摘要: Fluorescence lifetime (FL)-based thermometry has been widely applied in diverse technological fields owing to its self-referencing capability, high sensitivity, and reliability. However, conventional FL thermometers generally rely on thermally activated nonradiative relaxation processes, limiting their use for cryogenic sensing below liquid nitrogen temperature (77 K). Herein, we report a new FL thermometric method based on Te4+-doped 0D Cs2ScCl5·H2O crystals, which operates independently of nonradiative relaxation and enables accurate and reliable cryogenic temperature detection down to liquid helium temperature (4 K), achieving a maximum relative sensitivity of 8.5% K–1 at 10 K. Specifically, we demonstrate that the photoluminescence (PL) in Cs2ScCl5·H2O: Te4+ originates from the 3P0,1 → 1S0 transitions of isolated Te4+ centers. This is evidenced by the strong temperature dependence of the dual-band emissions at low temperatures (induced by a dynamic Jahn-Teller effect) and the PL lifetime dictated by the interplay between spin-forbidden and spin−orbital allowed transitions. These findings not only provide fundamental insights into the excited-state dynamics of Te4+ in metal halides, but also establish a general approach for ultralow-temperature FL sensing utilizing main-group s-electron ions.