Thermal evolution of upconversion luminescence in a positive-thermal-expansion tungstate: A structure-related interpretation based on lattice distortion and oxygen-related defect competition

Thermally enhanced upconversion luminescence has been widely reported in negative‑thermal‑expansion tungstates, whereas whether and how this phenomenon can occur in positive-thermal-expansion tungstates remains largely unexplored. Here we show that the thermal response of upconversion luminescence in monoclinic MIn(WO 4 ) 2 : 1%Er 3 + , 10%Yb 3+ (M=Li, Na, K) is strongly dependent on the alkali metal ion at the M site. Under 980 nm excitation, the green emission of the K‑containing sample (KIW) continuously increases up to 573 K without quenching, whereas the Li and Na samples show only a transient rise followed by thermal decay. Combining variable‑temperature XRD, XPS, EPR, DFT calculations, and luminescence decay measurements, we attribute this divergence to a competition between two temperature‑activated processes. Lattice distortion enhances radiative transitions by modulating the local crystal field around Er 3+ , while oxygen vacancies introduce nonradiative channels. KIW combines a flexible lattice that sustains continuous distortion with a high oxygen vacancy formation energy that suppresses defect generation, so radiative enhancement dominates across the whole temperature range. The Li and Na hosts, with more rigid lattices and lower defect formation energies, develop oxygen vacancies readily and their distortion saturates, leading to quenching at elevated temperatures. Using the distinct temperature responses of the 520 nm and 650 nm emissions in KIW, a ratiometric optical thermometer achieves an absolute sensitivity of 0.1052 K −1 . This work provides a mechanistic understanding of thermally enhanced upconversion in positive‑thermal‑expansion tungstates and demonstrates a structure‑based strategy for designing high‑temperature‑stable luminescent materials.