selected grains from SEM images using Nano Measurer 1.2 software, with corresponding statistical data given in Fig. 2 . For the BBN-Ca0.05 sample, the average grain size measures around 3.76 µm, noticeably smaller than the 5.27 µm average grain size of the undoped BBN sample. Evidently, Ca doping plays a crucial role in refining the grain structure of the BBN compound. At a Ca 2+ doping concentration of x = 0.05, the average grain size decreases by 28.7%. This refinement arises from the radius mismatch between Ca 2+ and matrix cations Bi 3+ , which inhibits grain boundary migration and suppresses abnormal grain growth. Additionally, energy-dispersive X-ray spectrometry (EDS) analysis of the BBN-Ca0.05 sample revealed elemental compositions closely matching theoretical values for Ba, Bi, Ca, Nb, and O element, seen in Fig. 3 . This agreement confirms that Ca 2+ successfully substituted into the crystal lattice, forming a solid solution. 3.2 Conductivity test Figure 4 shows the AC impedance spectra of the BaBi 1 − x Ca x Nb 5 O 15−δ (x = 0, 0.05) samples at 673 K. As illustrated in Fig. 4 , the impedance curves of both the BBN and BBN-Ca0.05 samples exhibit three flattened semicircles, which correspond to the responses of grains, grain boundaries, and electrode interfaces in descending order of frequency. To determine the resistance and capacitance parameters, an equivalent circuit composed of three series-connected R//CPE elements, shown in Fig. 4 , was used to fit the AC impedance spectra of BaBi 1 − x Ca x Nb 5 O 15−δ samples. The fitting lines closely align with the experimental data points, confirming the validity of the proposed equivalent circuit model. The impedance spectrum fitting results of the BaBi 1 − x Ca x Nb 5 O 15−δ (x = 0, 0.05) samples at 673 K are shown in Table 1. The bulk capacitance (C b ) of around 10 − 10 F aligns with typical values for grain capacitance in oxide ion conductors, thereby confirming the sample’s ionic conductivity. The bulk conductivity of BaBi 1 − x Ca x Nb 5 O 15−δ (x = 0, 0.05) sample was calculated using the formula σ b = L/R b S [ 21 ] , where L represents the sample thickness and S is the electrode surface area. Figure 5 displays the Arrhenius plot of the bulk conductivity for the BaBi 1 − x Ca x Nb 5 O 15−δ (x = 0, 0.05) samples. Within the measured temperature range, the bulk conductivity of the BaBi 1 − x Ca x Nb 5 O 15−δ samples increases with rising temperature. The incorporation of Ca 2+ dopants further enhance conductivity, as evidenced by the BBN-Ca0.05 sample achieving a conductivity of 3.79×10 − 4 S/cm at 773 K, 5.1 times higher than that of the undoped parent BBN compound. To investigate the underlying causes and elucidate the oxygen ion diffusion mechanism in the BaBi 1 − x Ca x Nb 5 O 15−δ samples, dielectric relaxation spectroscopy, a technique highly sensitive to compositional and microstructural variations, was employed. 3.3 Dielectric Modulus Spectroscopy Dielectric modulus spectroscopy is a widely recognized and effective technique for characterizing the oxygen ion relaxation process, and there exists the following conversion relationship between the complex modulus (M*) and impedance (Z*) [ 22 ] : M*=jωC 0 Z* (1) Here, C 0 is the capacitance value in a vacuum and ω = 2π f is the angle frequency. Figure 6 shows the variation of the imaginary modulus (M") spectrum of the BBN-Ca0.05 sample with the natural logarithm of the measurement frequency (ln f ). An increase in measurement temperature induces a significant shift of the dielectric modulus peak to higher frequencies, confirming that the underlying relaxation process is thermally activated. According to the principles of dielectric relaxation theory, the peak position in the dielectric relaxation spectrum is determined by the angular frequency (ω) satisfying ωτ = 2π f p τ = 1, with τ denoting the characteristic relaxation time [ 23 ] .This critical condition signifies a resonance between the external electric field's oscillation rate and the intrinsic response rate (1/τ) of charge carriers (oxygen vacancies). At this point, the system exhibits maximal energy dissipation, manifesting as a pronounced peak in the dielectric loss spectrum. The relaxation time (τ) was derived via a non-linear fitting method. Figure 7 displays the Arrhenius curves (lnτ ~ 1000/T) of BaBi 1 − x Ca x Nb 5 O 15−δ (x = 0, 0.05) samples. Based on Arrhenius’ law (τ = τ 0 exp( E /K B T [ 24 ] ), where τ is the relaxation time and E is the activation energy), the activation energy E of the BaBi 1 − x Ca x Nb 5 O 15−δ samples was calculated using the linear fitting method. The relaxation activation energy E of the BBN and BBN-Ca0.05 sample is 0.47 eV and 0.33 eV, respectively. The relaxation activation energy for oxygen ion diffusion in BBN-Ca0.05 sample is lower than that in the undoped BBN sample, indicating that Ca 2+ doping reduces the diffusion barrier height and thereby enhances oxygen ion mobility. 4. Discussion The ionic conductivity of oxide ion conductors is critically dependent on oxygen vacancy concentration, which serves as the fundamental prerequisite for efficient ionic transport. To enhance oxygen ion conductivity, maximizing the oxygen vacancy density within the crystal lattice is essential. A-site doping represents a well-established strategy to systematically increase oxygen vacancy concentrations. In this study, Ca 2+ ions were used for A-site doping to partially substitute Bi 3+ in the BBN host lattice, as illustrated by the following Kroger-Vink equation: $$\:2\varvec{C}\varvec{a}\varvec{O}\underrightarrow{{\varvec{B}\varvec{i}}_{2}{\varvec{O}}_{3}}\:2{\varvec{C}\varvec{a}}_{\varvec{B}\varvec{i}}^{\varvec{{\prime\:}}}+{\varvec{V}}_{\varvec{O}}^{\bullet\:\bullet\:}+2{\varvec{O}}_{\varvec{O}}$$ 2 From the above defect Eq. 2 , the incorporation of 5 mol% Ca 2+ ions generate 2.5 mol% oxygen vacancies in the resulting BaBi 1 − x Ca x Nb 5 O 15−δ samples. Compared to the undoped BBN compound, there exist more oxygen vacancies in the BBN-Ca0.05 sample. According to the reported results, ionic conductivity does not scale linearly with total oxygen vacancy concentration, as only mobile vacancies effectively participate in charge transport, while others may be trapped or energetically unfavorable for migration. Combined with the point defect theory, the relaxation peak height and the relaxation time τ are usually described the concentration and mobility for the mobile defect. An elevated relaxation peak signifies an increased concentration of mobile defects, and concomitantly, a reduced relaxation time implies superior mobility, facilitating faster ion hopping. Figure 8 displays the curves of the dielectric loss tanδ versus the natural logarithm ln f for the BBN and BBN-Ca0.05 sample. As shown in the figure, the BBN-Ca0.05 sample exhibits a higher relaxation peak height than the undoped BBN compound, indicating a greater concentration of mobile oxygen vacancies. Furthermore, the dielectric loss peak for the doped BBN-Ca0.05 sample is observed to shift towards a higher frequency range. The relaxation time (τ) is determined from the dielectric loss peak, which occurs under the condition ωτ = 2π f p τ = 1 [ 25 ] . Consequently, a higher characteristic frequency f p corresponds to a shorter relaxation time. As depicted in Fig. 7 , the loss peak for the BBN-Ca0.05 sample shifts to a higher frequency range compared to the undoped BBN compound. This indicates that the relaxation time for oxygen ion diffusion is significantly reduced upon Ca-doping, implying enhanced ionic mobility in BBN-Ca0.05 sample. The preceding analysis indicates that the BBN-Ca0.05 sample possesses both a higher concentration and enhanced mobility of mobile oxygen vacancies. This is considered to be the primary reason for its superior oxygen ion conductivity compared to the undoped BBN compound. The substitution of Ca 2+ for Bi 3+ in the BBN compound introduces oxygen vacancies to maintain charge neutrality, which is favorable for enhancing oxygen ion diffusion. However, from an electronic-structure perspective, the intrinsic [Xe]4f 14 5d 10 6s 2 configuration of Bi 3+ features a stereochemically active 6s 2 lone pair. This lone pair occupies a significant volume, leading to lattice distortions that can impede oxygen ion migration. In contrast, Ca 2+ ion possesses a noble gas electron configuration ([Ar]3d 0 4s 0 ) and lacks any lone pairs. The incorporation of Ca 2+ suppresses the Bi 3+ -driven lattice distortion, resulting in a more regular crystal framework. This structural regularization creates continuous, open channels conducive to faster ion transport. Furthermore, the spherically symmetric electron cloud of Ca 2+ forms highly symmetrical Ca-O bonds, which further lowers the oxygen ion migration barrier and synergistically enhances the oxygen ion conductivity of BBN-based materials. 5. Conclusion A conventional solid-state reaction approach was employed to synthesize BaBi 1 − x Ca x Nb 5 O 15−δ samples with x = 0 and x = 0.05, followed by a systematic exploration of how Ca 2+ ion doping influences the phase structure, electrical characteristics, and relaxation behavior of the parent BBN compound. The key findings are summarized as follows: (1) Doping with 5 mol% Ca 2+ significantly enhanced the bulk conductivity. At 673 K, the bulk conductivity reached 3.79×10 − 4 S/cm, which is approximately 5.1 times higher than that of the undoped BBN parent compound. This demonstrates that trace-level doping is an effective strategy for improving the bulk electrical performance of BBN-based materials. (2) Analysis of the dielectric modulus spectra, both the pristine BBN and Ca-doped BBN-0.05 samples exhibited characteristic dielectric relaxation phenomena. Linear fitting of the dielectric modulus spectra obtained relaxation activation energies of 0.47 eV and 0.33 eV for oxygen ion diffusion in the BBN and BBN-Ca0.05 samples, respectively. The enhanced bulk conductivity observed in the BBN-Ca0.05 sample can be primarily attributed to the synergy of a higher density of mobile oxygen vacancies and the elevated mobility of these vacancies. Declarations CRediT authorship contribution statement Xinye Yun and Han Wang : Writing – original draft, Investigation, Data curation. Xinyu Hu and Jiamiao Tuo : Software, Investigation, Formal analysis, Conceptualization. Shuyao Cao and Qinfu Zhao : Methodology, Software, Investigation. Weiguo Wang : Writing – review & editing, Software, Investigation, Funding acquisition. Lei Chen : Software, Methodology, Investigation, Conceptualization. Declaration of 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 Xinye Yun and Han Wang: Writing – original draft, Investigation, Data curation. Xinyu Hu and Jiamiao Tuo: Software, Investigation, Formal analysis, Conceptualization. Shuyao Cao and Qinfu Zhao: Methodology, Software, Investigation. Weiguo Wang: Writing – review & editing, Software, Investigation, Funding acquisition. Lei Chen: Software, Methodology, Investigation, Conceptualization. Acknowledgments This work has been subsidized by the National Natural Science Foundation of China (No.12064044, 11604286, 12504037), Natural Science Basic Research Program of Shaanxi Province (No.2024JC-YBQN-0720, 2025JC-YBMS-467, 2025JC-YBQN-079, 2025JC-YBQN-039), National College Students Innovation and Entrepreneurship Training Program (S202410719093), Yan’an University Research Student Education Innovation Project (YCX2023024). Data Availability Data will be made available on request. References Aimi A, Onodera H, Shimonishi Y, Fujimoto K, Yoshida S (2024) High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li 2–x La (1+ x)/3 M 2 O 6 F (M = Nb, Ta). Chem Mater 36:3717–3725. https://doi.org/10.1021/acs.chemmater.3c03288 Ueno N, Yaguchi H, Fujii K, Yashima M (2024) High conductivity and diffusion mechanism of oxide ions in triple fluorite-like layers of oxyhalides. 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Phys Rev B 65:064304. https://doi.org/10.1103/PhysRevB.65.064304 Fang Q, Wang X, Zhang G, Zhou H (2003) Damping mechanism in the novel La 2 Mo 2 O 9 -based oxide ion conductors. J ALLOY COMPD 355:177–182. https://doi.org/10.1016/S0925-8388(03)00278-0 Böttcher CJF, Bordewijk P (1978) Theory of Electric Polarization. Science 8:476–479. https://doi.org/10.1016/C2009-0-15579-4 Yang F, Wu P, Sinclair D (2017) Suppression of electrical conductivity and switching of conduction mechanisms in (Na 0.5 Bi 0.5 TiO 3 ) 1-x (BiAlO 3 ) x solid solutions. J MATER CHEM C 5:7243–7252. https://doi.org/10.1039/C7TC02519J 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. 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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-8539661","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":581874710,"identity":"1f7046f9-ccbf-47d0-8bfc-ed8f76ca5d20","order_by":0,"name":"Xinye Yun","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Xinye","middleName":"","lastName":"Yun","suffix":""},{"id":581874711,"identity":"55a05c73-5867-4476-bd44-97e4f34bfcdd","order_by":1,"name":"Han Wang","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Han","middleName":"","lastName":"Wang","suffix":""},{"id":581874712,"identity":"7597f7b6-99a2-44c1-b740-515cff5bf2a0","order_by":2,"name":"Xinyu Hu","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Xinyu","middleName":"","lastName":"Hu","suffix":""},{"id":581874713,"identity":"83659231-3d2a-4c6c-8591-afd79f096a6f","order_by":3,"name":"Jiamiao Tuo","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Jiamiao","middleName":"","lastName":"Tuo","suffix":""},{"id":581874714,"identity":"67962adc-96fd-4dae-a130-aef47717d16f","order_by":4,"name":"Shuyao Cao","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Shuyao","middleName":"","lastName":"Cao","suffix":""},{"id":581874715,"identity":"2f5ca913-7f82-40e5-bbdc-0b6672a617a5","order_by":5,"name":"Qinfu Zhao","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Qinfu","middleName":"","lastName":"Zhao","suffix":""},{"id":581874717,"identity":"4f3c9a43-7f54-4b58-94aa-8a6dbbf0f3b3","order_by":6,"name":"Weiguo Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAsklEQVRIiWNgGAWjYJCCA0Asx8befIA0LcZ8PMcSSLMpcZ5EjgJxSg1upF88+OPPnfQ2hhwGhh8V24jRklNwmLftWW4bw9kDjD1nbhPWYnY7J+EwY8Ph3DbGvgRmxjYitQAddjidjZnHgFgt6QcO8LAdTmBjI1aL/f03DCC/GLbxsCUcJMovkj3HH38Ehpi8/PzHBx/8qCBCCwMDjwEDJDZhJGHA/oAExaNgFIyCUTAiAQAKoUQWNS8qOgAAAABJRU5ErkJggg==","orcid":"","institution":"Yan'an University","correspondingAuthor":true,"prefix":"","firstName":"Weiguo","middleName":"","lastName":"Wang","suffix":""},{"id":581874726,"identity":"3a1f04ed-2d31-496e-9fad-95cb21bb7a8f","order_by":7,"name":"Lei Chen","email":"","orcid":"","institution":"Yan'an University","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2026-01-07 09:53:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8539661/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8539661/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101751351,"identity":"0eef193e-2321-4ccc-a3a6-4d534ca5ccf3","added_by":"auto","created_at":"2026-02-03 10:19:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":87411,"visible":true,"origin":"","legend":"\u003cp\u003ea) XRD patterns of the BBN and BBN-Ca0.05 samples at room temperature; b) the magnified view of the 2θ range of 27°–28°.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/7dc946f5e515a7a9975d81e2.png"},{"id":101450726,"identity":"91d0e9ca-90e9-414a-99f7-ce46ace8a714","added_by":"auto","created_at":"2026-01-29 20:16:37","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":119555,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images for the BaBi\u003csub\u003e1-x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15-δ\u003c/sub\u003e samples. a) BBN; b) BBN-Ca0.05.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/449cb5e3579471bf87c7141e.jpeg"},{"id":101450724,"identity":"11ecff03-7758-45eb-a56d-e415cc3255b4","added_by":"auto","created_at":"2026-01-29 20:16:37","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":103706,"visible":true,"origin":"","legend":"\u003cp\u003eEDS analysis of the BBN-Ca0.05 sample at room temperature.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/b882b31f167f5273fdede097.png"},{"id":101751316,"identity":"dc6ac358-3146-4eb3-a84f-f127ff56e94c","added_by":"auto","created_at":"2026-02-03 10:19:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":115676,"visible":true,"origin":"","legend":"\u003cp\u003eAC impedance diagrams of BBN and BBN-Ca0.05 samples at 673K.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/4d1d3af6c0117d52d097f387.png"},{"id":101450728,"identity":"58fdb099-ac6c-4d6b-bc89-5f40f6c9da08","added_by":"auto","created_at":"2026-01-29 20:16:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":69105,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius diagram of the bulk conductivity for the BBN and BBN-Ca0.05 samples.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/b38800df3fe0cc11b89c3b0f.png"},{"id":101450730,"identity":"6e680945-ef6f-4832-a353-c39461d251c7","added_by":"auto","created_at":"2026-01-29 20:16:37","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":106296,"visible":true,"origin":"","legend":"\u003cp\u003eThe plots of the dielectric loss modulus (m versus measuring frequency ln\u003cem\u003ef \u003c/em\u003efor the BBN-Ca0.05 sample.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/2d53ce0ddfdc28754ac2f625.png"},{"id":101450731,"identity":"fa7f10a7-43e9-4036-8d38-00d8d4c5b0b6","added_by":"auto","created_at":"2026-01-29 20:16:37","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":58949,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius plot of the relaxation time of BaBi\u003csub\u003e1-x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15-δ \u003c/sub\u003e(x= 0,0.05) samples.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/ea8f7ee3afdd69a8eb256986.png"},{"id":101751425,"identity":"87cfa5e5-9507-4144-9bd1-389903fc58e7","added_by":"auto","created_at":"2026-02-03 10:20:11","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":82077,"visible":true,"origin":"","legend":"\u003cp\u003eThe curves of the dielectric loss tanδ and measuring frequency ln\u003cem\u003ef \u003c/em\u003efor the BBNand BBN-Ca0.05 at 673 K.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/442a78786fc8436c02403895.png"},{"id":102748973,"identity":"14e75224-03e4-41d0-a519-794a135ddfb0","added_by":"auto","created_at":"2026-02-16 09:11:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1517212,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8539661/v1/98da2ba0-b7d1-45e8-bbdf-df0e765865c1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Investigation of electrical properties for Ca-modified BaBiNb5O15 compound","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe development of novel solid electrolytes has predominantly centered on oxygen-ion conductors with fluorite, perovskite, pyrochlore, and lanthanum molybdate (LaMOX) structures \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Fluorite-type materials demonstrate high ionic conductivity at elevated temperatures, structural robustness, tunable defect chemistry, and favorable chemical compatibility, establishing them as benchmark systems \u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. Perovskite-structured variants offer exceptional compositional flexibility \u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e, where A/B-site doping enables optimization of conductivity alongside superior intermediate-temperature performance and mechanical integrity \u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. Pyrochlore-based compounds, such as Gd\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e compound, exhibit remarkable thermal stability, reduction resistance, and adjustable defect concentrations, sustaining efficient ion transport under extreme conditions \u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. LaMOX electrolytes combine high ionic mobility, low activation energy, and electrode compatibility in the intermediate-temperature range, rendering them promising for solid oxide fuel cells (SOFCs). These materials collectively underpin advancements in SOFCs, oxygen sensors, and solid-state ionic devices \u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAlthough there are many types of oxygen ion conductors widely studied and applied in SOFCs nowadays, there are also inevitable drawbacks, such as excessively high operating temperatures, which cause electronic conductivity, the difficulty in synthesizing pure phases during phase transitions, and poor compatibility with electrode materials, all of which limit their commercial development \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Therefore, seeking electrolyte materials with high ionic rates and good chemical and mechanical stability is an important research direction for achieving intermediate-temperature in solid oxide fuel cells \u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTungsten bronze-structured dielectric ceramics represent a significant class of functional materials distinguished by their complex crystallography, pronounced dielectric relaxation behavior, and ferroelectric phase transitions. These characteristics confer superior functionality, including pyroelectricity, nonlinear optical activity, and piezoelectricity, relative to conventional oxygen-ion conductors. The structural flexibility of tungsten bronze frameworks arises from interstitial sites within oxygen octahedra, enabling multivalent cation occupancy (A/B/C sites) that facilitates tailored defect engineering \u003csup\u003e[\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. This architectural versatility permits precise control over phase transition temperatures and polarization mechanisms, as evidenced in systems like Sr\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eBa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e (SBN) \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, PbNb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e, and K\u003csub\u003e3\u003c/sub\u003eLi\u003csub\u003e2\u003c/sub\u003eNb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e \u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. Notably, SBN compound exhibits enhanced pyroelectric and electro-optical coefficients compared to calcium ferrite-based materials, positioning it for high-temperature piezoelectric and photonic applications.\u003c/p\u003e \u003cp\u003eExtensive research has investigated the electrical properties of oxygen-ion conductors based on barium bismuth niobium oxide (BaBiNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e, BBN), which adopts a tungsten bronze-type structure. While the intrinsic electrical conductivity of stoichiometric BBN compound is relatively low, tuning non-stoichiometry through controlled bismuth deficiency enables manipulation of oxygen vacancy concentration. This strategy significantly enhances ionic conductivity by facilitating oxygen ion migration. Furthermore, BBN-based conductors exhibit inherent advantages for oxygen ion transport: the high polarizability of Bi\u003csup\u003e3+\u003c/sup\u003e ions reduces oxygen migration energy, while weak Bi-O bond strength and minimal lattice restraint promote ion mobility \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. The crystal structure also accommodates abundant cation vacancies and free volume, further accelerating oxygen ion diffusion. Additionally, these materials demonstrate excellent thermal stability. Consequently, BBN-based compounds show significant promise as solid electrolytes for the intermediate-temperature oxygen ion applications.\u003c/p\u003e \u003cp\u003eTherefore, this study focuses on BBN-based tungsten bronze structural ceramics. Owing to their unique crystal framework and ionic transport properties, these materials have emerged as promising candidates for solid electrolytes. However, the pristine BBN material exhibits critical limitations, including restricted ionic mobility, inadequate electrical stability, and an unresolved oxygen relaxation mechanism, which impede practical applications in intermediate-temperature energy storage and conversion devices. To address these challenges, acceptor doping at the Bi\u003csup\u003e3+\u003c/sup\u003e site represents a well-established strategy for enhancing the electrical properties of BBN compound, wherein monovalent Na\u003csup\u003e+\u003c/sup\u003e or divalent Sr\u003csup\u003e2+\u003c/sup\u003e substitution induces compensatory oxygen vacancies. For instance, BaBi\u003csub\u003e0.98\u003c/sub\u003eSr\u003csub\u003e0.02\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e achieves a bulk conductivity of 1.40\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm at 773 K \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, a 1.4-fold improvement over pristine BBN under identical conditions. Complementary B-site engineering via Ti\u003csup\u003e4+\u003c/sup\u003e substitution for Nb\u003csup\u003e5+\u003c/sup\u003e further demonstrates efficacy, with BaBiNb\u003csub\u003e0.95\u003c/sub\u003eTi\u003csub\u003e0.05\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e exhibiting grain conductivity of 1.28\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm at 623 K, ~\u0026thinsp;5 times higher than undoped BBN compound \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCritical gaps persist in understanding how lower-valence cation doping at Nb\u003csup\u003e5+\u003c/sup\u003e/Bi\u003csup\u003e3+\u003c/sup\u003e sites influences the structural and electrochemical stability of BBN, despite established benefits of A/B-site engineering. Herein, we rationally select Ca\u003csup\u003e2+\u003c/sup\u003e as a dopant candidate based on two critical considerations: (i) Ionic radius compatibility-Ca\u003csup\u003e2+\u003c/sup\u003e (0.134 nm, CN\u0026thinsp;=\u0026thinsp;12) closely matches Bi\u003csup\u003e3+\u003c/sup\u003e (0.144 nm, CN\u0026thinsp;=\u0026thinsp;12), minimizing lattice distortion while preserving the tungsten bronze framework integrity; (ii) Electronic structure modulation-the lower valence state of Ca\u003csup\u003e2+\u003c/sup\u003e relative to Bi\u003csup\u003e3+\u003c/sup\u003e introduces charge compensation mechanisms that potentially optimize oxygen ion mobility. This work establishes a foundation for understanding how controlled Ca\u003csup\u003e2+\u003c/sup\u003e doping influences defect chemistry and transport properties in the BBN-based electrolytes.\u003c/p\u003e"},{"header":"2. Experimental Section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Experimental synthesis\u003c/h2\u003e \u003cp\u003eBaBiNb\u003csub\u003e5\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) ceramics were synthesized using a conventional solid-state reaction method. High-purity BaCO\u003csub\u003e3\u003c/sub\u003e, Bi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Nb\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e, and CaCO\u003csub\u003e3\u003c/sub\u003e powders were pre-dried at 473 K for 10 h to eliminate adsorbed moisture and CO₂. Stoichiometric quantities of the dried precursors were mixed via ball-milling in anhydrous ethanol for 12 h. The resulting slurry was air-dried, then calcined at 973 K for 6 h in air. After furnace cooling, the powder was reground, repelletized with 5 wt% polyvinyl alcohol binder, and pressed into discs at around 300 MPa. Finally, pellets were sintered at 1423 K for 6 h under sacrificial powder beds of matching composition to suppress volatilization of low-melting-point elements (Bi), yielding samples for dielectric relaxation and conductivity measurements.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Structure and Property Characterization\u003c/h2\u003e \u003cp\u003ePhase composition analysis of BaBiNb\u003csub\u003e5\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples was conducted using a PANalytical X\u0026rsquo;Pert-PRO MPD diffractometer with Cu\u003cem\u003eK\u003c/em\u003eα radiation (λ\u0026thinsp;=\u0026thinsp;0.154 nm), scanning from 10\u0026deg; to 80\u0026deg; at a step size of 0.0167\u0026deg;. Scanning electron microscopy (SEM; Czech TESCAN MIRA LMS) characterized surface microstructures. Thermal etching was performed at 1323 K for 15 minutes. Silver electrodes were deposited on both pellet faces and annealed at 973 K for 30 min to obtain the working electrodes. Electrical conductivity measurements employed the AC impedance spectroscopy (HIOKI IM3536) across 10 Hz-1 MHz and 473\u0026ndash;773 K.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Experimental Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Structure characteristic\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) shows the room-temperature powder XRD patterns of BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples with x\u0026thinsp;=\u0026thinsp;0 and x\u0026thinsp;=\u0026thinsp;0.05. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, there are no additional impurity peaks in the XRD pattern of the BaBi\u003csub\u003e0.95\u003c/sub\u003eCa\u003csub\u003e0.05\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (BBN-Ca0.05) sample when contrasted with the undoped BBN compound. This absence indicates that the primary perovskite structure remains unchanged upon Ca\u003csup\u003e2+\u003c/sup\u003e ion doping, confirming successful substitution of Ca\u003csup\u003e2+\u003c/sup\u003e for Bi\u003csup\u003e3+\u003c/sup\u003e sites and the formation of a single-phase solid solution, BaBiNb\u003csub\u003e4.95\u003c/sub\u003eCa\u003csub\u003e0.05\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e. The XRD pattern of the BBN-Ca0.05 sample exhibits a diffraction peak shift to higher angles relative to the parent BBN compound (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). According to the Bragg\u0026rsquo;s equation, this increase in diffraction angle indicates reduced interplanar spacing (\u003cem\u003ed\u003c/em\u003e) and lattice contraction, consistent with the smaller ionic radius of Ca\u003csup\u003e2+\u003c/sup\u003e substituting for Bi\u003csup\u003e3+\u003c/sup\u003e. This contraction confirms successful calcium incorporation into the crystal lattice, forming a substitutional solid solution.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the microscopic morphology characteristics and grain size statistics of the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples. SEM analysis reveals a dense microstructure with minimal porosity and cracking in the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples. The interconnected network of larger grains incorporates smaller grains occupying interstitial positions within the matrix. The observed short columnar morphology correlates with c-axis-preferred anisotropic growth, consistent with the characteristic crystallographic behavior of tungsten bronze-type materials. Grain size distribution was quantitatively analyzed by measuring forty randomly selected grains from SEM images using Nano Measurer 1.2 software, with corresponding statistical data given in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e2\u003c/span\u003e. For the BBN-Ca0.05 sample, the average grain size measures around 3.76 \u0026micro;m, noticeably smaller than the 5.27 \u0026micro;m average grain size of the undoped BBN sample. Evidently, Ca doping plays a crucial role in refining the grain structure of the BBN compound. At a Ca\u003csup\u003e2+\u003c/sup\u003e doping concentration of x\u0026thinsp;=\u0026thinsp;0.05, the average grain size decreases by 28.7%. This refinement arises from the radius mismatch between Ca\u003csup\u003e2+\u003c/sup\u003e and matrix cations Bi\u003csup\u003e3+\u003c/sup\u003e, which inhibits grain boundary migration and suppresses abnormal grain growth. Additionally, energy-dispersive X-ray spectrometry (EDS) analysis of the BBN-Ca0.05 sample revealed elemental compositions closely matching theoretical values for Ba, Bi, Ca, Nb, and O element, seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e3\u003c/span\u003e. This agreement confirms that Ca\u003csup\u003e2+\u003c/sup\u003e successfully substituted into the crystal lattice, forming a solid solution.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Conductivity test\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the AC impedance spectra of the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) samples at 673 K. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the impedance curves of both the BBN and BBN-Ca0.05 samples exhibit three flattened semicircles, which correspond to the responses of grains, grain boundaries, and electrode interfaces in descending order of frequency. To determine the resistance and capacitance parameters, an equivalent circuit composed of three series-connected R//CPE elements, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e4\u003c/span\u003e, was used to fit the AC impedance spectra of BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples. The fitting lines closely align with the experimental data points, confirming the validity of the proposed equivalent circuit model. The impedance spectrum fitting results of the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) samples at 673 K are shown in Table\u0026nbsp;1. The bulk capacitance (C\u003csub\u003eb\u003c/sub\u003e) of around 10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e F aligns with typical values for grain capacitance in oxide ion conductors, thereby confirming the sample\u0026rsquo;s ionic conductivity. The bulk conductivity of BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) sample was calculated using the formula σ\u003csub\u003eb\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;L/R\u003csub\u003eb\u003c/sub\u003eS \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, where L represents the sample thickness and S is the electrode surface area.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e5\u003c/span\u003e displays the Arrhenius plot of the bulk conductivity for the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) samples. Within the measured temperature range, the bulk conductivity of the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples increases with rising temperature. The incorporation of Ca\u003csup\u003e2+\u003c/sup\u003e dopants further enhance conductivity, as evidenced by the BBN-Ca0.05 sample achieving a conductivity of 3.79\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm at 773 K, 5.1 times higher than that of the undoped parent BBN compound. To investigate the underlying causes and elucidate the oxygen ion diffusion mechanism in the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples, dielectric relaxation spectroscopy, a technique highly sensitive to compositional and microstructural variations, was employed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Dielectric Modulus Spectroscopy\u003c/h2\u003e \u003cp\u003eDielectric modulus spectroscopy is a widely recognized and effective technique for characterizing the oxygen ion relaxation process, and there exists the following conversion relationship between the complex modulus (M*) and impedance (Z*) \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e :\u003c/p\u003e \u003cp\u003e \u003cb\u003eM*=jωC\u003c/b\u003e \u003csub\u003e \u003cb\u003e0\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eZ* (1)\u003c/b\u003e \u003c/p\u003e \u003cp\u003eHere, C\u003csub\u003e0\u003c/sub\u003e is the capacitance value in a vacuum and ω\u0026thinsp;=\u0026thinsp;2π\u003cem\u003ef\u003c/em\u003e is the angle frequency. Figure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the variation of the imaginary modulus (M\") spectrum of the BBN-Ca0.05 sample with the natural logarithm of the measurement frequency (ln\u003cem\u003ef\u003c/em\u003e). An increase in measurement temperature induces a significant shift of the dielectric modulus peak to higher frequencies, confirming that the underlying relaxation process is thermally activated. According to the principles of dielectric relaxation theory, the peak position in the dielectric relaxation spectrum is determined by the angular frequency (ω) satisfying ωτ\u0026thinsp;=\u0026thinsp;2π\u003cem\u003ef\u003c/em\u003e\u003csub\u003ep\u003c/sub\u003eτ\u0026thinsp;=\u0026thinsp;1, with τ denoting the characteristic relaxation time \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.This critical condition signifies a resonance between the external electric field's oscillation rate and the intrinsic response rate (1/τ) of charge carriers (oxygen vacancies). At this point, the system exhibits maximal energy dissipation, manifesting as a pronounced peak in the dielectric loss spectrum. The relaxation time (τ) was derived via a non-linear fitting method. Figure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e7\u003c/span\u003e displays the Arrhenius curves (lnτ\u0026thinsp;~\u0026thinsp;1000/T) of BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e (x\u0026thinsp;=\u0026thinsp;0, 0.05) samples. Based on Arrhenius\u0026rsquo; law (τ\u0026thinsp;=\u0026thinsp;τ\u003csub\u003e0\u003c/sub\u003eexp(\u003cem\u003eE\u003c/em\u003e/K\u003csub\u003eB\u003c/sub\u003eT \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e), where τ is the relaxation time and \u003cem\u003eE\u003c/em\u003e is the activation energy), the activation energy \u003cem\u003eE\u003c/em\u003e of the BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples was calculated using the linear fitting method. The relaxation activation energy \u003cem\u003eE\u003c/em\u003e of the BBN and BBN-Ca0.05 sample is 0.47 eV and 0.33 eV, respectively. The relaxation activation energy for oxygen ion diffusion in BBN-Ca0.05 sample is lower than that in the undoped BBN sample, indicating that Ca\u003csup\u003e2+\u003c/sup\u003e doping reduces the diffusion barrier height and thereby enhances oxygen ion mobility.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe ionic conductivity of oxide ion conductors is critically dependent on oxygen vacancy concentration, which serves as the fundamental prerequisite for efficient ionic transport. To enhance oxygen ion conductivity, maximizing the oxygen vacancy density within the crystal lattice is essential. A-site doping represents a well-established strategy to systematically increase oxygen vacancy concentrations. In this study, Ca\u003csup\u003e2+\u003c/sup\u003e ions were used for A-site doping to partially substitute Bi\u003csup\u003e3+\u003c/sup\u003e in the BBN host lattice, as illustrated by the following Kroger-Vink equation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:2\\varvec{C}\\varvec{a}\\varvec{O}\\underrightarrow{{\\varvec{B}\\varvec{i}}_{2}{\\varvec{O}}_{3}}\\:2{\\varvec{C}\\varvec{a}}_{\\varvec{B}\\varvec{i}}^{\\varvec{{\\prime\\:}}}+{\\varvec{V}}_{\\varvec{O}}^{\\bullet\\:\\bullet\\:}+2{\\varvec{O}}_{\\varvec{O}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFrom the above defect Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the incorporation of 5 mol% Ca\u003csup\u003e2+\u003c/sup\u003e ions generate 2.5 mol% oxygen vacancies in the resulting BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples. Compared to the undoped BBN compound, there exist more oxygen vacancies in the BBN-Ca0.05 sample.\u003c/p\u003e \u003cp\u003eAccording to the reported results, ionic conductivity does not scale linearly with total oxygen vacancy concentration, as only mobile vacancies effectively participate in charge transport, while others may be trapped or energetically unfavorable for migration. Combined with the point defect theory, the relaxation peak height and the relaxation time τ are usually described the concentration and mobility for the mobile defect. An elevated relaxation peak signifies an increased concentration of mobile defects, and concomitantly, a reduced relaxation time implies superior mobility, facilitating faster ion hopping. Figure\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e8\u003c/span\u003e displays the curves of the dielectric loss tanδ versus the natural logarithm ln\u003cem\u003ef\u003c/em\u003e for the BBN and BBN-Ca0.05 sample. As shown in the figure, the BBN-Ca0.05 sample exhibits a higher relaxation peak height than the undoped BBN compound, indicating a greater concentration of mobile oxygen vacancies. Furthermore, the dielectric loss peak for the doped BBN-Ca0.05 sample is observed to shift towards a higher frequency range. The relaxation time (τ) is determined from the dielectric loss peak, which occurs under the condition ωτ\u0026thinsp;=\u0026thinsp;2π\u003cem\u003ef\u003c/em\u003e\u003csub\u003ep\u003c/sub\u003eτ\u0026thinsp;=\u0026thinsp;1 \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. Consequently, a higher characteristic frequency \u003cem\u003ef\u003c/em\u003e\u003csub\u003ep\u003c/sub\u003e corresponds to a shorter relaxation time. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the loss peak for the BBN-Ca0.05 sample shifts to a higher frequency range compared to the undoped BBN compound. This indicates that the relaxation time for oxygen ion diffusion is significantly reduced upon Ca-doping, implying enhanced ionic mobility in BBN-Ca0.05 sample. The preceding analysis indicates that the BBN-Ca0.05 sample possesses both a higher concentration and enhanced mobility of mobile oxygen vacancies. This is considered to be the primary reason for its superior oxygen ion conductivity compared to the undoped BBN compound.\u003c/p\u003e \u003cp\u003eThe substitution of Ca\u003csup\u003e2+\u003c/sup\u003e for Bi\u003csup\u003e3+\u003c/sup\u003e in the BBN compound introduces oxygen vacancies to maintain charge neutrality, which is favorable for enhancing oxygen ion diffusion. However, from an electronic-structure perspective, the intrinsic [Xe]4f\u003csup\u003e14\u003c/sup\u003e5d\u003csup\u003e10\u003c/sup\u003e6s\u003csup\u003e2\u003c/sup\u003e configuration of Bi\u003csup\u003e3+\u003c/sup\u003e features a stereochemically active 6s\u003csup\u003e2\u003c/sup\u003e lone pair. This lone pair occupies a significant volume, leading to lattice distortions that can impede oxygen ion migration. In contrast, Ca\u003csup\u003e2+\u003c/sup\u003e ion possesses a noble gas electron configuration ([Ar]3d\u003csup\u003e0\u003c/sup\u003e4s\u003csup\u003e0\u003c/sup\u003e) and lacks any lone pairs. The incorporation of Ca\u003csup\u003e2+\u003c/sup\u003e suppresses the Bi\u003csup\u003e3+\u003c/sup\u003e-driven lattice distortion, resulting in a more regular crystal framework. This structural regularization creates continuous, open channels conducive to faster ion transport. Furthermore, the spherically symmetric electron cloud of Ca\u003csup\u003e2+\u003c/sup\u003e forms highly symmetrical Ca-O bonds, which further lowers the oxygen ion migration barrier and synergistically enhances the oxygen ion conductivity of BBN-based materials.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eA conventional solid-state reaction approach was employed to synthesize BaBi\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e samples with x\u0026thinsp;=\u0026thinsp;0 and x\u0026thinsp;=\u0026thinsp;0.05, followed by a systematic exploration of how Ca\u003csup\u003e2+\u003c/sup\u003e ion doping influences the phase structure, electrical characteristics, and relaxation behavior of the parent BBN compound. The key findings are summarized as follows:\u003c/p\u003e \u003cp\u003e(1) Doping with 5 mol% Ca\u003csup\u003e2+\u003c/sup\u003e significantly enhanced the bulk conductivity. At 673 K, the bulk conductivity reached 3.79\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm, which is approximately 5.1 times higher than that of the undoped BBN parent compound. This demonstrates that trace-level doping is an effective strategy for improving the bulk electrical performance of BBN-based materials.\u003c/p\u003e \u003cp\u003e(2) Analysis of the dielectric modulus spectra, both the pristine BBN and Ca-doped BBN-0.05 samples exhibited characteristic dielectric relaxation phenomena. Linear fitting of the dielectric modulus spectra obtained relaxation activation energies of 0.47 eV and 0.33 eV for oxygen ion diffusion in the BBN and BBN-Ca0.05 samples, respectively. The enhanced bulk conductivity observed in the BBN-Ca0.05 sample can be primarily attributed to the synergy of a higher density of mobile oxygen vacancies and the elevated mobility of these vacancies.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e \u003cb\u003eCRediT authorship contribution statement\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eXinye Yun and Han Wang\u003c/b\u003e: Writing \u0026ndash; original draft, Investigation, Data curation. \u003cb\u003eXinyu Hu\u003c/b\u003e and \u003cb\u003eJiamiao Tuo\u003c/b\u003e: Software, Investigation, Formal analysis, Conceptualization. \u003cb\u003eShuyao Cao and Qinfu Zhao\u003c/b\u003e: Methodology, Software, Investigation. \u003cb\u003eWeiguo Wang\u003c/b\u003e: Writing \u0026ndash; review \u0026amp; editing, Software, Investigation, Funding acquisition. \u003cb\u003eLei Chen\u003c/b\u003e: Software, Methodology, Investigation, Conceptualization.\u003c/p\u003e\u003cp\u003e \u003ch2\u003eDeclaration of Competing 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\u003eXinye Yun and Han Wang: Writing \u0026ndash; original draft, Investigation, Data curation. Xinyu Hu and Jiamiao Tuo: Software, Investigation, Formal analysis, Conceptualization. Shuyao Cao and Qinfu Zhao: Methodology, Software, Investigation. Weiguo Wang: Writing \u0026ndash; review \u0026amp; editing, Software, Investigation, Funding acquisition. Lei Chen: Software, Methodology, Investigation, Conceptualization.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis work has been subsidized by the National Natural Science Foundation of China (No.12064044, 11604286, 12504037), Natural Science Basic Research Program of Shaanxi Province (No.2024JC-YBQN-0720, 2025JC-YBMS-467, 2025JC-YBQN-079, 2025JC-YBQN-039), National College Students Innovation and Entrepreneurship Training Program (S202410719093), Yan\u0026rsquo;an University Research Student Education Innovation Project (YCX2023024).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAimi A, Onodera H, Shimonishi Y, Fujimoto K, Yoshida S (2024) High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li\u003csub\u003e2\u0026ndash;x\u003c/sub\u003eLa\u003csub\u003e(1+ x)/3\u003c/sub\u003eM\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003eF (M\u0026thinsp;=\u0026thinsp;Nb, Ta). 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J MATER CHEM C 5:7243\u0026ndash;7252. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/C7TC02519J\u003c/span\u003e\u003cspan address=\"10.1039/C7TC02519J\" 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":"BaBiNb5O15, oxygen ion conductor, acceptor doping, dielectric modulus spectroscopy, mobility","lastPublishedDoi":"10.21203/rs.3.rs-8539661/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8539661/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBaBiNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e, a tungsten bronze-structured oxygen ion conductor, shows great promise for medium-temperature solid oxide fuel cells due to its excellent electrical conductivity and thermal stability. The effects of Ca\u003csup\u003e2+\u003c/sup\u003e ion doping on the structural, microstructural, and electrical conductivity characteristics of the BaBiNb\u003csub\u003e5\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eCa\u003csub\u003ex\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e compounds were analyzed. Measurements using AC impedance spectroscopy revealed that at 773 K, the grain conductivity of BaBi\u003csub\u003e0.95\u003c/sub\u003eCa\u003csub\u003e0.05\u003c/sub\u003eNb₅O\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e sample can reach 3.79\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm, representing an approximate 5.1-fold increase compared to the undoped BaBiNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e compound. Dielectric modulus analysis identified an activation energy of 0.37 eV for oxygen ion diffusion, with relaxation parameters indicating that dielectric loss peaks originate from vacancy-mediated oxide ion motion in the BaBi\u003csub\u003e0.95\u003c/sub\u003eCa\u003csub\u003e0.05\u003c/sub\u003eNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u0026minus;δ\u003c/sub\u003e sample. The enhanced ionic conductivity arises from increased mobile oxygen vacancy density, improved mobility, lower activation energy, and reduced relaxation time. These findings elucidate the mechanism of oxygen vacancy diffusion and the origin of oxygen relaxation behavior in BaBiNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e compound, advancing fundamental understanding of oxygen ion conductor characteristics while guiding future investigations into the electrical properties of BaBiNb\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e15\u003c/sub\u003e-based materials.\u003c/p\u003e","manuscriptTitle":"Investigation of electrical properties for Ca-modified BaBiNb5O15 compound","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-29 20:16:32","doi":"10.21203/rs.3.rs-8539661/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":"102f4246-ebb4-4bdb-a9a9-8912044b93b3","owner":[],"postedDate":"January 29th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-15T21:38:33+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-29 20:16:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8539661","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8539661","identity":"rs-8539661","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}