Lead-free halide double perovskites are gaining attention as sustainable and stable alternatives to lead-based counterparts. Here, we report a detailed investigation of the vacancy-ordered double perovskite Cs2SnCl6, synthesized via a simple chemical precipitation method. X-ray diffraction confirmed its cubic \({\text{Fm}}\overline{3}{\text{m}}\) symmetry, with a refined lattice parameter of a = 10.3847(4) Å. Thermogravimetric analysis demonstrated outstanding thermal stability up to 614.4°C, highlighting its robustness for high-temperature processing. Optical measurements revealed a wide direct bandgap of 3.60 eV and an Urbach energy of 0.425 eV, indicating low structural disorder and strong intrinsic excitonic effects. Photoluminescence analysis showed a broad emission centered at 433.88 nm with bi-exponential decay lifetimes of τ1 ≈ 7.85 ns and τ2 ≈ 281.3 ns, confirming the role of self-trapped excitons. Impedance spectroscopy between 303 K and 403 K identified thermally activated ionic transport with two distinct activation energies (0.32 eV and 0.10 eV), corresponding to a transition from trap-limited to bulk-dominated conduction around 350 K. Complementary dielectric and modulus analyses further evidenced non-Debye relaxation governed by hopping conduction and Maxwell–Wagner–Sillars polarization. These findings establish Cs2SnCl6 as a thermally robust and multifunctional material with promising potential in UV photodetectors, ion-conducting devices, and scintillation technologies.
