Simultaneous Enhancement and Modulation of Upconversion by Thermal Stimulation in Sc₂Mo₃O₁₂ Crystals

Rational control of photoluminescence against a change in temperature is important for fundamental research and technological applications. Herein, we report an anomalous temperature dependence of upconversion luminescence in Yb/Ho co-doped Sc₂Mo₃O₁₂ crystals. By leveraging negative thermal expansion of the crystal lattice, energy transfer between the lanthanide dopants is promoted as the temperature is increased from 303 to 573 K, resulting in an ∼5-fold enhancement of the emission. Meanwhile, the emission profile is also substantially altered due to the concurrent thermal quenching of selective energy states, corresponding to a clear shift in color from green to red. Via correlation of the red-to-green emission intensity ratio of Ho³⁺ dopant ions with temperature, a ratiometric luminescence thermometer is constructed with a maximum sensitivity of 2.75% K^-1 at 543 K. As the Sc₂Mo₃O₁₂ crystals are thermally stable and nonhygroscopic, our findings highlight a general approach for highly reversible control of upconversion by temperature in ambient air.