Low-Temperature Hydrothermal Synthesis of Rare-Earth-Doped Sc2Mo3O12 Crystals for Thermally Enhanced Upconversion Luminescence and Optical Temperature Sensing

Thermal quenching severely limits the application of upconversion phosphors in high-temperature environments. Negative thermal expansion (NTE) materials have attracted considerable attention as promising hosts for enhancing high-temperature luminescence, whereas low-temperature synthesis of Sc2Mo3O12 remains rarely reported. In this work, phase-pure orthorhombic Sc2Mo3O12 crystals were synthesized via a low-temperature hydrothermal method. The effects of precursor molar ratio, reaction temperature, reaction time, and surfactant on phase formation were systematically investigated. Under the optimized synthesis conditions, efficient multicolor upconversion luminescence (UCL) was achieved under 980 nm excitation by employing Yb3+ as the sensitizer and Er3+, Tm3+, or Ho3+ as the activator ions. Only phase-pure Sc2Mo3O12 exhibited efficient UCL. Furthermore, the temperature- dependent luminescence properties were investigated over the range of 310–670 K, where the upconversion emission intensity continuously increased with temperature and a maximum relative sensitivity (Sr) of 0.0028 K−1 was obtained at 310 K based on the fluorescence intensity ratio (FIR) technique. These results demonstrate that phase-pure Sc2Mo3O12 prepared by a low-temperature hydrothermal method exhibits thermally enhanced UCL and promising optical thermometric performance, highlighting its potential as an NTE host for high-temperature optical temperature sensing.