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 ~ 700 °C, a phase-pure elpasolite structure (\(Fm\overline{3 }m\)) 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 predominantly attributed to self-trapped exciton (STE) emission. Such picture is further supported by the excitation-power-dependent PL measurements: the spectral profile remains essentially unchanged across the investigated excitation range, whereas the emission intensity exhibits an approximately linear dependence on excitation power (k ≈ 0.92). This behavior is consistent with STE-mediated emission within the explored excitation window. Besides, with an activation energy of nearly 111 meV—a value which is in complete agreement with effective STE stabilization and strong exciton–phonon coupling, thermally-activated STE dynamics are revealed by temperature-dependent PL measurements. More importantly, no clear fingerprints of defect-driven relaxation governing the process are found. Instead, femtosecond transient absorption measurements reveal sub-picosecond exciton thermalization, followed by phonon-assisted STE formation occurring within 10–15 ps. Only weak linear photoconductivity and extremely low currents are revealed by the I–V characteristics (265 nm illumination versus dark), indicating negligible free-carrier transport and supporting the formation of strongly localized self-trapped excitons. An activation energy of 0.625 eV together with a single bulk relaxation process are revealed by 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 ordered 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.
