Photoresponsive Charge–Lattice Interplay in Layered Lead-Free Cs₃Sb₂Cl₉ Perovskite Crystals

Abstract

The search for stable, nontoxic alternatives to lead-based halide perovskites has directed significant attention toward the Cs₃Sb₂Cl₉ compound, a promising lead-free material with a layered crystal structure. In this study, we comprehensively investigate the structural, thermal, optical, vibrational, and electrical properties of high-quality Cs₃Sb₂Cl₉ single crystals. X-ray diffraction (XRD) analysis confirms the formation of a well-crystallized trigonal phase (P-3m1), with lattice parameters consistent with prior reports. Thermogravimetric analysis (TGA) reveals thermal stability up to 321 °C. Optical absorption measurements reveal an indirect bandgap of 2.76 eV and an Urbach energy of 0.37 eV, indicating moderate static disorder and exciton–phonon coupling within the layered 2D framework. Photoluminescence (PL) spectra under dark and illuminated conditions exhibit pronounced intensity enhancement and redshift upon photoexcitation, supported by Voigt profile analysis revealing reduced line widths under illumination. Raman spectroscopy reveals characteristic vibrational modes associated with Sb–Cl bonding, with illumination inducing spectral sharpening and enhanced mode intensities, indicative of dynamic lattice stabilization. Electrochemical impedance spectroscopy (EIS) highlights improved charge transport and reduced recombination losses under illumination, supported by decreased activation energies and suppressed ionic diffusion. These findings reveal a strong interplay between light exposure and charge-lattice dynamics, demonstrating Cs₃Sb₂Cl₉’s potential for optoelectronic applications where environmental stability and photophysical responsiveness are critical.

1. Introduction

The pursuit of stable, environmentally benign, and cost-effective materials for optoelectronic and energy applications has led to an intensified focus on lead-free halide perovskites. Among them, cesium antimony halide perovskites, with the general formula Cs₃Sb₂X₉ (X = Cl, Br, I), have garnered significant attention as potential alternatives to the archetypal lead-based perovskites, due to their low toxicity, rich structural polymorphism, and promising optical characteristics. − In contrast to the conventional three-dimensional (3D) connectivity observed in lead halide perovskites, Cs₃Sb₂X₉ compounds crystallize in low-dimensional architecturesnamely trigonal (P-3m1) and orthorhombic (Pnma) structureswhich profoundly influence their electronic structure and carrier dynamics. ,

In particular, Cs₃Sb₂Cl₉ has emerged as a model system for exploring structure–property relationships in halide perovskites devoid of heavy metals. In contrast to the iodide analogue Cs₃Sb₂I₉, which adopts a 0D structure composed of isolated [Sb₂I₉]^3– dimers, Cs₃Sb₂Cl₉ crystallizes in a layered two-dimensional (2D) trigonal (P-3m1) framework. In this structure, corner-sharing [SbCl₆] octahedra form extended two-dimensional layers separated by cesium cations, leading to pronounced structural anisotropy and quantum-confinement effects typical of low-dimensional halide perovskites. Such structural motifs inherently induce strong excitonic effects, quantum confinement, and self-trapped exciton behavior, which can be exploited for applications ranging from photodetectors and light-emitting devices to photocatalysis. − Recent studies have demonstrated that precursor chemistry, reaction temperature, and ligand environment can be finely tuned to modulate the crystal phase, particle morphology, and dimensionality of Cs₃Sb₂Cl₉ nanocrystals. , Furthermore, bismuth substitution at the Sb site has been shown to induce a phase transition and bandgap tuning, offering an additional lever to engineer the material’s optoelectronic response.

Optical investigations have shown that Cs₃Sb₂Cl₉ is an indirect-band gap semiconductor, with E _g values typically ranging from 2.7 to 2.9 eV, depending on its crystallinity and polymorph. ,− The small energy difference between the indirect and direct transitions often leads to apparent direct-gap features in experimental spectra, accounting for the variability reported in the literature. Although the wide bandgap may restrict its applicability as a primary photovoltaic absorber, it renders Cs₃Sb₂Cl₉ highly suitable for UV photodetectors, luminescent sensors, and photocatalytic comaterials where large optical gaps are advantageous. Its photophysical responseencompassing band-edge absorption, Urbach tailing, and photoenhanced luminescencereflects the strong coupling between static disorder, excitonic effects, and lattice vibrations within the layered 2D framework. ,−

Despite this growing body of research, comprehensive correlative studies that bridge the structural features, optical properties, and photoelectronic response of Cs₃Sb₂Cl₉ single crystals remain scarce. In this context, the present work aims to provide an integrated investigation of the crystallographic, optical, photoluminescent, Raman, and impedance properties of Cs₃Sb₂Cl₉ under both dark and illumination conditions. Through a combination of XRD, UV–vis absorption, PL and Raman spectral fitting, and complex impedance analysis, we elucidate the underlying mechanisms of photoinduced modulation, carrier relaxation, and ion migration, thus contributing a deeper understanding of this lead-free halide perovskite for next-generation optoelectronic and photonic technologies.

2. Experimental Details

2.1. Materials

Cesium chloride (CsCl, 99.9%), Antimony­(III) chloride (SbCl₃, ≥99.0%), and Hydrochloric acid (HCl, 37%) were obtained from Sigma-Aldrich and used as received without further purification.

2.2. Synthesis

Cs₃Sb₂Cl₉ single crystals were synthesized through a controlled slow-cooling method aimed at obtaining high-quality, well-defined crystals. Initially, three equivalents of CsCl were completely dissolved in 40 mL of concentrated HCl at 120 °C under constant magnetic stirring until a clear solution was achieved. Subsequently, two equivalents of SbCl₃ were added while maintaining the same temperature and stirring conditions, yielding a homogeneous and transparent solution. The resulting mixture was then slowly cooled from 120 °C to room temperature at a rate of 0.3 °C h^–1 without any seed introduction. This gradual temperature decrease facilitated spontaneous nucleation and the formation of uniform crystals. Once at room temperature, the solution was kept undisturbed for several days under ambient conditions to allow crystal maturation.

2.3. Characterizations

The structural features of the Cs₃Sb₂Cl₉ single crystals were examined by powder X-ray diffraction (PXRD) using a Philips PW 1710 diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) at ambient temperature. Data were collected over a 2θ range of 10°–50° with a step size of 0.05°, confirming the high crystallinity and phase purity of the material.

Thermal behavior was analyzed by differential scanning calorimetry (DSC) using a TA Instruments Q100 apparatus under a steady nitrogen atmosphere. The temperature was ramped from 0 to 800 °C at a constant heating rate of 5 °C min^–1.

Optical absorption measurements were performed on polished single crystals of Cs₃Sb₂Cl₉ using a Hach–Lange DR 500 UV–Visible spectrophotometer operating in transmission mode. The crystals were mounted vertically in a quartz sample holder with a fixed optical path length of 1 mm. Spectra were recorded in the 350–700 nm range at room temperature. The optical absorption coefficient (α) was calculated from the measured transmittance using the relation α = (1/d) ln­(1/T), where d is the sample thickness and T is the transmittance. This approach allowed accurate assessment of the optical band gap and Urbach energy while preserving the intrinsic optical quality of the single crystals.

Photoluminescence (PL) measurements were performed using a Shimadzu RF-1501 spectrofluorometer with an excitation wavelength of 350 nm. For the “dark” condition, the spectrum was collected in a fully closed and unilluminated chamber. For the “illuminated” condition, the sample was first exposed to a calibrated white LED source (400–700 nm, ∼100 mW cm^–2) for 15 s with the excitation shutter closed. Immediately after this preillumination, the LED was switched off, the excitation at 350 nm was opened, and the PL spectrum was recorded under identical instrumental settings. Thus, the recorded PL signal corresponds to the photoconditioned state of the crystal, without any contribution from stray white-light photons. Control measurements with the excitation beam blocked showed no detectable signal in the 400–600 nm range, confirming the absence of parasitic LED scattering during PL acquisition.

Raman analysis was performed on single Cs₃Sb₂Cl₉ crystals using a HORIBA LABRAM HR Evolution spectrometer with a 633 nm laser and a grating of 1800 grooves mm^–1. All spectra were obtained at room temperature with optimized laser power to prevent sample heating. Spectral profiles were fitted with Lorentzian functions, and comparative measurements were made under dark and illuminated conditions to assess any photo-induced lattice modifications.

Electrochemical impedance spectroscopy (EIS) was employed to investigate the ionic and electronic transport properties of Cs₃Sb₂Cl₉. Dense, disc-shaped pellets (8.00 mm diameter, 0.91 mm thickness) were prepared by uniaxial pressing. Both surfaces were polished and coated with silver paste to ensure good electrical contact, and platinum wires were attached to form a two-electrode configuration. Measurements were carried out using a Zahner electrochemical workstation over a frequency range of 0.1 Hz–1 MHz with an AC amplitude of 0.5 V. Data were collected under both dark and illuminated conditions using the same calibrated white LED source (∼100 mW cm^–2) positioned 10 cm from the sample to guarantee uniform illumination. The results were analyzed using an equivalent circuit model incorporating Warburg elements to extract diffusion-related parameters, and all measurements were repeated on multiple samples to verify reproducibility.

3. Results and Discussion

3.1. Structural and Thermal Characterization

Figure a displays the experimental PXRD pattern of the synthesized Cs₃Sb₂Cl₉ crystals, measured at room temperature, along with the reference pattern simulated from the crystallographic data of Cs₃Sb₂Cl₉ (Materials Project ID: mp-27629, DOI: 10.17188/1201827). The diffraction data were refined using CELREF3 software, with all reflectionsincluding those of weak intensitysuccessfully indexed to the trigonal P-3m1 (no. 164) phase. The refined lattice parameters are a = 7.7909 Å, b = 7.7835 Å, and c = 9.5398 Å (Table ), which are in excellent agreement with previously reported values for Cs₃Sb₂Cl₉ synthesized under similar conditions. The low root-mean-square deviation (RMS = 0.018 in 2θ) confirms the reliability of the refinement and the absence of impurity phases. The previously unindexed low-intensity peaks correspond to higher-index reflections of the same structure rather than to secondary phases. For completeness, Figure S1 in the Supporting Information shows the full refinement output generated by CELREF3, demonstrating the excellent match between the observed and calculated patterns.

1.

(a) Experimental and simulated PXRD patterns of Cs₃Sb₂Cl₉. (b) Crystal structure of Cs₃Sb₂Cl₉ (P-3m1) viewed down the a-axis, showing corner-sharing [SbCl₆] octahedra forming 2D layers separated by Cs⁺ cations. (c) Thermogravimetric analysis curve of Cs₃Sb₂Cl₉.

1. Refined Lattice Parameters and Structural Data of Cs₃Sb₂Cl₉ Obtained from PXRD Analysis (P-3m1 Phase).

Parameter	Value	Standard Error	Unit
a	7.7909	±0.0104	Å
b	7.7835	±0.0101	Å
c	9.5398	±0.0090	Å
α	89.97	±0.15	°
β	89.97	±0.16	°
γ	119.95	±0.10	°
V	501.2	±1.2	Å3
RMS (Σ(2θ O–C)2/N)	0.018
Space group	P-3m1

Figure b illustrates the crystallographic structure of Cs₃Sb₂Cl₉, visualized using the VESTA software based on the refined crystallographic data. The compound adopts a layered two-dimensional (2D) trigonal structure (space group P-3m1 ), characteristic of A₃B₂X₉ perovskite derivatives. In this configuration, the [SbCl₆] octahedra are corner-sharing, forming extended two-dimensional layers parallel to the ab-plane and separated by cesium cations. The cesium sublattice ensures charge neutrality and defines the interlayer spacing, which governs the degree of structural anisotropy. This corner-connected 2D network gives rise to reduced dimensionality and enhanced exciton–phonon coupling, features that are typical of layered antimony halide perovskites. Such low-dimensional connectivity has been widely reported to induce strong quantum-confinement effects and promote self-trapped exciton formation, thereby influencing the compound’s optical and dielectric responses. ,−

Moreover, the structural configuration of Cs₃Sb₂Cl₉ is highly sensitive to external parameters such as temperature, pressure, or cation substitution. Previous studies have demonstrated a possible phase transition from trigonal to orthorhombic symmetry upon doping or thermal modulation, accompanied by changes in lattice distortion and octahedral tilting.

However, under the current synthesis conditions, the pure trigonal phase is preserved, ensuring reliable comparison with prior structural and functional investigations.

Figure c displays the TGA profile of Cs₃Sb₂Cl₉. The thermal decomposition behavior reveals that the material remains thermally stable up to approximately 321 °C, beyond which a sharp weight loss is observed, indicating structural degradation. This decomposition threshold is consistent with the thermal behavior reported in related antimony-based halide perovskites. , The absence of any significant mass loss below this temperature affirms the robust thermal stability of Cs₃Sb₂Cl₉, a crucial attribute for applications in optoelectronic or photothermal devices where elevated operational temperatures may be encountered.

3.2. Optical Properties and Band Structure Analysis

Figure a displays the room-temperature UV–visible absorption spectrum of the synthesized Cs₃Sb₂Cl₉ single crystals, recorded over the 300–600 nm range. The absorption edge exhibits a pronounced onset in the near-ultraviolet region, reflecting strong optical transitions associated with the intrinsic electronic structure. To determine the optical band gap energy (E _g), the absorption data were analyzed using the Tauc formalism for both direct- and indirect-allowed transitions according to the relation:

$${(\alpha \mathrm{h}\nu )}^n=\beta (\mathrm{h}\nu -E_{\mathrm{g}})$$ 1

where α is the absorption coefficient, hν the photon energy, β a material-dependent constant, and n = 2 or 1/2 for direct and indirect transitions, respectively. The plots of (αhν)² and (αhν)^1/2 versus hν are presented in Figure b,c. The direct fit yields E _g (direct) ≈ 2.96 eV, whereas the indirect fit provides E _g (indirect) ≈ 2.76 eV with a broader and more robust linear region. The latter behavior indicates that Cs₃Sb₂Cl₉ is an indirect-band gap semiconductor, consistent with prior reports for the trigonal P-3m1 phase. The small (∼0.2 eV) difference between direct and indirect thresholds may account for the apparent direct-gap features occasionally reported in the literature. This wide optical band gap places Cs₃Sb₂Cl₉ in the UV-to-visible range, highlighting its potential for optoelectronic and photocatalytic applications.

2.

(a) Room-temperature UV–visible absorption spectrum of Cs₃Sb₂Cl₉ single crystals. (b) Tauc plot for a direct-allowed transition, yielding E _g (direct) = 2.96 eV. (c) Tauc plot for an indirect-allowed transition, yielding E _g (indirect) = 2.76 eV. (d) Urbach plot of ln­(α) versus photon energy, giving an Urbach energy of 0.37 eV.

To further probe the optical quality and degree of energetic disorder within the material, the Urbach energy (E _u) was extracted from the exponential absorption tail. This tail arises due to localized electronic states within the bandgap, typically induced by structural imperfections, phonon coupling, or static disorder. In disordered semiconductors, the absorption coefficient α near the band edge follows the Urbach rule:

$$\alpha ={\alpha }_0\exp \left(\frac{\mathrm{h}\nu }{E_{\mathrm{u}}}\right)$$ 2

which can be linearized by taking the natural logarithm:

$$\ln (\alpha )=\ln ({\alpha }_0)+\frac{1}{E_u}\mathrm{h}\nu$$ 3

Figure d presents the plot of ln­(α) versus photon energy (hν), and the slope of the linear region yields an Urbach energy of 0.37 eV. This value indicates moderate static disorder and exciton–phonon coupling inherent to the layered two-dimensional (2D) octahedral framework of Cs₃Sb₂Cl₉ (P-3m1). Such behavior is typical of low-dimensional halide perovskites, where quantum confinement and lattice fluctuations contribute to the exponential tail of the absorption edge rather than to a high density of structural defects. ,− This interpretation is consistent with the corrected crystallographic model based on corner-sharing [SbCl₆] octahedra forming extended two-dimensional layers separated by cesium cations.

3.3. Behavior and Spectral Deconvolution under Light and Dark Conditions

Figure illustrates the room-temperature photoluminescence (PL) spectra of Cs₃Sb₂Cl₉ single crystals measured under both dark and illuminated conditions. In the dark, the PL spectrum exhibits a well-defined emission band centered at ≈ 440 nm, corresponding to near-band-edge recombination of photoexcited excitons. Upon continuous white-light illumination, the PL intensity increases markedly and the emission maximum undergoes a red shift to ≈ 446 nm, accompanied by a slight spectral narrowing (fwhm decreasing from ≈65 nm to ≈61 nm). This red shift toward lower transition energies indicates a photoinduced modification of the local potential landscape, typically associated with exciton–phonon coupling and lattice relaxation effects rather than shallow-trap filling, which would normally induce a blue shift.

3.

Photoluminescence spectra under dark and illuminated conditions with Voigt fits.

To quantitatively analyze these emission characteristics, both spectra were fitted using the Voigt functiona convolution of Gaussian and Lorentzian line shapeswhich provides a physically rigorous model accounting for both inhomogeneous and homogeneous broadening mechanisms simultaneously. The Voigt profile V(λ) is expressed as ,

$$\mathrm{V}(\lambda )=\mathrm{A}\times \mathrm{Re}[\mathit{w}((\lambda -\mu +\mathrm{i}\gamma )/(\sigma \surd 2))]/(\sigma \surd (2\pi ))$$ 4

where A is the amplitude, μ the center wavelength, σ the standard deviation of the Gaussian component, γ the Lorentzian half-width at half-maximum (HWHM), and w(z) the Faddeeva function (complex error function). The Gaussian broadening reflects dynamic lattice disorder and exciton–phonon interactions, whereas the Lorentzian component originates from intrinsic radiative recombination and localized defect states.

Table summarizes the fitted parameters for both conditions. The shift in μ under illumination (≈ 6 nm toward higher wavelengths) evidence a photoinduced lattice relaxation and the possible formation of self-trapped excitons (STEs) or recombination through deeper localized states, consistent with the strong coupling between electronic and vibrational degrees of freedom in low-dimensional antimony halide perovskites. The full width at half-maximum (fwhm), estimated from the approximate expression (5):

$$\mathrm{FWHM}\approx 0.5346\times 2\gamma +\surd (0.2166\times {(2\gamma )}^2+{(2.3548\sigma )}^2)$$ 5

2. Fitted Voigt Parameters of PL Spectra under Dark and Illumination.

Condition	Amplitude (a.u.)	Center Wavelength (nm)	Sigma (nm)	Gamma (nm)	fwhm (nm)
Dark	99.05442	439.9984	0.000295	32.55993	65.12007
Illumination	132.06870	445.8258	0.006909	30.62340	61.24700

As observed from the fitted parameters, a moderate reduction in the fwhm is observed under illumination, indicating reduced inhomogeneous broadening and partial trap passivation, possibly resulting from local heating and enhanced carrier screening.

The notable increase in PL intensity under illumination corroborates the activation of additional radiative channels and supports the photoresponsive nature of Cs₃Sb₂Cl₉. These findings, together with the impedance and optical analyses, reveal a strong correlation between light-induced excitonic dynamics and charge transport enhancement, reinforcing the potential of Cs₃Sb₂Cl₉ as a lead-free, low-dimensional perovskite-like material for light-harvesting, photodetector, and optoelectronic applications. The detailed spectral deconvolution presented here provides deep insight into the recombination dynamics and supports the structure–property relationships discussed in earlier sections.

3.4. Raman Spectroscopic Investigation under Dark and Illumination Conditions

Figure a displays the Raman spectra of Cs₃Sb₂Cl₉ single crystals measured under both dark and illuminated conditions at room temperature. The observed vibrational modes provide direct insight into the lattice dynamics, local bonding environments, and structural distortions within the compound. The experimental Raman peaks under dark conditions are observed at approximately 66.6, 90.7, 126.6, 254.4, and 307.1 cm^–1, in excellent agreement with literature values for layered Cs₃Sb₂Cl₉. ,,

4.

(a) Raman spectra of Cs₃Sb₂Cl₉ under dark and illuminated conditions. (b) Deconvoluted Raman fit under dark conditions. (c) Deconvoluted Raman fit under illumination.

According to group theory analysis and previous experimental studies, these modes are attributed to distinct vibrational origins within the [SbCl₆] octahedral framework. The bands at 66.6 and 90.7 cm^–1 correspond to external lattice modes (T and L modes), mainly involving translational and librational motions of Cs⁺ cations coupled with octahedral rotations. The 126.6 cm^–1 band is assigned to the ν₂(Sb–Cl) bending mode, while the 254.4 cm^–1 feature corresponds to the ν₃ antisymmetric Sb–Cl stretching vibration of the [SbCl₆] octahedra within the layered 2D network. Finally, the 307.1 cm^–1 mode is attributed to δ­(Sb–Cl–Sb) bridging deformation within the layered 2D framework. ,,,−

These detailed assignments ensure full correlation between the Raman-active modes and the structural model (P-3m1), which features corner-connected octahedra rather than isolated dimers. Notably, the low-frequency region (<100 cm^–1) corresponds to external lattice vibrations, while higher-frequency modes (>120 cm^–1) arise from internal deformations of the [SbCl₆] octahedra.

To quantitatively evaluate the spectral modifications induced by illumination, each Raman spectrum was deconvoluted using the Gauss–Lorentz model implemented in LabSpec software.

This model represents a numerical implementation of the Voigt-type mixed Gaussian–Lorentzian profile, which accounts for both inhomogeneous (Gaussian) and homogeneous (Lorentzian) broadening contributions. Therefore, it is physically equivalent to the Voigt function used in the PL analysis, ensuring methodological consistency across all spectroscopic fittings in this study. Figure b,c presents the fitted Raman profiles for the dark and illuminated conditions, respectively, while Table lists the peak parameters (position, intensity, fwhm, and mode assignment). The deconvolution confirms that the main Raman-active modes remain spectrally positioned in similar regions under both conditions, yet subtle shifts in peak position, line width, and intensity provide compelling evidence of photoinduced structural dynamics.

3. Raman Deconvolution Parameters in Dark and Illuminated Conditions.

Condition	Peak position (cm–1)	Intensity/Area (a.u.)	fwhm (cm–1)	Gaussian fraction, g (−)	Formula	Mode assigment
Dark	90.740	0.737937	34.7110	0.966529	GaussLoren()	External lattice (T/L)
Dark	254.4	0.944674	23.7553	0.172641	GaussLoren()	ν3(Sb–Cl) antisymmetric stretch
Dark	307.082	0.735165	24.5749	0.106122	GaussLoren()	δ(Sb–Cl–Sb) bridging deformation
Dark	66.641	0.13247	13.7174	1	GaussLoren()	External lattice (T/L)
Dark	126.567	0.194446	26.5841	0.002232	GaussLoren()	ν2(Sb–Cl) bending
Illumination	92.747	0.885018	34.6363	0.984712	GaussLoren()	External lattice (T/L)
Illumination	256.402	1.133870	23.7597	0.171205	GaussLoren()	ν3(Sb–Cl) antisymmetric stretch
Illumination	309.083	0.881931	24.5811	0.11051	GaussLoren()	δ(Sb–Cl–Sb) bridging deformation
Illumination	68.586	0.161363	13.8565	1	GaussLoren()	External lattice (T/L)
Illumination	128.508	0.233563	26.5222	0.03711	GaussLoren()	ν2(Sb–Cl) bending

Under illumination, the experimental Raman bands are observed at approximately 68.6, 92.7, 128.5, 256.4, and 309.1 cm^–1. An enhancement in the intensity of the main Raman modes is evident, particularly those centered near 128.5 cm^–1 (ν₂ bending) and 256.4 cm^–1 (ν₃ antisymmetric stretching). This intensity increase indicates enhanced polarizability due to the excitation of charge carriers and a redistribution of the local electronic density, which modifies the Raman scattering cross-section. The high-frequency mode at 309.1 cm^–1, corresponding to the δ­(Sb–Cl–Sb) bridging deformation, also shows a noticeable intensity increase and a slight redshift under illumination, consistent with light-induced modulation of Sb–Cl bonding strength and minor lattice softening. Such redshifts and narrowing of the fwhm suggest local lattice relaxation and anharmonic interactions stimulated by photoexcitation. These photoinduced spectral variations reflect the strong coupling between photogenerated carriers and lattice vibrations, typical of low-dimensional halide perovskites where transient structural relaxation occurs under illumination. The enhanced Raman activity and spectral sharpening under light exposure are in harmony with the PL and impedance observations, which revealed improved carrier transport, reduced recombination losses, and trap passivation under illumination. Collectively, these findings highlight a light-responsive modulation of both electronic and vibrational properties in Cs₃Sb₂Cl₉, underscoring the importance of lattice–carrier coupling and phonon-mediated processes in defining the optoelectronic behavior of lead-free halide perovskites. This detailed spectroscopic insight deepens the structure–property correlation established in the preceding sections and further supports the viability of Cs₃Sb₂Cl₉ as a robust and tunable platform for next-generation photonic and energy applications.

3.5. Impedance Spectroscopy and Photo-Induced Charge Transport Dynamics

EIS was employed to investigate the charge transport dynamics of Cs₃Sb₂Cl₉ single crystals under both dark and illuminated conditions, over a temperature range spanning 303 to 403 K. Figure a,b depicts the corresponding Nyquist plots, which illustrate the complex impedance response in terms of the real (Z′) and imaginary (Z″) components. Under dark conditions, semicircular arcs were observed, with radii that shrink as the temperature increases, indicative of thermally activated conduction processes. This behavior is characteristic of semiconductors with mixed ionic–electronic transport mechanisms. An equivalent circuit model, shown in the insets of Figure a,b, was used to fit the experimental data using ZView software. The circuit consists of a series resistance (R _s), a transport/recombination resistance (R _tr/rec), charge transfer resistance (R _CT), chemical capacitance (C _μ), double-layer capacitance (C _DL), and a Warburg element (W _s).

5.

Nyquist plots of Cs₃Sb₂Cl₉ under (a) dark and (b) illumination conditions.

The Warburg element accounts for ion diffusion and is represented by the complex impedance expression:

$$\mathit{W}=A_{\mathrm{W}}\frac{\tanh {(j\omega T_{\mathrm{W}})}^P}{{(j\omega T_{\mathrm{W}})}^P}$$ 6

where A _W is the Warburg coefficient, T _W is the Warburg time constant, ω is the angular frequency, and P ≈ 0.5 for semi-infinite linear diffusion.

Under illumination (Figure b), a substantial reduction in arc radii is evident at all temperatures, reflecting enhanced photoconductivity due to the generation of additional free carriers. Both R _tr/rec and R _CT decrease noticeably, confirming improved carrier transport and suppressed recombination under light. While the Warburg element remains present under illumination, its influence diminishes, suggesting a decrease in ionic mobility, possibly due to photopassivation of mobile vacancies or defect states. − The consistent decrease of impedance with increasing temperature under both conditions confirms the thermally activated nature of charge transport in Cs₃Sb₂Cl₉. −

To probe ion diffusion quantitatively, the real (|Z′|) and imaginary (|Z″|) components of impedance were plotted versus ω^–1/2 [Figures S2–S3]. A linear regime is observed in |Z′| at low and intermediate frequencies, consistent with Warburg-type diffusion. The slope of the |Z′|−ω^–1/2 plot yields the Warburg coefficient A _W, which decreases with temperature under both conditions, reflecting enhanced ion mobility. Furthermore, A _W values are lower under illumination, indicating photoinduced suppression of ionic diffusion. At very low frequencies, the rise in |Z″| is attributed to interfacial charge accumulation at grain boundaries. The linear extrapolation of |Z″| confirms diffusion-limited behavior, typical in mixed-conduction materials. −

The Warburg impedance W accounts for semi-infinite diffusion behavior, characterized by A _W and T _W, both of which are compiled in Table (under dark) and Table (under light). Crucially, the Warburg time constant is related to the ionic diffusion activation energy E _a through the thermally activated expression: ,

$$\frac{1}{T_{\mathrm{W}}}=\frac{k_{\mathrm{B}}{a}^2\alpha }{h{L}_D^2}T{e}^{-E_{\mathrm{a}}/k_{\mathrm{B}}T}$$ 7

where a is the lattice parameter, k _B is Boltzmann’s constant, α is a geometrical factor, L _D is the effective diffusion length, and h is Planck’s constant. Arrhenius plots of $\ln (\mathrm{T}\times {\tau }_{\mathrm{w}}^{-1})$ vs 1000/T [Figure a,b] reveal two linear regimes, with distinct activation energies: E _a1 = 0.22 eV at low temperatures and E _a2 = 0.51 eV at high temperatures in darkness. − Under illumination, E _a decreases to 0.11 and 0.36 eV, respectively, highlighting enhanced transport and reduced trap-mediated barriers. −

4. Warburg Parameters and Ionic Transport Activation Energies of Cs₃Sb₂Cl₉ under Dark Conditions.

T (K)	R s (Ω)	R tr/rec (106 Ω)	R CT (106 Ω)	A W (106 Ω)	T W (10–3 s)	P	C dl (10–12 F)	C μ (10–13 F)
303	11478	13.6	2.72	1.19	1.1	0.1	2.60	4.34
328	11478	9.31	2.30	0.8	1.8	0.1	3.55	6.10
353	11478	6.35	1.87	0.5	3.3	0.1	4.73	8.73
378	11478	3.94	1.54	0.3	7	0.1	6.31	12.6
403	11478	2.16	1.28	0.25	25	0.1	8.25	18.3

5. Warburg Parameters and Ionic Transport Activation Energies of Cs₃Sb₂Cl₉ under Illumination.

T (K)	R S (Ω)	R tr/rec (106 Ω)	R CT (105 Ω)	AW (105 Ω)	TW (10–4s)	P	C dl (10–13 F)	C μ (10–11 F)
303	6061	7.73	9.40	7.3	2.4	0.1	4.15	1.02
328	6061	5.47	9.10	5.6	2.2	0.1	5.82	1.37
353	6061	3.65	8.70	4.8	3	0.1	8.51	1.99
378	6061	2.22	8.30	2.9	4	0.1	13	2.47
403	6061	1.39	7.90	1.6	11	0.1	20	2.96

6.

Arrhenius-like plot of $\ln \left(\frac{1}{\mathit{T}\times {\mathit{T}}_{\mathrm{W}}}\right)$ vs T ^–1 for Cs₃Sb₂Cl₉ (a) under dark and (b) under illumination.

Dielectric loss spectra (ε″) as a function of frequency and temperature [Figure a,b] further corroborate this behavior. In darkness, ε″ increases with temperature at low frequencies, suggesting thermally activated interfacial polarization. , Log–log plots reveal two characteristic regimes: a dispersive low-frequency slope of ≈−0.97 due to interfacial effects (Maxwell–Wagner), and a high-frequency plateau (slope ∼ 0) consistent with intrinsic dielectric loss. ,, Under illumination, ε″ increases across the spectrum without slope change, reflecting photoinduced carrier accumulation while maintaining similar dielectric relaxation mechanisms. − These results confirm a light-driven modulation of both ionic and dielectric properties in Cs₃Sb₂Cl₉, consistent with prior PL and Raman observations.

7.

Dielectric loss $({\epsilon }^{″})$ spectra (a) under dark and (b) under illumination conditions at different temperatures.

4. Conclusions

In summary, this work presents a comprehensive experimental investigation of Cs₃Sb₂Cl₉ single crystals, highlighting their structural integrity, thermal robustness, and optoelectronic functionality under both dark and illuminated conditions. The material crystallizes in a layered trigonal structure with excellent thermal stability, maintaining its integrity up to 321 °C. Optical analysis reveals a wide indirect bandgap of 2.76 eV and an Urbach energy of 0.37 eV, indicating strong intrinsic absorption in the near-UV region and moderate static disorder arising from exciton–phonon coupling within the layered framework. PL measurements, supported by Voigt deconvolution, show illumination-induced enhancement and spectral narrowing, consistent with trap passivation and improved carrier recombination dynamics. Raman spectroscopy further reveals that light exposure modulates the lattice vibrational landscape, enhancing mode definition and reducing inhomogeneous broadening. EIS corroborates these observations, unveiling light-enhanced ionic/electronic transport, reduced recombination resistance, and lower activation energies. The combined results confirm the crucial role of photoexcitation in tuning the structural, vibrational, and transport properties of Cs₃Sb₂Cl₉. These synergistic effects position Cs₃Sb₂Cl₉ as a promising lead-free candidate for stable optoelectronic devices and photoactive systems, offering a compelling balance between environmental safety, thermal endurance, and light-responsive electronic behavior.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c07725.

• Additional PXRD refinement results (Figure S1), supplementary impedance spectroscopy analyses including |Z′| and |Z″| versus ω^–1/2 plots (Figures S2–S3), and complementary transport and diffusion parameters under dark and illuminated conditions (PDF)

The authors extend their appreciation to the Scientific Research Deanship at the University of Ha’il, Saudi Arabia, for funding this research work through project number RG-24061.

The authors declare no competing financial interest.