Charge Localization and Intrinsic Self-Trapped Exciton Photophysics in Undoped Cs₂NaScCl₆ Elpasolite Single Crystals

Charge Localization and Intrinsic Self-Trapped Exciton Photophysics in Undoped Cs₂NaScCl₆ Elpasolite Single Crystals | 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 Article Charge Localization and Intrinsic Self-Trapped Exciton Photophysics in Undoped Cs₂NaScCl₆ Elpasolite Single Crystals Mohamed Bouzidi, Abdullah A. Alatawi, Turki Alkathiri, Sultan Albarakati, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9369678/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Even though the intrinsic photophysics of all-inorganic lead-free halide double perovskites remains poorly understood in the absence of dopants, they have emerged as robust platforms for exciton physics. This paper provides a comprehensive investigation of undoped Cs₂NaScCl₆ combining electrical, structural and steady-state and time-resolved spectroscopy characterization. With a rigid [ScCl₆]³⁻octahedral framework which is thermally stable up to ~ 760–870°C, a phase-pure elpasolite structure ( \(\:Fm\stackrel{-}{3}m\) ) is confirmed by both Raman spectroscopy and X-ray diffraction. Pointing to low electronic disorder, a small Urbach energy of 0.123 eV and a direct bandgap of 4.63 eV are revealed by UV–visible spectroscopy. With a mono-exponential lifetime of 1.087 µs, a broad blue photoluminescence band is indicated at ~ 453 nm by Cs₂NaScCl₆, which is fully assigned to self-trapped exciton (STE) emission. With no evidence of defect-mediated relaxation, sub-picosecond carrier cooling followed by 10–15 ps STE formation is shown through Femtosecond transient absorption measurements. Only weak linear photoconductivity and extremely low currents are revealed by I–V characteristics (265 nm illumination vs dark), which is in agreement with not only negligible free-carrier transport but also strongly-localized excitations. However, activation energy of 0.625 eV and single bulk relaxation with Arrhenius-type conductivity are yielded using impedance spectroscopy. Therefore, Cs₂NaScCl₆ is collectively established by such results as a model wide-bandgap elpasolite where deeply localized STEs are driven within a structurally and electronically pristine lattice through exciton–phonon coupling, which suggests not only a well-defined platform for fundamental studies of STE-mediated emission in lead-free halide perovskites but also a robust host for rare-earth doping. Physical sciences/Materials science Physical sciences/Optics and photonics Physical sciences/Physics Halide Double Perovskites Self-Trapped Excitons (STE) Ultrafast Transient Absorption Impedance Spectroscopy Cs₂NaScCl₆ Single Crystals Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Lead-free halide double perovskites of the general formula A₂B(I)B(III)X₆ are considered to be a promising alternative to toxic Pb-based perovskites as they represent a compelling class of materials combining attributes such as environmental friendliness, wide bandgaps, thermal stability and chemical robustness [ 1 – 6 ]. The rigid cubic \(\:Fm\stackrel{-}{3}m\) symmetry where the highly-ordered lattice produces exceptionally-clean electronic states and strongly suppresses defect formation distinguishes chloride elpasolites within this family [ 7 , 8 ]. A unique photophysical behavior that is fundamentally different from that of traditional semiconductor perovskites is brought about by the structural ideality which favors STE formation through pronounced exciton–phonon coupling [ 9 , 10 ]. Without intentional doping, some halide double perovskites such as Cs₂NaScCl₆ exhibit intense broadband intrinsic STE photoluminescence unlike the majority of the members of this class which depend on dopant-induced emissions or narrow-bandgap transitions [ 11 – 14 ]. To investigate exciton localization in highly ionic wide-gap lattices, this remarkable behavior has turned Cs₂NaScCl₆-type hosts into benchmark systems. Added to that, studies have lately demonstrated that STE emission can be tuned across the visible and near-infrared range by moderate perturbations of the octahedral environment, which underlines the sensitivity of their optical response to local symmetry and lattice rigidity [ 11 – 14 ]. A thorough understanding of the intrinsic properties of undoped Cs₂NaScCl₆ is still missing in spite of these advances. Key questions concerning the interplay of macroscopic charge transport, ultrafast carrier dynamics and lattice vibrations added to the fundamental exciton relaxation pathways and the native electronic purity are left open as previous studies largely focused on alloyed or doped compositions [ 15 – 17 ]. Thus, a unified experimental framework simultaneously probing the electrical response, the vibrational properties, the optical absorption, the structural order and the steady-state and time-resolved emission is crucial for establishing Cs₂NaScCl₆ as a true model system for STE physics. This paper is the first comprehensive multi-dimensional investigation of pristine Cs₂NaScCl₆ single crystals integrating current–voltage (I–V) measurements, AC impedance spectroscopy, femtosecond transient absorption, Raman spectroscopy, steady-state and time-resolved photoluminescence, UV–visible absorption and structural refinement. Such holistic approach shows that the material possesses purely thermally-activated hopping conduction, a single bulk dielectric relaxation, an ultrafast sub-picosecond carrier cooling, strongly-localized microsecond-lived STEs, an ultrawide bandgap added to a very low Urbach energy and an exceptional structural integrity. Undoped Cs₂NaScCl₆ is established through all these results as an archetypal wide-bandgap elpasolite with intrinsically clean electronic and excitonic landscapes, which offers a rigorous baseline for future strain-engineered, dopant-engineered or defect-engineered derivatives aimed at optoelectronic and optical applications. Experimental Section Materials. Cesium chloride (CsCl, 99.9%, Sigma-Aldrich), sodium chloride (NaCl, ≥ 99.0%, Sigma-Aldrich), scandium(III) oxide (Sc₂O₃, 99.99% trace metals basis, Sigma-Aldrich), hydrochloric acid (HCl, 37%, EMSURE®, Merck), and 2-propanol (≥ 99.0%, Sigma-Aldrich) were used as received with no further purification. Synthesis. By means of a hydrothermal crystallization route, the single crystals of Cs₂NaScCl₆ were grown in a chloride-rich acidic medium. The starting materials used were scandium oxide (Sc₂O₃), sodium chloride (NaCl) and Cesium chloride (CsCl). 0.25 mmol of Sc₂O₃, 0.4 mmol of NaCl and 1 mmol of CsCl were typically loaded into a 20 mL Teflon-lined stainless-steel autoclave. In order to favor the formation of the elpasolite chloride framework and to suppress the crystallization of separate NaCl phases, the overall Cs/Na/Sc molar ratio was adjusted to 2/0.8/1, which is slightly deficient in Na⁺with respect to the ideal stoichiometry. To provide a highly chlorinated and strongly acidic environment that promotes not only stabilization of [ScCl₆]³⁻octahedra but also dissolution of Sc₂O₃ and to act as a mineralizing solvent, 0.8 mL of concentrated HCl was subsequently added to the liner. In order to allow the growth and nucleation of Cs₂NaScCl₆ single crystals, the autoclave was sealed, placed in an oven and heated to 180°C under autogenous pressure for 12 h. The furnace was switched off and the autoclave was left to cool slowly to room temperature after the heating step. Colorless transparent block-like crystals of Cs₂NaScCl₆ were found upon opening on the walls and at the bottom of the Teflon liner. The crystals were dried at 60°C in the air after being separated from the mother liquor by decantation and briefly rinsed with cold isopropanol several times to remove soluble by-products and residual acid. No mechanical grinding was applied. Without any aliovalent or rare-earth dopant, all electrical, optical and structural measurements were carried out on the as-grown undoped Cs₂NaScCl₆ single crystals. Characterizations. Using a Rigaku MiniFlex 600 diffractometer equipped with a Cu Kα source (λ = 1.5406 Å), the crystal structure of Cs₂NaScCl₆ was characterized by powder X-ray diffraction. At room temperature, diffraction patterns were recorded over the 2θ range 10–50° with a step size of 0.05°. To confirm the \(\:Fm\stackrel{-}{3}m\) cubic symmetry, the data were refined using the CELREF3 program. By thermogravimetric analysis, thermal stability was evaluated under nitrogen flow by means of a TGA-9000 system. The thermogravimetric analysis (TGA) profile was collected while almost 9.4 mg of the sample was heated at 5°C·min⁻¹ from room temperature to 1000°C. Steady-state photoluminescence (PL) measurements were conducted using a Shimadzu RF-1501 spectrofluorometer, whereas the optical absorption spectra were measured at room temperature by means of a Shimadzu UV-2550 spectrophotometer. The emission was collected in the 380–700 nm range while excitation was provided at 265 nm. PL spectra were recorded at the excitation wavelength under continuous UV illumination and in complete darkness. Using identical acquisition parameters, all measurements were carried out at room temperature. A FLOROCUBE time-correlated single-photon counting (TCSPC) system equipped with a NanoLED pulsed excitation source at 265 nm was employed to measure the time-resolved photoluminescence (TRPL). The instrument response function was recorded for deconvolution of the decay profiles, whereas the emitted signal was collected using a TBX-04-D detector. Raman spectroscopy was used to examine the vibrational properties of Cs₂NaScCl₆ single crystals by means of a HORIBA LabRAM HR Evolution confocal micro-Raman system equipped with a 532 nm diode-pumped solid-state (DPSS) laser as the excitation source. A spot size of almost 1–2 µm was provided by the laser beam which was focused onto the sample through a 50× long-working-distance objective. The backscattered radiation was detected using a thermoelectrically-cooled CCD camera and dispersed by an 1800 grooves·mm⁻¹ grating. Acquisition parameters (number of scans, laser power below 1 mW at the sample and accumulation time) were optimized so as to avoid photo-induced degradation or local heating. The standard Si phonon at 520.7 cm⁻¹ was used to carry out the calibration of the spectrometer. Under ambient conditions, all Raman spectra were collected at room temperature. Ultrafast transient absorption (TAS) measurements were conducted by means of an amplified Ti:sapphire laser system (repetition rate 1 kHz, pulse duration ~ 100 fs, center wavelength 800 nm). An optical parametric amplifier (OPA) tuned in the ultraviolet region was employed to generate pump pulses centered at 360 nm while a broadband white-light continuum extending from 320 to 700 nm was used as the probe beam and produced in a sapphire plate. A time window was allowed from ~ 200 fs to several nanoseconds by the temporal delays between probe and pump which were introduced using a motorized delay line. A dual-channel Si photodiode array was employed to detect and disperse the transmitted probe light in a spectrograph, which enables simultaneous acquisition of pumped and unpumped signals for improved spectral stability. To avoid nonlinear or multiphoton effects, all TAS measurements were performed under identical excitation fluences at room temperature (300 K). At selected probe wavelengths (340, 360, and 380 nm), kinetic traces were analyzed to describe the slow and fast relaxation channels by means of a bi-exponential decay model after being extracted from the TA dataset. To obtain a fine homogeneous powder suitable for electrical measurements, an agate mortar was used to gently crush a small quantity of Cs₂NaScCl₆ single crystals. A compact with a relative density of nearly 94% was yielded by the resulting powder which was pressed into a dense pellet (thickness ~ 0.92 mm, diameter 8 mm) by means of a hydraulic press operating at 50 Torr. To ensure stable and low-resistance electrical contact with the copper electrodes, silver paint was applied to the two faces of the pellet. A TH2828A precision impedance analyzer was employed to conduct the complex impedance measurements over the 10⁻¹–10⁶ Hz frequency range. At a controlled heating rate of 2 K min⁻¹, measurements were recorded between 300 and 375 K and the pellet–electrode assembly was mounted in a temperature-controlled chamber (TP94, Linkam, UK). Throughout the experiments, an AC excitation voltage of 0.5 V was used so as to remain within the linear response regime. An Integrated Photovoltaic Test System Assembly operating in two configurations (under continuous 265 nm UV illumination and in complete darkness) was employed to record the I–V characteristics of the Cs₂NaScCl₆ single crystals. Using two opposite silver electrodes, the crystals were contacted in order to form a planar two-terminal device. The applied voltage was swept at room temperature in the range − 20 to + 20 V. Picoampere sensitivity was used to measure the resulting current. To avoid photo-heating effects and to ensure reproducibility, the spot size and the illumination intensity were kept constant during all measurements. After stabilizing the device at each applied bias, all I–V curves were acquired under steady-state conditions. Results and discussion In addition to the reference diffraction pattern extracted from the crystallographic entry CCDC 2054287 corresponding to the ideal elpasolite-type double perovskite structure (space group \(\:Fm\stackrel{-}{3}m\) ), the room-temperature powder X-ray diffraction (XRD) pattern of the as-synthesized Cs₂NaScCl₆ sample is shown in Fig. 1 (a) [ 18 ]. The reference pattern agrees with the experimental reflections, demonstrating that the material crystallizes as a single phase-pure cubic perovskite without any detectable secondary phases. All diffraction peaks can be indexed to the (111), (022), (222), (004), (024) and (044) planes which characterizes the rock-salt-ordered arrangement of the [NaCl₆]⁵⁻ and [ScCl₆]³⁻ octahedra [ 18 ]. As displayed in Fig. S1 , the full-pattern refinement by means of the CELREF3 software was used to perform a more rigorous structural analysis [ 19 ]. Corresponding to a cell volume of 1153.37 ų, a cubic unit cell with a = 10.4871 ± 0.0207 Å is indicated in Table 1 by the refined lattice parameters obtained from the CELREF3 output file. An excellent match between the experimental and calculated peak positions is confirmed by the refinement residuals which are remarkably low with a root-mean-square deviation of 0.0267°. The high-symmetry \(\:Fm\stackrel{-}{3}m\) crystallographic phase fully agrees with the expected structure of the Cs₂NaScCl₆ elpasolite family as it is unambiguously demonstrated by the orthogonality of the angles (α = β = γ = 90°) and the equality of the three lattice parameters (a = b = c) [ 18 ]. The long-range ordering of the alternating [NaCl₆]⁵⁻ and [ScCl₆]³⁻ octahedra connected via corner-sharing is highlighted by the three-dimensional structural model generated using VESTA [ 20 ] as shown in Fig. 1 (b). The electrostatic stabilization to the framework is provided by the Cs⁺ ions reside in the cuboctahedral cavities. The intrinsic structural rigidity of this halide double perovskite is revealed by the absence of noticeable octahedral tilting or distortion. The foundation for the STE–dominated photophysics discussed in the coming sections is laid by the exciton localization behavior which is strongly influenced by the nearly ideal octahedral environment. Under a heating rate of 5°C min⁻¹, the TGA curve of Cs₂NaScCl₆ recorded from 30 to 1200°C is displayed in Fig. 1 (c). With the mass remaining constant at ~ 100 wt %, the sample shows an exceptionally stable profile up to ≈ 670°C, which confirms not only the high thermal robustness of the double-perovskite lattice but also the absence of physisorbed species. Marking the onset of thermal decomposition, a 5 wt % loss is reached around ≈ 760°C while a slight deviation from the baseline appears only above such a temperature. After that, with the maximum degradation rate centered at ≈ 865°C, a single sharp mass-loss event takes place between ≈ 770 and 900°C, which ultimately leaves a residual mass of ≈ 5.9 wt % at 1200°C. A similar abrupt degradation near ~ 740°C is revealed by such behavior which corresponds to the decomposition temperature reported in the literature for Cs₂NaScCl₆ single crystals [ 14 ]. The structural rigidity and robustness of the [NaCl₆]⁵⁻ and [ScCl₆]³⁻ corner-sharing octahedral framework are emphasized by the notably high thermal stability observed which exceeds that of many other lead-free double perovskites. A solid foundation is offered by this stability for investigating the intrinsic optoelectronic and excitonic properties of undoped Cs₂NaScCl₆, especially those dominated by STEs and exciton–phonon interactions. The UV–visible absorption spectrum of Cs₂NaScCl₆ recorded between 250 and 500 nm at room temperature is displayed in Fig. 2 (a). Halide double perovskites with highly symmetric octahedral frameworks are characterized by a clean and well-defined band-to-band transition, which is shown through a steep absorption edge observed at approximately 268–270 nm. The localized excitonic or STE–related absorption arising from the [ScCl₆]³⁻ and [NaCl₆]⁵⁻ octahedra is typically associated with several weak sub-band features in the 300–420 nm range which are also exhibited by the spectrum. These features are frequently noticed in Cs₂NaScCl₆-type hosts where strong exciton–phonon coupling induces shallow localized states slightly below the conduction band, thus broadening the absorption tail [ 21 , 22 ]. Constructed under the assumption of a direct allowed electronic transition, the corresponding Tauc plot [ 23 ] is plain to see in the inset of Fig. 2 (a). A direct bandgap of 4.63 eV which agrees with the large bandgap expected for highly ionic chloride-based perovskites is yielded by a linear extrapolation of (αhν)² versus photon energy. Such a value aligns well with the electronic structure of Cs₂NaScCl₆ where the conduction band minimum is governed by Sc–Cl antibonding states, which brings about an inherently-low absorption in the visible region and a wide electronic gap [ 17 ]. The excellent structural quality of the synthesized material is confirmed as a low density of deep defect states and minimal electronic disorder are further revealed by the sharpness of the absorption edge. As shown in Fig. 2 (b), the Urbach analysis was carried out by plotting ln(α) against photon energy so as to gain additional insight into the tailing region of the absorption edge. The Urbach energy (E u ) of 0.123 eV which not only means a modest degree of thermal and structural lattice disorder but is also relatively small for halide perovskites is yielded by the resulting linear fit. A low density of band-tail states is revealed by the electronic transitions near the band edge which are only weakly perturbed by phonons and localized defects as suggested by the E u value. This is in agreement not only with the strong lattice rigidity previously established through the structural characterization but also with the nearly ideal octahedral symmetry. The combined results from the Urbach and Tauc analyses confirm that its optical response is not primarily governed by defect-mediated absorption pathways but rather by shallow STE-related states and band-to-band transitions and also highlight the intrinsic electronic purity of undoped Cs₂NaScCl₆. The steady-state PL spectra of Cs₂NaScCl₆ recorded at room temperature not only under continuous UV illumination at 265 nm but also in the dark are compared in Fig. 3 (a). The characteristic signature of radiative recombination from STEs within the highly ionic [ScCl₆]³⁻ octahedral sub-lattice proves to be a broad featureless blue emission band centered at ≈ 453 nm exhibited by both spectra [ 13 ]. The remarkable rigidity of the Sc–Cl octahedral network and the strong localization of the excitonic wavefunction preventing thermally-driven or spectral-wandering shifts even under prolonged illumination are reflected by the similarity in peak position under both excitation conditions. Using a Gaussian function as follows, both PL bands were fitted to quantitatively analyze the emission profiles [ 24 ]: $$\:I\left(\lambda\:\right)=A{e}^{\left[-\raisebox{1ex}{${\left(\lambda\:-{\lambda\:}_{0}\right)}^{2}$}\!\left/\:\!\raisebox{-1ex}{${2\sigma\:}^{2}$}\right.\right]}+C$$ 1 where A stands for the peak amplitude, λ₀ is the peak wavelength, σ refers to the standard deviation related to the bandwidth and C represents the baseline offset. Table 2 provides a summary of the fitted parameters demonstrating that the full width remains essentially unchanged at half maximum (FWHM ≈ 74 nm) added to the emission maxima which remain strictly invariant for both spectra at λ 0 = 453.1 nm. However, under illumination, there is a slight decline in the PL amplitude. This agrees with a weak photo-induced quenching of shallow traps or a small reversible redistribution of STE populations. The high photostability of the STE state in Cs₂NaScCl₆ is further affirmed by the absence of any significant broadening or spectral shifts upon illumination, which is consistent not only with its structurally-rigid elpasolite framework but also with its low Urbach energy (0.123 eV). Figure 3 (b) displays the exciton dynamics which were investigated further using time-resolved PL (TRPL). The following single-exponential function characterizing a dominant radiative pathway involving a single emissive STE state describes the decay profile well [ 25 , 26 ]: $$\:I\left(t\right)={y}_{0}+{A}_{1}\:\text{e}\text{x}\text{p}(-\frac{t}{{\tau\:}_{1}})$$ 2 where y 0 = 0 refers to the baseline, A 1 ​ represents the initial amplitude and τ 1 ​ is the exciton lifetime. With a lifetime of τ 1 = 1 .08727 ± 0.00064 µs and parameters A 1 = 15005.08 ± 6.27, an excellent goodness of fit (χ 2 = 2158.81, R 2 = 0.99981) is yielded by the fitting results. The long-lived radiative relaxation which originates from a deeply localized self-trapped state stabilized by strong exciton–phonon coupling within the Sc–Cl octahedra is affirmed by the microsecond-scale τ₁ value, which completely agrees with STE-mediated emission previously reported for Cs₂NaScCl₆ single crystals [ 13 ]. The absence of competing defect-related or trap-assisted recombination channels is further demonstrated by the perfect mono-exponential behavior and high R², which is consistent not only with the narrow Urbach tail but also with the structurally derived low-disorder environment. The TRPL, the Gaussian fitting analysis and the steady-state PL results together offer a coherent picture of the strongly-localized and intrinsically-stable STE emission in undoped Cs₂NaScCl₆. Added to the absence of secondary decay channels and the microsecond exciton lifetime, the invariance of the emission energy under continuous illumination emphasizes not only the potential of this chloride elpasolite as a robust intrinsic blue-emitting material among lead-free perovskites but also its remarkable photophysical purity. The Raman spectra of Cs₂NaScCl₆ recorded at room temperature under continuous illumination and in the dark are shown in Fig. 4 (a). Centered at ≈ 51, 147, 209 and 292 cm⁻¹, the four well-resolved vibrational bands exhibited by the two spectra are fully consistent with the vibrational fingerprints expected for the \(\:Fm\stackrel{-}{3}m\) elpasolite structure [ 27 ]. Arising from the lattice motions involving Cs⁺ ions and the internal vibrations of the discrete [ScCl₆]³⁻ octahedra, four Raman-active modes (A₁g + Eg + 2F₂g) are predicted for Cs₂NaScCl₆ according to factor-group analysis. Such assignments completely agree with the bibliographic reference Raman study of Zissi and Papatheodorou who reported identical vibrational features for the solid Cs₂NaScCl₆ [ 27 ]. A lattice F₂g mode involving collective translational motions of Cs⁺against the halide framework brought about the lowest frequency band at ≈ 51 cm⁻¹. The bands at ≈ 209 and ≈ 292 cm⁻¹ are assigned to the E g bending mode and the higher-energy F₂g asymmetric stretching mode respectively. However, the intense feature at ≈ 147 cm⁻¹ matches the symmetric breathing A₁g mode of the [ScCl₆]³⁻octahedra [ 27 ]. The absence of any local structural distortion or symmetry lowering within the elpasolite lattice and the near-ideal octahedral environment around Sc³⁺are affirmed by the presence of all expected modes with well-defined positions and sharp linewidths. This is in full agreement with the structural refinement results discussed above. Using a pseudo-Voigt (Gauss–Lorentz) function of the form, all Raman peaks were fitted so as to further quantify the vibrational response as follows [ 28 ]: $$\:I\left(\nu\:\right)=\alpha\:\left[gL\left(\nu\:\right)+\left(1-g\right)G\left(\nu\:\right)\right]$$ 3 where a stands for the peak amplitude, g specifies the Lorentzian contribution and L(ν) denotes the Lorentzian component while G(ν) refers to the Gaussian component. Figure 4 (b) displays the full fitting profiles generated by LabSpec, whereas Table 3 provides a summary of the fitted parameters for both dark and illuminated spectra [ 29 ]. With variations below 0.001 cm⁻¹, all peak positions remain strictly invariant under illumination as shown by the fitting results. Continuous illumination does not induce any detectable structural modification in the Cs₂NaScCl₆ lattice as confirmed by such invariance of vibrational frequencies. Under illumination, the absence of increased anharmonicity or phonon softening is demonstrated by the full-width-at-half-maximum (FWHM) values which also remain nearly unchanged for all modes. The unaffected intrinsic damping and phonon coherence mechanisms are shown by the Lorentzian fraction which remains identical within fitting uncertainty. The only illumination-induced change emerges in the peak amplitudes which slightly decline (≈ 5–10%) under light exposure. Instead of any electronic or structural modification of the lattice, small illumination-dependent modifications to Raman cross-sections or marginal changes in local phonon populations usually lead to such minor and fully reversible intensity variation. Notably, no shift or mode broadening is noticed. This strongly contrasts with materials showing defect-related phonon perturbations or photo-induced lattice distortions. Overall, Cs₂NaScCl₆ possesses a remarkably rigid [ScCl₆]³⁻ octahedral framework as affirmed by the Raman results, which agrees with its exceptional thermal stability and ideal \(\:Fm\stackrel{-}{3}m\) symmetry. The high structural homogeneity of the material and the low phonon disorder are demonstrated by the combination of (i) narrow peak widths, (ii) well-defined vibrational modes and (iii) negligible dark–light variations. The optical observations presented above, specifically the highly stable STE emission, the low Urbach energy (0.123 eV) and the sharp absorption edge, are directly supported by such vibrational characteristics, thus offering a unified vibrational and structural basis for the exceptional photophysical purity of the undoped Cs₂NaScCl₆. Femtosecond transient absorption (TA) measurements were carried out so as to unravel the ultrafast relaxation pathways underlying the STE-dominated photophysics of Cs₂NaScCl₆. The pump–probe spectra were recorded at selected time delays (0.2, 1, 5, 20, and 50 ps) following UV excitation as displayed in Fig. 5 (a). Characterizing excited-state population within the Sc–Cl charge-transfer manifold, a broad photo-induced absorption band centered around ≈ 360 nm is shown by all spectra. As the delay increases, only the amplitude decreases and the spectral shape remains invariant with time, which reveals a monotonic depopulation of the excited manifold without any spectral shifting or broadening [ 30 ]. This behavior reflects that, with no intermediary defect-related states contributing to the spectral evolution, the carriers relax along a single well-defined relaxation pathway, which fully agrees with the structurally-ideal elpasolite lattice established and the low Urbach energy (0.123 eV). The corresponding TA kinetics extracted at 340 nm, 360 nm and 380 nm are shown in Figs. 5 (b)–(d) respectively. A bi-exponential model of the form reproduces the temporal evolution well in all cases [ 31 ]: $$\:{\Delta\:}A\left(t\right)={A}_{1}{e}^{\raisebox{1ex}{$-t$}\!\left/\:\!\raisebox{-1ex}{${\tau\:}_{1}$}\right.}+{A}_{2}{e}^{\raisebox{1ex}{$-t$}\!\left/\:\!\raisebox{-1ex}{${\tau\:}_{2}$}\right.}+C$$ 4 where A 1 ​ and A 2 ​ represent the amplitudes of the fast and slow relaxation channels, τ 1 ​ and τ 2 ​ stand for their characteristic lifetimes and C is a small offset. The fits yield a slower few-picosecond component (τ₂ ≈ 10–15 ps) associated with the stabilization and formation of STEs after a sub-picosecond component (τ₁ ≈ several 10⁻¹ ps) attributed to ultrafast carrier cooling within the highly ionic [ScCl₆]³⁻charge-transfer band across all probe wavelengths. The universality of the relaxation mechanism throughout the probed spectral region is confirmed by the excellent agreement between the experiment and the fit reflected in smooth residuals and consistent τ₁/τ₂ values at all wavelengths. As shown in Fig. 5 (e), this behavior is further emphasized by the global TA map. Without any spectral deformation or dispersive shift, a single, intense and positive ΔA band that decays uniformly in time is shown by the map. Thus, two important conclusions are reinforced: (i) without sampling shallow or mid-gap defect states, the system quickly funnels into a deep STE potential well and (ii) the early-time dynamics are governed by carrier–phonon interactions driving rapid cooling. Often indicating defect trapping, the absence of long-lived broad features or photoinduced bleaching which further attests to the excellent electronic purity of Cs₂NaScCl₆ is in line with Raman and PL results. The energy-level diagram of Fig. 5 (f) provides a summary of a unified picture of the excited-state relaxation mechanism. Electrons are promoted from the valence band into the Sc–Cl charge-transfer band upon UV excitation. Relaxing toward lower-energy configurations, hot carriers dissipate excess energy within a fraction of a picosecond through strong exciton–phonon coupling. Forming deeply bound STEs stabilized by local lattice relaxation around the [ScCl₆]³⁻ octahedral, these carriers become localized over the next few picoseconds, which is in agreement with the structural rigidity proven by Raman spectroscopy. Eventually, the mono-exponential TRPL decay and the intense blue emission noticed in steady-state PL are produced by these STEs which radiatively recombine on the microsecond timescale. Dominated by microsecond radiative recombination, picosecond STE formation, sub-picosecond carrier thermalization and direct CTB excitation, a clean defect-free ultrafast relaxation landscape is shown in the undoped Cs₂NaScCl₆ as displayed in Figs. 5 (a)–(f). Such coherent sequence of relaxation processes not only underscores the exceptional photophysical purity and stability of the Cs₂NaScCl₆ elpasolite framework but also agrees completely with all spectroscopic signatures presented in this paper. Recorded in the temperature range 300–375 K, the complex impedance response of Cs₂NaScCl₆ is summarized in Fig. 6 (a). With increasing frequency at all temperatures, a characteristic decrease is exhibited by the real part Z′(ω) noticed in Fig. 6 (a). The highly insulating nature of the undoped chloride double perovskite is affirmed by Z′ which reaches values as high as ≈ 4 × 10⁴ Ω at 300 K in the low-frequency region. Z′ decreases sharply and tends towards a nearly constant value at high ω when the frequency rises, which matches the intrinsic bulk response of the material. Together with a shift of the dispersion region (the onset of the Z′ drop) towards higher frequencies, a pronounced reduction in Z′ over the entire frequency range results from the temperature increase (from 300 to 375 K). Charge transport in Cs₂NaScCl₆ is thermally-activated since such behavior is representative of materials showing a negative temperature coefficient of resistance (NTCR) [ 32 – 35 ]. As displayed in Fig. 6 (b), the imaginary part –Z″(ω) gives complementary insight into the relaxation processes. Characterized by intensity and position evolving with temperature, a well-defined relaxation peak is shown by all spectra. A dominant dipolar or hopping-related relaxation within the bulk is reflected by a strong maximum which is observed at 300 K at intermediate frequencies [ 36 ]. The peak position shifts systematically to higher frequencies and the peak amplitude declines as the temperature increases, which is in agreement with a reduction in the characteristic relaxation time τ = 1/ω max . A thermally-assisted hopping mechanism within the rigid [ScCl₆]³⁻ octahedral sub-lattice is further supported by this shift which demonstrates that, at elevated temperatures, charge carriers or dipoles respond to the AC field more rapidly. The corresponding Nyquist diagrams (Z′ vs –Z″) are shown in Fig. 6 (c). A single depressed semicircular arc whose diameter decreases markedly as soon as temperature rises is yielded by each temperature. With no resolvable grain-boundary or electrode effects in this temperature range, a single bulk contribution governs the impedance response as indicated by the presence of a single semicircle. A distribution of relaxation times which is typical of disordered or polycrystalline insulating halides is revealed by the depression of the arcs below the ideal semicircle [ 37 ]. Using the equivalent circuit R₁ ‖ CPE₁ in Fig. S2 for the representative case of 325 K, the Nyquist plots were fitted so as to quantitatively analyze the impedance behavior. Table 4 provides a summary of the extracted parameters. With CPE-P values close to unity (0.93–1.02), the CPE parameters remain nearly constant across the temperature range while the fitted bulk resistance R₁ declines dramatically from 3.84 × 10⁴ Ω at 300 K to 2.40 × 10² Ω at 375 K, which agrees with the strong thermal activation seen in Z′(ω) and –Z″(ω). With only slight deviations coming from a mild distribution of microscopic relaxation environments, the former result reflects that the bulk response behaves almost as an ideal capacitor [ 38 ]. The absence of additional R–CPE pairs in the fits not only affirms that the conduction process is governed primarily by the intrinsic bulk properties of Cs₂NaScCl₆ but it also shows that grain-boundary effects are negligible. The inverse of the bulk resistance was plotted as ln(1/R₁) against 1/T in Fig. 6 (d) to further examine the thermal activation of charge transport. The data follow a linear Arrhenius dependence of the form as shown: $$\:\text{ln}\left(\sigma\:\right)=\text{ln}\left({\sigma\:}_{0}\right)-\frac{{E}_{a}}{{k}_{B}T}$$ 5 where σ = 1/R₁. An activation energy of: E a ​=0.625 eV is yielded by the slope of the fitted line. This relatively high activation energy is fully consistent not only with the strong carrier localization imposed by the rigid [ScCl₆]³⁻octahedral framework but also with the wide bandgap (4.63 eV). Moreover, it is in agreement with the photophysical response observed in previous sections: the absence of defect-mediated trapping pathways, the microsecond-scale PL lifetime and the formation of deeply bound STEs. All these observations collectively indicate that Cs₂NaScCl₆ behaves as a highly-resistive, electronically-pure and thermally-activated dielectric where conduction does not predominantly proceed via free-carrier transport but rather using activated hopping. Overall, the impedance spectroscopy results offer a coherent electrical counterpart to the structural, optical, and ultrafast spectroscopic analyses reported. Cs₂NaScCl₆ possesses an exceptionally stable and homogeneous lattice which reinforces its classification as a robust intrinsic halide insulator with negligible electronic disorder and strong exciton–phonon coupling owing to the combination of a nearly ideal capacitive behavior, high activation energy, Arrhenius-type conductivity and single bulk relaxation process. I–V measurements were conducted in the dark and under continuous 265 nm illumination in order not only to further assess the macroscopic electronic transport properties of Cs₂NaScCl₆ but also to complement the impedance spectroscopy analysis. The resulting I–V curves are shown in Fig. 7 (a). Once again, the highly insulating character of the undoped chloride host is confirmed by the current which remains extremely low in the dark across the entire ± 10 V bias range. This result is consistent with (i) the absence of free-carrier signatures in steady-state and transient absorption spectra, (ii) the large optical bandgap (4.63 eV) and (iii) the high activation energy extracted from impedance measurements (E a = 0.625 eV). The I–V curve maintains a linear (ohmic) behavior in both bias directions, whereas a measurable increase in current is observed upon UV excitation. The absence of any space-charge-limited current (SCLC) regime demonstrates that the photocurrent originates from a slight enhancement of thermally-activated hopping within the Sc–Cl charge-transfer manifold and that illumination does not lead to a sufficient carrier density so as to modify the conduction mechanism [ 39 ]. The photocurrent defined as ΔI = I light – I dark as a function of applied voltage is presented in Fig. 7 (b). With a slope matching a very low photoconductivity, ΔI increases linearly with V, which is in line not only with the microsecond-scale STE recombination seen in TRPL but also with the weak photo-induced absorption amplitude observed in ultrafast TA measurements. The modest magnitude of ΔI confirms that photoexcitation does not primarily lead to mobile free carriers but it rather brings about the formation of strongly-localized STEs. As proven earlier, these STEs negligibly contribute to long-range electronic conduction since they deeply relax within the local ScCl₆ octahedral distortion well. Therefore, the combined I–V and impedance spectroscopy findings reflect a unified picture: Cs₂NaScCl₆ behaves as a strongly ionic wide-bandgap dielectric with negligible intrinsic carrier mobility where both photo-induced and dark currents are not limited by electronic band conduction but rather by thermally-activated hopping. Conclusion and Outlook The undoped Cs₂NaScCl₆ is found to be an exceptionally pure and structurally rigid halide double perovskite in which the photophysical response is governed by exciton–phonon coupling. This material combines a highly ordered \(\:Fm\stackrel{-}{3}m\) lattice that suppresses defect-mediated pathways, a remarkably low Urbach energy and a wide 4.63 eV bandgap. A coherent picture emerges across the femtosecond TA, steady-state PL and TRPL: before the deep STE formation and long-lived microsecond emission, photoexcitation drives ultrafast carrier cooling with no evidence of free-carrier transport. The thermally-activated hopping conductivity of the bulk and its strongly-insulating nature are further affirmed by I–V measurements and impedance. The structural robustness and the intrinsic excitonic purity of Cs₂NaScCl₆ will potentially turn it into a promising host lattice and an ideal model system for quantum–optical applications, defect engineering and controlled rare-earth doping. The well-defined STE landscape shown in this work offers a solid foundation not only for advancing the fundamental understanding of exciton localization in wide-bandgap halide perovskites but also for designing next-generation lead-free luminescent materials. Declarations Competing interests The authors declare no competing interests. Funding: This research has been funded by Scientific Research Deanship at Northern Border University, Saudi Arabia through project number (NBU-CRP-2026-2461). Author Contribution Mohamed Bouzidi: Conceptualization; Methodology; Synthesis of single crystals; Structural and optical characterization; Formal analysis; Data curation; Visualization; Writing – original draft. Abdullah A. Alatawi: Photoluminescence measurements; Time-resolved spectroscopy (TRPL); Data analysis; Interpretation of optical results; Review & editing. Turki Alkathiri: Electrical measurements; Impedance spectroscopy; Data processing; Validation of electrical analysis. Sultan Albarakati: Experimental support; Assistance in characterization; Structural analysis. Norah Alwadai: Optical measurements; UV–visible spectroscopy; Data analysis; Visualization. Ahmed F. Almutairi: Electrical transport measurements; I–V characterization; Data analysis. Refka Ghodhbani: Scientific discussion; Methodological support; Validation of results. Mohamed Ben Bechir: Supervision; Scientific oversight; Conceptual guidance; Review & editing of the manuscript. Acknowledgement The authors extend their appreciation to Northern Border University, Saudi Arabia, for supporting this work through project number (NBU-CRP-2026-2461). Data Availability The datasets generated and/or analyzed during the current study are available in the Zenodo repository, https://doi.org/10.5281/zenodo.17853026 References Kumar, A. A. & Lee, N. Mater. Horiz. 12 7749–7778. (2025). Wang, S., Li, H., Qi, L. & Pan, K. J. Mater. Chem. C 13 19080–19105. (2025). Gao, W. et al. Laser Photonics Rev. 19 2500113. (2025). Bechir, M. B. & Alresheedi, F. RSC Adv. 14 1634–1648. (2024). Li, M. et al. ACS Sens. 10 2224–2233. (2025). Wei, Y. et al. Advanced Functional Materials (2025). Volonakis, G. et al. J. Phys. Chem. Lett. 8 772–778. (2017). Tripathi, M. N., Saha, A. & Singh, S. Mater. Res. Express 6 115517. (2019). De Paula, A. M. et al. J. Am. Chem. Soc. 147 28923–28931. (2025). Cong, M. et al. Sci. Bull. 65 1078–1084. (2020). Zhao, C., Gao, Y., Song, T., Wang, J. & Qiu, J. J. Phys. Chem. Lett. 14 9011–9018. (2023). Yang, G. et al. ACS Appl. Mater. Interfaces 15 24629–24637. (2023). Zhang, R. et al. Adv. Opt. Mater. 9 (2021). Liu, Y. et al. ACS Energy Lett. 10 2150–2159. (2025). Wang, X., Zhang, X., Yan, S., Liu, H. & Zhang, Y. Angew. Chem. Int. Ed. 61 (2022). Huang, W. et al. EcoMat 6 (2024). Jiang, X. et al. Inorg. Chem. 63 10756–10766. (2024). Zhao, C., Gao, Y., Song, T., Wang, J. & Qiu, J. J. Phys. Chem. Lett. 14 9011–9018. (2023). Bechir, M. B. & Rhaiem, A. B. Phys. E Low-Dimensional Syst. Nanostruct. 130 114686. (2021). Bechir, M. B. & Alresheedi, F. Phys. Chem. Chem. Phys. 26 1274–1283. (2023). Li, Q. et al. Chem. Eng. J. 521 166819. (2025). Li, S., Luo, J., Liu, J. & Tang, J. J. Phys. Chem. Lett. 10 1999–2007. (2019). Bechir, M. B., Dhaou, M. H. & Altrifi, S. M. Mater. Res. Bull. 167 112381. (2023). Jamshidi, M. & Gardner, J. M. Dalton Trans. 53 10544–10552. (2024). Chen, J., Lv, J., Liu, X., Lin, J. & Chen, X. Phys. Chem. Chem. Phys. 25 7574–7588. (2023). Taddei, M. et al. ACS Energy Lett. 9 2508–2516. (2024). Zissi, G. D. & Papatheodorou, G. N. Chem. Phys. Lett. 308 51–57. (1999). Hoffman, A. E. J. et al. APL Mater. 11 (2023). Karoui, K., Bechir, M. B., Bulou, A., Guidara, K. & Rhaiem, A. B. J. Mol. Struct. 1114 161–170. (2016). Bouzidi, M. et al. Opt. Mater. 168 117535. (2025). Aljaloud, A. S. et al. J. Electron. Mater. 54 11142–11154. (2025). Almalawi, D. R. et al. J. Mater. Sci. Mater. Electron. 36 (2025). Jebnouni, A. et al. J. Mater. Sci. Mater. Electron. 36 (2025). Almutairi, F. N., Naouari, R. & Oueslati, A. J. Mater. Sci. Mater. Electron. 35 (2024). Yadav, J. et al. Trans. Electr. Electron. Mater. 26 345–355. (2025). Bouzidi, M. et al. Appl. Organomet. Chem. 39 (2024). Sengupta, P. et al. J. Appl. Phys. 127 (2020). Maji, P., Chatterjee, S. & Das, S. Ceram. Int. 45 6012–6020. (2018). Gavranovic, S., Zmeskal, O., Weiter, M. & Pospisil, J. Commun. Phys. 8 (2025). Tables Tables 1 to 4 are available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Tables.pdf SupportingInformation.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 07 May, 2026 Reviews received at journal 26 Apr, 2026 Reviewers agreed at journal 20 Apr, 2026 Reviewers agreed at journal 15 Apr, 2026 Reviews received at journal 15 Apr, 2026 Reviewers agreed at journal 14 Apr, 2026 Reviewers invited by journal 14 Apr, 2026 Editor invited by journal 13 Apr, 2026 Editor assigned by journal 10 Apr, 2026 Submission checks completed at journal 10 Apr, 2026 First submitted to journal 09 Apr, 2026 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. 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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-9369678","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":626133646,"identity":"f8d42992-4919-46e4-82b3-3ba5382eb0c7","order_by":0,"name":"Mohamed Bouzidi","email":"","orcid":"","institution":"University of Ha'il","correspondingAuthor":false,"prefix":"","firstName":"Mohamed","middleName":"","lastName":"Bouzidi","suffix":""},{"id":626133647,"identity":"804d53d7-1e69-4877-9cf3-2022036fdb93","order_by":1,"name":"Abdullah A. 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(b) Crystal structure of the Cs₂NaScCl₆ double perovskite. (c) TGA thermal behavior.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/6596ca00f70ca7214e928a1e.png"},{"id":107705532,"identity":"f9c24e7b-83f8-4db2-ba78-5cc0ad428dab","added_by":"auto","created_at":"2026-04-24 09:13:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":562946,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV–Vis absorption spectrum and Tauc plot. (b) Urbach tail analysis (ln α vs hν).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/02cee2f0c9e5af6d717ba212.png"},{"id":107705502,"identity":"5448e4fc-fcd6-42d7-a86d-cb09cb6c6880","added_by":"auto","created_at":"2026-04-24 09:13:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":674275,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Room-temperature PL spectra (dark vs illuminated) with Gaussian fitting. (b) Time-resolved PL (TRPL) decay curve with mono-exponential fit.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/f5a924e56515b9699659ebb6.png"},{"id":107547445,"identity":"e61a95c7-3031-4dff-add6-b6cdcc08b776","added_by":"auto","created_at":"2026-04-22 13:35:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":582334,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Raman spectra recorded in dark and illuminated conditions. (b) Pseudo-Voigt Raman peak fitting profiles.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/f65050a56ca86a8807db286f.png"},{"id":107705896,"identity":"84d84071-20d6-4db0-a636-84000a90e496","added_by":"auto","created_at":"2026-04-24 09:15:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2414747,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Transient absorption spectra at selected delays. (b) TA kinetics at 340 nm. (c) TA kinetics at 360 nm. (d) TA kinetics at 380 nm. (e) 2D TA map ΔA(λ,t). (f) Schematic energy-level diagram of relaxation pathways.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/b7677b9ff3c6080ea17a7ce4.png"},{"id":107706415,"identity":"ceb7e865-ff6e-40b6-a0d5-689a2e61ea8b","added_by":"auto","created_at":"2026-04-24 09:18:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1325452,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Frequency dependence of the real part Z′(ω). (b) Frequency dependence of –Z″(ω). (c) Nyquist plots at different temperatures. (d) Arrhenius plot of bulk conductivity.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/8b42d6968e91e8e6e029b254.png"},{"id":107547447,"identity":"1273ab70-2496-491e-8e23-41d750302d01","added_by":"auto","created_at":"2026-04-22 13:35:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":450636,"visible":true,"origin":"","legend":"\u003cp\u003e(a) I–V characteristics in dark and under UV illumination. (b) Photocurrent ΔI as a function of voltage.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/5a2778d72596c79678787231.png"},{"id":107868709,"identity":"c149e4f7-0c64-45f1-8f72-36d2c7aee828","added_by":"auto","created_at":"2026-04-27 07:32:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8031602,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/e2c19334-7eea-4620-95bd-c624715d84b4.pdf"},{"id":107705301,"identity":"1c3bcac6-8358-44ff-bbee-9af2bd7403bc","added_by":"auto","created_at":"2026-04-24 09:11:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":280599,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/8b721c85f9c1fc0cec9b740a.pdf"},{"id":107706260,"identity":"cff56a85-3ea4-4dba-95a1-f85621f58482","added_by":"auto","created_at":"2026-04-24 09:17:46","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":690392,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9369678/v1/5b8c4b6a28e8377a2c04d636.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Charge Localization and Intrinsic Self-Trapped Exciton Photophysics in Undoped Cs₂NaScCl₆ Elpasolite Single Crystals","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLead-free halide double perovskites of the general formula A₂B(I)B(III)X₆ are considered to be a promising alternative to toxic Pb-based perovskites as they represent a compelling class of materials combining attributes such as environmental friendliness, wide bandgaps, thermal stability and chemical robustness [\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The rigid cubic \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e symmetry where the highly-ordered lattice produces exceptionally-clean electronic states and strongly suppresses defect formation distinguishes chloride elpasolites within this family [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. A unique photophysical behavior that is fundamentally different from that of traditional semiconductor perovskites is brought about by the structural ideality which favors STE formation through pronounced exciton\u0026ndash;phonon coupling [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWithout intentional doping, some halide double perovskites such as Cs₂NaScCl₆ exhibit intense broadband intrinsic STE photoluminescence unlike the majority of the members of this class which depend on dopant-induced emissions or narrow-bandgap transitions [\u003cspan additionalcitationids=\"CR12 CR13\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. To investigate exciton localization in highly ionic wide-gap lattices, this remarkable behavior has turned Cs₂NaScCl₆-type hosts into benchmark systems. Added to that, studies have lately demonstrated that STE emission can be tuned across the visible and near-infrared range by moderate perturbations of the octahedral environment, which underlines the sensitivity of their optical response to local symmetry and lattice rigidity [\u003cspan additionalcitationids=\"CR12 CR13\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA thorough understanding of the intrinsic properties of undoped Cs₂NaScCl₆ is still missing in spite of these advances. Key questions concerning the interplay of macroscopic charge transport, ultrafast carrier dynamics and lattice vibrations added to the fundamental exciton relaxation pathways and the native electronic purity are left open as previous studies largely focused on alloyed or doped compositions [\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Thus, a unified experimental framework simultaneously probing the electrical response, the vibrational properties, the optical absorption, the structural order and the steady-state and time-resolved emission is crucial for establishing Cs₂NaScCl₆ as a true model system for STE physics.\u003c/p\u003e \u003cp\u003eThis paper is the first comprehensive multi-dimensional investigation of pristine Cs₂NaScCl₆ single crystals integrating current\u0026ndash;voltage (I\u0026ndash;V) measurements, AC impedance spectroscopy, femtosecond transient absorption, Raman spectroscopy, steady-state and time-resolved photoluminescence, UV\u0026ndash;visible absorption and structural refinement. Such holistic approach shows that the material possesses purely thermally-activated hopping conduction, a single bulk dielectric relaxation, an ultrafast sub-picosecond carrier cooling, strongly-localized microsecond-lived STEs, an ultrawide bandgap added to a very low Urbach energy and an exceptional structural integrity. Undoped Cs₂NaScCl₆ is established through all these results as an archetypal wide-bandgap elpasolite with intrinsically clean electronic and excitonic landscapes, which offers a rigorous baseline for future strain-engineered, dopant-engineered or defect-engineered derivatives aimed at optoelectronic and optical applications.\u003c/p\u003e"},{"header":"Experimental Section","content":"\u003cp\u003e \u003cb\u003eMaterials.\u003c/b\u003e Cesium chloride (CsCl, 99.9%, Sigma-Aldrich), sodium chloride (NaCl, \u0026ge;\u0026thinsp;99.0%, Sigma-Aldrich), scandium(III) oxide (Sc₂O₃, 99.99% trace metals basis, Sigma-Aldrich), hydrochloric acid (HCl, 37%, EMSURE\u0026reg;, Merck), and 2-propanol (\u0026ge;\u0026thinsp;99.0%, Sigma-Aldrich) were used as received with no further purification.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis.\u003c/b\u003e By means of a hydrothermal crystallization route, the single crystals of Cs₂NaScCl₆ were grown in a chloride-rich acidic medium. The starting materials used were scandium oxide (Sc₂O₃), sodium chloride (NaCl) and Cesium chloride (CsCl). 0.25 mmol of Sc₂O₃, 0.4 mmol of NaCl and 1 mmol of CsCl were typically loaded into a 20 mL Teflon-lined stainless-steel autoclave. In order to favor the formation of the elpasolite chloride framework and to suppress the crystallization of separate NaCl phases, the overall Cs/Na/Sc molar ratio was adjusted to 2/0.8/1, which is slightly deficient in Na⁺with respect to the ideal stoichiometry.\u003c/p\u003e \u003cp\u003eTo provide a highly chlorinated and strongly acidic environment that promotes not only stabilization of [ScCl₆]\u0026sup3;⁻octahedra but also dissolution of Sc₂O₃ and to act as a mineralizing solvent, 0.8 mL of concentrated HCl was subsequently added to the liner. In order to allow the growth and nucleation of Cs₂NaScCl₆ single crystals, the autoclave was sealed, placed in an oven and heated to 180\u0026deg;C under autogenous pressure for 12 h.\u003c/p\u003e \u003cp\u003eThe furnace was switched off and the autoclave was left to cool slowly to room temperature after the heating step. Colorless transparent block-like crystals of Cs₂NaScCl₆ were found upon opening on the walls and at the bottom of the Teflon liner. The crystals were dried at 60\u0026deg;C in the air after being separated from the mother liquor by decantation and briefly rinsed with cold isopropanol several times to remove soluble by-products and residual acid. No mechanical grinding was applied. Without any aliovalent or rare-earth dopant, all electrical, optical and structural measurements were carried out on the as-grown undoped Cs₂NaScCl₆ single crystals.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCharacterizations.\u003c/b\u003e Using a Rigaku MiniFlex 600 diffractometer equipped with a Cu Kα source (λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;), the crystal structure of Cs₂NaScCl₆ was characterized by powder X-ray diffraction. At room temperature, diffraction patterns were recorded over the 2θ range 10\u0026ndash;50\u0026deg; with a step size of 0.05\u0026deg;. To confirm the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e cubic symmetry, the data were refined using the CELREF3 program. By thermogravimetric analysis, thermal stability was evaluated under nitrogen flow by means of a TGA-9000 system. The thermogravimetric analysis (TGA) profile was collected while almost 9.4 mg of the sample was heated at 5\u0026deg;C\u0026middot;min⁻\u0026sup1; from room temperature to 1000\u0026deg;C. Steady-state photoluminescence (PL) measurements were conducted using a Shimadzu RF-1501 spectrofluorometer, whereas the optical absorption spectra were measured at room temperature by means of a Shimadzu UV-2550 spectrophotometer. The emission was collected in the 380\u0026ndash;700 nm range while excitation was provided at 265 nm. PL spectra were recorded at the excitation wavelength under continuous UV illumination and in complete darkness. Using identical acquisition parameters, all measurements were carried out at room temperature. A FLOROCUBE time-correlated single-photon counting (TCSPC) system equipped with a NanoLED pulsed excitation source at 265 nm was employed to measure the time-resolved photoluminescence (TRPL). The instrument response function was recorded for deconvolution of the decay profiles, whereas the emitted signal was collected using a TBX-04-D detector. Raman spectroscopy was used to examine the vibrational properties of Cs₂NaScCl₆ single crystals by means of a HORIBA LabRAM HR Evolution confocal micro-Raman system equipped with a 532 nm diode-pumped solid-state (DPSS) laser as the excitation source. A spot size of almost 1\u0026ndash;2 \u0026micro;m was provided by the laser beam which was focused onto the sample through a 50\u0026times; long-working-distance objective. The backscattered radiation was detected using a thermoelectrically-cooled CCD camera and dispersed by an 1800 grooves\u0026middot;mm⁻\u0026sup1; grating. Acquisition parameters (number of scans, laser power below 1 mW at the sample and accumulation time) were optimized so as to avoid photo-induced degradation or local heating. The standard Si phonon at 520.7 cm⁻\u0026sup1; was used to carry out the calibration of the spectrometer. Under ambient conditions, all Raman spectra were collected at room temperature. Ultrafast transient absorption (TAS) measurements were conducted by means of an amplified Ti:sapphire laser system (repetition rate 1 kHz, pulse duration\u0026thinsp;~\u0026thinsp;100 fs, center wavelength 800 nm). An optical parametric amplifier (OPA) tuned in the ultraviolet region was employed to generate pump pulses centered at 360 nm while a broadband white-light continuum extending from 320 to 700 nm was used as the probe beam and produced in a sapphire plate. A time window was allowed from ~\u0026thinsp;200 fs to several nanoseconds by the temporal delays between probe and pump which were introduced using a motorized delay line. A dual-channel Si photodiode array was employed to detect and disperse the transmitted probe light in a spectrograph, which enables simultaneous acquisition of pumped and unpumped signals for improved spectral stability. To avoid nonlinear or multiphoton effects, all TAS measurements were performed under identical excitation fluences at room temperature (300 K). At selected probe wavelengths (340, 360, and 380 nm), kinetic traces were analyzed to describe the slow and fast relaxation channels by means of a bi-exponential decay model after being extracted from the TA dataset. To obtain a fine homogeneous powder suitable for electrical measurements, an agate mortar was used to gently crush a small quantity of Cs₂NaScCl₆ single crystals. A compact with a relative density of nearly 94% was yielded by the resulting powder which was pressed into a dense pellet (thickness\u0026thinsp;~\u0026thinsp;0.92 mm, diameter 8 mm) by means of a hydraulic press operating at 50 Torr. To ensure stable and low-resistance electrical contact with the copper electrodes, silver paint was applied to the two faces of the pellet. A TH2828A precision impedance analyzer was employed to conduct the complex impedance measurements over the 10⁻\u0026sup1;\u0026ndash;10⁶ Hz frequency range. At a controlled heating rate of 2 K min⁻\u0026sup1;, measurements were recorded between 300 and 375 K and the pellet\u0026ndash;electrode assembly was mounted in a temperature-controlled chamber (TP94, Linkam, UK). Throughout the experiments, an AC excitation voltage of 0.5 V was used so as to remain within the linear response regime. An Integrated Photovoltaic Test System Assembly operating in two configurations (under continuous 265 nm UV illumination and in complete darkness) was employed to record the I\u0026ndash;V characteristics of the Cs₂NaScCl₆ single crystals. Using two opposite silver electrodes, the crystals were contacted in order to form a planar two-terminal device. The applied voltage was swept at room temperature in the range\u0026thinsp;\u0026minus;\u0026thinsp;20 to +\u0026thinsp;20 V. Picoampere sensitivity was used to measure the resulting current. To avoid photo-heating effects and to ensure reproducibility, the spot size and the illumination intensity were kept constant during all measurements. After stabilizing the device at each applied bias, all I\u0026ndash;V curves were acquired under steady-state conditions.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eIn addition to the reference diffraction pattern extracted from the crystallographic entry CCDC 2054287 corresponding to the ideal elpasolite-type double perovskite structure (space group \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e), the room-temperature powder X-ray diffraction (XRD) pattern of the as-synthesized Cs₂NaScCl₆ sample is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The reference pattern agrees with the experimental reflections, demonstrating that the material crystallizes as a single phase-pure cubic perovskite without any detectable secondary phases. All diffraction peaks can be indexed to the (111), (022), (222), (004), (024) and (044) planes which characterizes the rock-salt-ordered arrangement of the [NaCl₆]⁵⁻ and [ScCl₆]\u0026sup3;⁻ octahedra [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs displayed in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, the full-pattern refinement by means of the CELREF3 software was used to perform a more rigorous structural analysis [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Corresponding to a cell volume of 1153.37 \u0026Aring;\u0026sup3;, a cubic unit cell with a\u0026thinsp;=\u0026thinsp;10.4871\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0207 \u0026Aring; is indicated in Table\u0026nbsp;1 by the refined lattice parameters obtained from the CELREF3 output file. An excellent match between the experimental and calculated peak positions is confirmed by the refinement residuals which are remarkably low with a root-mean-square deviation of 0.0267\u0026deg;. The high-symmetry \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e crystallographic phase fully agrees with the expected structure of the Cs₂NaScCl₆ elpasolite family as it is unambiguously demonstrated by the orthogonality of the angles (α\u0026thinsp;=\u0026thinsp;β\u0026thinsp;=\u0026thinsp;γ\u0026thinsp;=\u0026thinsp;90\u0026deg;) and the equality of the three lattice parameters (a\u0026thinsp;=\u0026thinsp;b = c) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe long-range ordering of the alternating [NaCl₆]⁵⁻ and [ScCl₆]\u0026sup3;⁻ octahedra connected via corner-sharing is highlighted by the three-dimensional structural model generated using VESTA [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). The electrostatic stabilization to the framework is provided by the Cs⁺ ions reside in the cuboctahedral cavities. The intrinsic structural rigidity of this halide double perovskite is revealed by the absence of noticeable octahedral tilting or distortion. The foundation for the STE\u0026ndash;dominated photophysics discussed in the coming sections is laid by the exciton localization behavior which is strongly influenced by the nearly ideal octahedral environment.\u003c/p\u003e \u003cp\u003eUnder a heating rate of 5\u0026deg;C min⁻\u0026sup1;, the TGA curve of Cs₂NaScCl₆ recorded from 30 to 1200\u0026deg;C is displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c). With the mass remaining constant at ~\u0026thinsp;100 wt %, the sample shows an exceptionally stable profile up to \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;670\u0026deg;C, which confirms not only the high thermal robustness of the double-perovskite lattice but also the absence of physisorbed species. Marking the onset of thermal decomposition, a 5 wt % loss is reached around \u0026asymp;\u0026thinsp;760\u0026deg;C while a slight deviation from the baseline appears only above such a temperature. After that, with the maximum degradation rate centered at \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;865\u0026deg;C, a single sharp mass-loss event takes place between \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;770 and 900\u0026deg;C, which ultimately leaves a residual mass of \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;5.9 wt % at 1200\u0026deg;C. A similar abrupt degradation near ~\u0026thinsp;740\u0026deg;C is revealed by such behavior which corresponds to the decomposition temperature reported in the literature for Cs₂NaScCl₆ single crystals [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The structural rigidity and robustness of the [NaCl₆]⁵⁻ and [ScCl₆]\u0026sup3;⁻ corner-sharing octahedral framework are emphasized by the notably high thermal stability observed which exceeds that of many other lead-free double perovskites. A solid foundation is offered by this stability for investigating the intrinsic optoelectronic and excitonic properties of undoped Cs₂NaScCl₆, especially those dominated by STEs and exciton\u0026ndash;phonon interactions.\u003c/p\u003e \u003cp\u003eThe UV\u0026ndash;visible absorption spectrum of Cs₂NaScCl₆ recorded between 250 and 500 nm at room temperature is displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). Halide double perovskites with highly symmetric octahedral frameworks are characterized by a clean and well-defined band-to-band transition, which is shown through a steep absorption edge observed at approximately 268\u0026ndash;270 nm. The localized excitonic or STE\u0026ndash;related absorption arising from the [ScCl₆]\u0026sup3;⁻ and [NaCl₆]⁵⁻ octahedra is typically associated with several weak sub-band features in the 300\u0026ndash;420 nm range which are also exhibited by the spectrum. These features are frequently noticed in Cs₂NaScCl₆-type hosts where strong exciton\u0026ndash;phonon coupling induces shallow localized states slightly below the conduction band, thus broadening the absorption tail [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConstructed under the assumption of a direct allowed electronic transition, the corresponding Tauc plot [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] is plain to see in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). A direct bandgap of 4.63 eV which agrees with the large bandgap expected for highly ionic chloride-based perovskites is yielded by a linear extrapolation of (αhν)\u0026sup2; versus photon energy. Such a value aligns well with the electronic structure of Cs₂NaScCl₆ where the conduction band minimum is governed by Sc\u0026ndash;Cl antibonding states, which brings about an inherently-low absorption in the visible region and a wide electronic gap [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The excellent structural quality of the synthesized material is confirmed as a low density of deep defect states and minimal electronic disorder are further revealed by the sharpness of the absorption edge.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), the Urbach analysis was carried out by plotting ln(α) against photon energy so as to gain additional insight into the tailing region of the absorption edge. The Urbach energy (E\u003csub\u003eu\u003c/sub\u003e) of 0.123 eV which not only means a modest degree of thermal and structural lattice disorder but is also relatively small for halide perovskites is yielded by the resulting linear fit. A low density of band-tail states is revealed by the electronic transitions near the band edge which are only weakly perturbed by phonons and localized defects as suggested by the E\u003csub\u003eu\u003c/sub\u003e value. This is in agreement not only with the strong lattice rigidity previously established through the structural characterization but also with the nearly ideal octahedral symmetry. The combined results from the Urbach and Tauc analyses confirm that its optical response is not primarily governed by defect-mediated absorption pathways but rather by shallow STE-related states and band-to-band transitions and also highlight the intrinsic electronic purity of undoped Cs₂NaScCl₆.\u003c/p\u003e \u003cp\u003eThe steady-state PL spectra of Cs₂NaScCl₆ recorded at room temperature not only under continuous UV illumination at 265 nm but also in the dark are compared in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a). The characteristic signature of radiative recombination from STEs within the highly ionic [ScCl₆]\u0026sup3;⁻ octahedral sub-lattice proves to be a broad featureless blue emission band centered at \u0026asymp;\u0026thinsp;453 nm exhibited by both spectra [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The remarkable rigidity of the Sc\u0026ndash;Cl octahedral network and the strong localization of the excitonic wavefunction preventing thermally-driven or spectral-wandering shifts even under prolonged illumination are reflected by the similarity in peak position under both excitation conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUsing a Gaussian function as follows, both PL bands were fitted to quantitatively analyze the emission profiles [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:I\\left(\\lambda\\:\\right)=A{e}^{\\left[-\\raisebox{1ex}{${\\left(\\lambda\\:-{\\lambda\\:}_{0}\\right)}^{2}$}\\!\\left/\\:\\!\\raisebox{-1ex}{${2\\sigma\\:}^{2}$}\\right.\\right]}+C$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e stands for the peak amplitude, \u003cem\u003eλ₀\u003c/em\u003e is the peak wavelength, \u003cem\u003eσ\u003c/em\u003e refers to the standard deviation related to the bandwidth and \u003cem\u003eC\u003c/em\u003e represents the baseline offset. Table\u0026nbsp;2 provides a summary of the fitted parameters demonstrating that the full width remains essentially unchanged at half maximum (FWHM\u0026thinsp;\u0026asymp;\u0026thinsp;74 nm) added to the emission maxima which remain strictly invariant for both spectra at λ\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;453.1 nm. However, under illumination, there is a slight decline in the PL amplitude. This agrees with a weak photo-induced quenching of shallow traps or a small reversible redistribution of STE populations. The high photostability of the STE state in Cs₂NaScCl₆ is further affirmed by the absence of any significant broadening or spectral shifts upon illumination, which is consistent not only with its structurally-rigid elpasolite framework but also with its low Urbach energy (0.123 eV).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) displays the exciton dynamics which were investigated further using time-resolved PL (TRPL). The following single-exponential function characterizing a dominant radiative pathway involving a single emissive STE state describes the decay profile well [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:I\\left(t\\right)={y}_{0}+{A}_{1}\\:\\text{e}\\text{x}\\text{p}(-\\frac{t}{{\\tau\\:}_{1}})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere y\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0 refers to the baseline, A\u003csub\u003e1\u003c/sub\u003e​ represents the initial amplitude and τ\u003csub\u003e1\u003c/sub\u003e​ is the exciton lifetime. With a lifetime of τ\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1 .08727\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00064 \u0026micro;s and parameters A\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;15005.08\u0026thinsp;\u0026plusmn;\u0026thinsp;6.27, an excellent goodness of fit (χ\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;2158.81, R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.99981) is yielded by the fitting results.\u003c/p\u003e \u003cp\u003eThe long-lived radiative relaxation which originates from a deeply localized self-trapped state stabilized by strong exciton\u0026ndash;phonon coupling within the Sc\u0026ndash;Cl octahedra is affirmed by the microsecond-scale τ₁ value, which completely agrees with STE-mediated emission previously reported for Cs₂NaScCl₆ single crystals [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The absence of competing defect-related or trap-assisted recombination channels is further demonstrated by the perfect mono-exponential behavior and high R\u0026sup2;, which is consistent not only with the narrow Urbach tail but also with the structurally derived low-disorder environment.\u003c/p\u003e \u003cp\u003eThe TRPL, the Gaussian fitting analysis and the steady-state PL results together offer a coherent picture of the strongly-localized and intrinsically-stable STE emission in undoped Cs₂NaScCl₆. Added to the absence of secondary decay channels and the microsecond exciton lifetime, the invariance of the emission energy under continuous illumination emphasizes not only the potential of this chloride elpasolite as a robust intrinsic blue-emitting material among lead-free perovskites but also its remarkable photophysical purity.\u003c/p\u003e \u003cp\u003eThe Raman spectra of Cs₂NaScCl₆ recorded at room temperature under continuous illumination and in the dark are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). Centered at \u0026asymp;\u0026thinsp;51, 147, 209 and 292 cm⁻\u0026sup1;, the four well-resolved vibrational bands exhibited by the two spectra are fully consistent with the vibrational fingerprints expected for the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e elpasolite structure [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Arising from the lattice motions involving Cs⁺ ions and the internal vibrations of the discrete [ScCl₆]\u0026sup3;⁻ octahedra, four Raman-active modes (A₁g\u0026thinsp;+\u0026thinsp;Eg\u0026thinsp;+\u0026thinsp;2F₂g) are predicted for Cs₂NaScCl₆ according to factor-group analysis. Such assignments completely agree with the bibliographic reference Raman study of Zissi and Papatheodorou who reported identical vibrational features for the solid Cs₂NaScCl₆ [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA lattice F₂g mode involving collective translational motions of Cs⁺against the halide framework brought about the lowest frequency band at \u0026asymp;\u0026thinsp;51 cm⁻\u0026sup1;. The bands at \u0026asymp;\u0026thinsp;209 and \u0026asymp;\u0026thinsp;292 cm⁻\u0026sup1; are assigned to the E\u003csub\u003eg\u003c/sub\u003e bending mode and the higher-energy F₂g asymmetric stretching mode respectively. However, the intense feature at \u0026asymp;\u0026thinsp;147 cm⁻\u0026sup1; matches the symmetric breathing A₁g mode of the [ScCl₆]\u0026sup3;⁻octahedra [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The absence of any local structural distortion or symmetry lowering within the elpasolite lattice and the near-ideal octahedral environment around Sc\u0026sup3;⁺are affirmed by the presence of all expected modes with well-defined positions and sharp linewidths. This is in full agreement with the structural refinement results discussed above.\u003c/p\u003e \u003cp\u003eUsing a pseudo-Voigt (Gauss\u0026ndash;Lorentz) function of the form, all Raman peaks were fitted so as to further quantify the vibrational response as follows [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:I\\left(\\nu\\:\\right)=\\alpha\\:\\left[gL\\left(\\nu\\:\\right)+\\left(1-g\\right)G\\left(\\nu\\:\\right)\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere a stands for the peak amplitude, g specifies the Lorentzian contribution and L(ν) denotes the Lorentzian component while G(ν) refers to the Gaussian component. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) displays the full fitting profiles generated by LabSpec, whereas Table\u0026nbsp;3 provides a summary of the fitted parameters for both dark and illuminated spectra [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. With variations below 0.001 cm⁻\u0026sup1;, all peak positions remain strictly invariant under illumination as shown by the fitting results. Continuous illumination does not induce any detectable structural modification in the Cs₂NaScCl₆ lattice as confirmed by such invariance of vibrational frequencies. Under illumination, the absence of increased anharmonicity or phonon softening is demonstrated by the full-width-at-half-maximum (FWHM) values which also remain nearly unchanged for all modes. The unaffected intrinsic damping and phonon coherence mechanisms are shown by the Lorentzian fraction which remains identical within fitting uncertainty. The only illumination-induced change emerges in the peak amplitudes which slightly decline (\u0026asymp;\u0026thinsp;5\u0026ndash;10%) under light exposure. Instead of any electronic or structural modification of the lattice, small illumination-dependent modifications to Raman cross-sections or marginal changes in local phonon populations usually lead to such minor and fully reversible intensity variation. Notably, no shift or mode broadening is noticed. This strongly contrasts with materials showing defect-related phonon perturbations or photo-induced lattice distortions.\u003c/p\u003e \u003cp\u003eOverall, Cs₂NaScCl₆ possesses a remarkably rigid [ScCl₆]\u0026sup3;⁻ octahedral framework as affirmed by the Raman results, which agrees with its exceptional thermal stability and ideal \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e symmetry. The high structural homogeneity of the material and the low phonon disorder are demonstrated by the combination of (i) narrow peak widths, (ii) well-defined vibrational modes and (iii) negligible dark\u0026ndash;light variations. The optical observations presented above, specifically the highly stable STE emission, the low Urbach energy (0.123 eV) and the sharp absorption edge, are directly supported by such vibrational characteristics, thus offering a unified vibrational and structural basis for the exceptional photophysical purity of the undoped Cs₂NaScCl₆.\u003c/p\u003e \u003cp\u003eFemtosecond transient absorption (TA) measurements were carried out so as to unravel the ultrafast relaxation pathways underlying the STE-dominated photophysics of Cs₂NaScCl₆. The pump\u0026ndash;probe spectra were recorded at selected time delays (0.2, 1, 5, 20, and 50 ps) following UV excitation as displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). Characterizing excited-state population within the Sc\u0026ndash;Cl charge-transfer manifold, a broad photo-induced absorption band centered around \u0026asymp;\u0026thinsp;360 nm is shown by all spectra. As the delay increases, only the amplitude decreases and the spectral shape remains invariant with time, which reveals a monotonic depopulation of the excited manifold without any spectral shifting or broadening [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. This behavior reflects that, with no intermediary defect-related states contributing to the spectral evolution, the carriers relax along a single well-defined relaxation pathway, which fully agrees with the structurally-ideal elpasolite lattice established and the low Urbach energy (0.123 eV).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe corresponding TA kinetics extracted at 340 nm, 360 nm and 380 nm are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b)\u0026ndash;(d) respectively. A bi-exponential model of the form reproduces the temporal evolution well in all cases [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:{\\Delta\\:}A\\left(t\\right)={A}_{1}{e}^{\\raisebox{1ex}{$-t$}\\!\\left/\\:\\!\\raisebox{-1ex}{${\\tau\\:}_{1}$}\\right.}+{A}_{2}{e}^{\\raisebox{1ex}{$-t$}\\!\\left/\\:\\!\\raisebox{-1ex}{${\\tau\\:}_{2}$}\\right.}+C$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere A\u003csub\u003e1\u003c/sub\u003e​ and A\u003csub\u003e2\u003c/sub\u003e​ represent the amplitudes of the fast and slow relaxation channels, τ\u003csub\u003e1\u003c/sub\u003e​ and τ\u003csub\u003e2\u003c/sub\u003e​ stand for their characteristic lifetimes and C is a small offset. The fits yield a slower few-picosecond component (τ₂ \u0026asymp; 10\u0026ndash;15 ps) associated with the stabilization and formation of STEs after a sub-picosecond component (τ₁ \u0026asymp; several 10⁻\u0026sup1; ps) attributed to ultrafast carrier cooling within the highly ionic [ScCl₆]\u0026sup3;⁻charge-transfer band across all probe wavelengths. The universality of the relaxation mechanism throughout the probed spectral region is confirmed by the excellent agreement between the experiment and the fit reflected in smooth residuals and consistent τ₁/τ₂ values at all wavelengths.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(e), this behavior is further emphasized by the global TA map. Without any spectral deformation or dispersive shift, a single, intense and positive ΔA band that decays uniformly in time is shown by the map. Thus, two important conclusions are reinforced: (i) without sampling shallow or mid-gap defect states, the system quickly funnels into a deep STE potential well and (ii) the early-time dynamics are governed by carrier\u0026ndash;phonon interactions driving rapid cooling. Often indicating defect trapping, the absence of long-lived broad features or photoinduced bleaching which further attests to the excellent electronic purity of Cs₂NaScCl₆ is in line with Raman and PL results.\u003c/p\u003e \u003cp\u003eThe energy-level diagram of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(f) provides a summary of a unified picture of the excited-state relaxation mechanism. Electrons are promoted from the valence band into the Sc\u0026ndash;Cl charge-transfer band upon UV excitation. Relaxing toward lower-energy configurations, hot carriers dissipate excess energy within a fraction of a picosecond through strong exciton\u0026ndash;phonon coupling. Forming deeply bound STEs stabilized by local lattice relaxation around the [ScCl₆]\u0026sup3;⁻ octahedral, these carriers become localized over the next few picoseconds, which is in agreement with the structural rigidity proven by Raman spectroscopy. Eventually, the mono-exponential TRPL decay and the intense blue emission noticed in steady-state PL are produced by these STEs which radiatively recombine on the microsecond timescale.\u003c/p\u003e \u003cp\u003eDominated by microsecond radiative recombination, picosecond STE formation, sub-picosecond carrier thermalization and direct CTB excitation, a clean defect-free ultrafast relaxation landscape is shown in the undoped Cs₂NaScCl₆ as displayed in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)\u0026ndash;(f). Such coherent sequence of relaxation processes not only underscores the exceptional photophysical purity and stability of the Cs₂NaScCl₆ elpasolite framework but also agrees completely with all spectroscopic signatures presented in this paper.\u003c/p\u003e \u003cp\u003eRecorded in the temperature range 300\u0026ndash;375 K, the complex impedance response of Cs₂NaScCl₆ is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a). With increasing frequency at all temperatures, a characteristic decrease is exhibited by the real part Z\u0026prime;(ω) noticed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a). The highly insulating nature of the undoped chloride double perovskite is affirmed by Z\u0026prime; which reaches values as high as \u0026asymp;\u0026thinsp;4 \u0026times; 10⁴ Ω at 300 K in the low-frequency region. Z\u0026prime; decreases sharply and tends towards a nearly constant value at high ω when the frequency rises, which matches the intrinsic bulk response of the material. Together with a shift of the dispersion region (the onset of the Z\u0026prime; drop) towards higher frequencies, a pronounced reduction in Z\u0026prime; over the entire frequency range results from the temperature increase (from 300 to 375 K). Charge transport in Cs₂NaScCl₆ is thermally-activated since such behavior is representative of materials showing a negative temperature coefficient of resistance (NTCR) [\u003cspan additionalcitationids=\"CR33 CR34\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), the imaginary part \u0026ndash;Z\u0026Prime;(ω) gives complementary insight into the relaxation processes. Characterized by intensity and position evolving with temperature, a well-defined relaxation peak is shown by all spectra. A dominant dipolar or hopping-related relaxation within the bulk is reflected by a strong maximum which is observed at 300 K at intermediate frequencies [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The peak position shifts systematically to higher frequencies and the peak amplitude declines as the temperature increases, which is in agreement with a reduction in the characteristic relaxation time τ\u0026thinsp;=\u0026thinsp;1/ω\u003csub\u003emax\u003c/sub\u003e. A thermally-assisted hopping mechanism within the rigid [ScCl₆]\u0026sup3;⁻ octahedral sub-lattice is further supported by this shift which demonstrates that, at elevated temperatures, charge carriers or dipoles respond to the AC field more rapidly.\u003c/p\u003e \u003cp\u003eThe corresponding Nyquist diagrams (Z\u0026prime; vs \u0026ndash;Z\u0026Prime;) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c). A single depressed semicircular arc whose diameter decreases markedly as soon as temperature rises is yielded by each temperature. With no resolvable grain-boundary or electrode effects in this temperature range, a single bulk contribution governs the impedance response as indicated by the presence of a single semicircle. A distribution of relaxation times which is typical of disordered or polycrystalline insulating halides is revealed by the depression of the arcs below the ideal semicircle [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Using the equivalent circuit R₁ ‖ CPE₁ in Fig. S2 for the representative case of 325 K, the Nyquist plots were fitted so as to quantitatively analyze the impedance behavior. Table\u0026nbsp;4 provides a summary of the extracted parameters.\u003c/p\u003e \u003cp\u003eWith CPE-P values close to unity (0.93\u0026ndash;1.02), the CPE parameters remain nearly constant across the temperature range while the fitted bulk resistance R₁ declines dramatically from 3.84 \u0026times; 10⁴ Ω at 300 K to 2.40 \u0026times; 10\u0026sup2; Ω at 375 K, which agrees with the strong thermal activation seen in Z\u0026prime;(ω) and \u0026ndash;Z\u0026Prime;(ω). With only slight deviations coming from a mild distribution of microscopic relaxation environments, the former result reflects that the bulk response behaves almost as an ideal capacitor [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The absence of additional R\u0026ndash;CPE pairs in the fits not only affirms that the conduction process is governed primarily by the intrinsic bulk properties of Cs₂NaScCl₆ but it also shows that grain-boundary effects are negligible.\u003c/p\u003e \u003cp\u003eThe inverse of the bulk resistance was plotted as ln(1/R₁) against 1/T in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d) to further examine the thermal activation of charge transport. The data follow a linear Arrhenius dependence of the form as shown:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\text{ln}\\left(\\sigma\\:\\right)=\\text{ln}\\left({\\sigma\\:}_{0}\\right)-\\frac{{E}_{a}}{{k}_{B}T}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere σ\u0026thinsp;=\u0026thinsp;1/R₁. An activation energy of: E\u003csub\u003ea\u003c/sub\u003e​=0.625 eV is yielded by the slope of the fitted line.\u003c/p\u003e \u003cp\u003eThis relatively high activation energy is fully consistent not only with the strong carrier localization imposed by the rigid [ScCl₆]\u0026sup3;⁻octahedral framework but also with the wide bandgap (4.63 eV). Moreover, it is in agreement with the photophysical response observed in previous sections: the absence of defect-mediated trapping pathways, the microsecond-scale PL lifetime and the formation of deeply bound STEs. All these observations collectively indicate that Cs₂NaScCl₆ behaves as a highly-resistive, electronically-pure and thermally-activated dielectric where conduction does not predominantly proceed via free-carrier transport but rather using activated hopping.\u003c/p\u003e \u003cp\u003eOverall, the impedance spectroscopy results offer a coherent electrical counterpart to the structural, optical, and ultrafast spectroscopic analyses reported. Cs₂NaScCl₆ possesses an exceptionally stable and homogeneous lattice which reinforces its classification as a robust intrinsic halide insulator with negligible electronic disorder and strong exciton\u0026ndash;phonon coupling owing to the combination of a nearly ideal capacitive behavior, high activation energy, Arrhenius-type conductivity and single bulk relaxation process.\u003c/p\u003e \u003cp\u003eI\u0026ndash;V measurements were conducted in the dark and under continuous 265 nm illumination in order not only to further assess the macroscopic electronic transport properties of Cs₂NaScCl₆ but also to complement the impedance spectroscopy analysis. The resulting I\u0026ndash;V curves are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a). Once again, the highly insulating character of the undoped chloride host is confirmed by the current which remains extremely low in the dark across the entire\u0026thinsp;\u0026plusmn;\u0026thinsp;10 V bias range. This result is consistent with (i) the absence of free-carrier signatures in steady-state and transient absorption spectra, (ii) the large optical bandgap (4.63 eV) and (iii) the high activation energy extracted from impedance measurements (E\u003csub\u003ea\u003c/sub\u003e = 0.625 eV).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe I\u0026ndash;V curve maintains a linear (ohmic) behavior in both bias directions, whereas a measurable increase in current is observed upon UV excitation. The absence of any space-charge-limited current (SCLC) regime demonstrates that the photocurrent originates from a slight enhancement of thermally-activated hopping within the Sc\u0026ndash;Cl charge-transfer manifold and that illumination does not lead to a sufficient carrier density so as to modify the conduction mechanism [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe photocurrent defined as ΔI\u0026thinsp;=\u0026thinsp;I\u003csub\u003elight\u003c/sub\u003e \u0026ndash; I\u003csub\u003edark\u003c/sub\u003e as a function of applied voltage is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b). With a slope matching a very low photoconductivity, ΔI increases linearly with V, which is in line not only with the microsecond-scale STE recombination seen in TRPL but also with the weak photo-induced absorption amplitude observed in ultrafast TA measurements. The modest magnitude of ΔI confirms that photoexcitation does not primarily lead to mobile free carriers but it rather brings about the formation of strongly-localized STEs. As proven earlier, these STEs negligibly contribute to long-range electronic conduction since they deeply relax within the local ScCl₆ octahedral distortion well.\u003c/p\u003e \u003cp\u003eTherefore, the combined I\u0026ndash;V and impedance spectroscopy findings reflect a unified picture: Cs₂NaScCl₆ behaves as a strongly ionic wide-bandgap dielectric with negligible intrinsic carrier mobility where both photo-induced and dark currents are not limited by electronic band conduction but rather by thermally-activated hopping.\u003c/p\u003e"},{"header":"Conclusion and Outlook","content":"\u003cp\u003eThe undoped Cs₂NaScCl₆ is found to be an exceptionally pure and structurally rigid halide double perovskite in which the photophysical response is governed by exciton\u0026ndash;phonon coupling. This material combines a highly ordered \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e lattice that suppresses defect-mediated pathways, a remarkably low Urbach energy and a wide 4.63 eV bandgap. A coherent picture emerges across the femtosecond TA, steady-state PL and TRPL: before the deep STE formation and long-lived microsecond emission, photoexcitation drives ultrafast carrier cooling with no evidence of free-carrier transport. The thermally-activated hopping conductivity of the bulk and its strongly-insulating nature are further affirmed by I\u0026ndash;V measurements and impedance. The structural robustness and the intrinsic excitonic purity of Cs₂NaScCl₆ will potentially turn it into a promising host lattice and an ideal model system for quantum\u0026ndash;optical applications, defect engineering and controlled rare-earth doping. The well-defined STE landscape shown in this work offers a solid foundation not only for advancing the fundamental understanding of exciton localization in wide-bandgap halide perovskites but also for designing next-generation lead-free luminescent materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThis research has been funded by Scientific Research Deanship at Northern Border University, Saudi Arabia through project number (NBU-CRP-2026-2461).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eMohamed Bouzidi: Conceptualization; Methodology; Synthesis of single crystals; Structural and optical characterization; Formal analysis; Data curation; Visualization; Writing \u0026ndash; original draft. Abdullah A. Alatawi: Photoluminescence measurements; Time-resolved spectroscopy (TRPL); Data analysis; Interpretation of optical results; Review \u0026amp; editing. Turki Alkathiri: Electrical measurements; Impedance spectroscopy; Data processing; Validation of electrical analysis. Sultan Albarakati: Experimental support; Assistance in characterization; Structural analysis. Norah Alwadai: Optical measurements; UV\u0026ndash;visible spectroscopy; Data analysis; Visualization. Ahmed F. Almutairi: Electrical transport measurements; I\u0026ndash;V characterization; Data analysis. Refka Ghodhbani: Scientific discussion; Methodological support; Validation of results. Mohamed Ben Bechir: Supervision; Scientific oversight; Conceptual guidance; Review \u0026amp; editing of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors extend their appreciation to Northern Border University, Saudi Arabia, for supporting this work through project number (NBU-CRP-2026-2461).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets generated and/or analyzed during the current study are available in the Zenodo repository, https://doi.org/10.5281/zenodo.17853026\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKumar, A. A. \u0026amp; Lee, N. \u003cem\u003eMater. Horiz.\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e 7749\u0026ndash;7778. (2025).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, S., Li, H., Qi, L. \u0026amp; Pan, K. \u003cem\u003eJ. Mater. Chem. C\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e 19080\u0026ndash;19105. 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Phys.\u003c/em\u003e \u003cb\u003e8\u003c/b\u003e (2025).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 4 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Halide Double Perovskites, Self-Trapped Excitons (STE), Ultrafast Transient Absorption, Impedance Spectroscopy, Cs₂NaScCl₆ Single Crystals","lastPublishedDoi":"10.21203/rs.3.rs-9369678/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9369678/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eEven though the intrinsic photophysics of all-inorganic lead-free halide double perovskites remains poorly understood in the absence of dopants, they have emerged as robust platforms for exciton physics. This paper provides a comprehensive investigation of undoped Cs₂NaScCl₆ combining electrical, structural and steady-state and time-resolved spectroscopy characterization. With a rigid [ScCl₆]\u0026sup3;⁻octahedral framework which is thermally stable up to ~\u0026thinsp;760\u0026ndash;870\u0026deg;C, a phase-pure elpasolite structure (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:Fm\\stackrel{-}{3}m\\)\u003c/span\u003e\u003c/span\u003e) is confirmed by both Raman spectroscopy and X-ray diffraction. Pointing to low electronic disorder, a small Urbach energy of 0.123 eV and a direct bandgap of 4.63 eV are revealed by UV\u0026ndash;visible spectroscopy. With a mono-exponential lifetime of 1.087 \u0026micro;s, a broad blue photoluminescence band is indicated at ~\u0026thinsp;453 nm by Cs₂NaScCl₆, which is fully assigned to self-trapped exciton (STE) emission. With no evidence of defect-mediated relaxation, sub-picosecond carrier cooling followed by 10\u0026ndash;15 ps STE formation is shown through Femtosecond transient absorption measurements. Only weak linear photoconductivity and extremely low currents are revealed by I\u0026ndash;V characteristics (265 nm illumination vs dark), which is in agreement with not only negligible free-carrier transport but also strongly-localized excitations. However, activation energy of 0.625 eV and single bulk relaxation with Arrhenius-type conductivity are yielded using impedance spectroscopy. Therefore, Cs₂NaScCl₆ is collectively established by such results as a model wide-bandgap elpasolite where deeply localized STEs are driven within a structurally and electronically pristine lattice through exciton\u0026ndash;phonon coupling, which suggests not only a well-defined platform for fundamental studies of STE-mediated emission in lead-free halide perovskites but also a robust host for rare-earth doping.\u003c/p\u003e","manuscriptTitle":"Charge Localization and Intrinsic Self-Trapped Exciton Photophysics in Undoped Cs₂NaScCl₆ Elpasolite Single Crystals","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-22 13:35:00","doi":"10.21203/rs.3.rs-9369678/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-05-07T11:32:19+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-26T17:19:55+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"202109565871218493951265509612024047231","date":"2026-04-20T08:50:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"116150453485131289962186465268502776634","date":"2026-04-15T12:33:54+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-15T11:17:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"43823437330977996319688075922412167445","date":"2026-04-15T01:28:17+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-14T23:38:51+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-04-13T13:20:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-10T10:13:34+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-10T10:12:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-04-09T13:58:45+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"47f5fe69-ec8c-47a7-b6d3-97e3318afa17","owner":[],"postedDate":"April 22nd, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-05-07T11:32:19+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":66635401,"name":"Physical sciences/Materials science"},{"id":66635402,"name":"Physical sciences/Optics and photonics"},{"id":66635403,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-05-19T01:54:05+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-22 13:35:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9369678","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9369678","identity":"rs-9369678","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}