| Summary: | Lead-free perovskite variants have emerged as promising candidates due to their self-trapped exciton emission. However, in ASnX<sub>3</sub> systems, facile oxidation of Sn(II) to Sn(IV) yields A<sub>2</sub>SnCl<sub>6</sub> vacancy-ordered derivatives. Paradoxically, despite possessing a direct bandgap, these variants exhibit diminished photoluminescence (PL). Doping engineering thus becomes essential for precise optical tailoring of A<sub>2</sub>SnX<sub>6</sub> materials. Herein, through integrated first-principles calculations and spectroscopic analysis, we elucidate the luminescence mechanism in Te<sup>4+</sup>-doped (NH<sub>4</sub>)<sub>2</sub>SnCl<sub>6</sub> lead-free perovskites. Density functional theory, X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) confirm Te<sup>4+</sup> substitution at Sn sites via favorable chemical potentials. Spectral interrogations, including absorption and emission profiles, reveal that the intense emission originates from the triplet STE recombination (<sup>3</sup>P<sub>1</sub> → <sup>1</sup>S<sub>0</sub>) of Te centers. Temperature-dependent PL spectra further demonstrate strong electron–phonon coupling that induces symmetry-breaking distortions to stabilize STEs. Complementary electronic band structure and molecular orbital calculations unveil the underlying photophysical pathway. Leveraging these distinct thermal responses of PL intensity and peak position, 0.5%Te:(NH<sub>4</sub>)<sub>2</sub>SnCl<sub>6</sub> emerges as a highly promising candidate for non-contact, dual-parameter optical thermometry over an ultra-broad range (100–400 K). This work provides fundamental insights into the exciton dynamics and thermal engineering of optical properties in this material system, establishing its significant potential for advanced temperature-sensing applications.
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