Visual optical thermometry with large color shifting based on a novel negative thermal expansion up-conversion molybdate KMgScMo3O12: Yb3+, Ho3.

Thermochromic luminescent materials with both naked-eye visibility and temperature-sensing potential have become a research focus, but high-temperature thermal quenching still severely limits their practical application. In this work, a multimodal optical temperature sensor with pronounced anti-thermal quenching behavior is developed based on the negative thermal expansion (NTE) phosphor KMgScMo3O12: Yb, Ho. The thermal behavior of the 5F4/5S2 and 5F5 levels of Ho3+ diverges significantly across the temperature range of 298 to 548 K. The 5F4/5S2 levels show a temperature-dependent decrease in emission intensity, compared to the thermally stable emission characteristic of the 5F5 level. Correspondingly, the upconversion (UC) emission shows a gradual chromatic transition from green to red. Based on the NTE characteristics of the material, the observed behavior is attributed to the combined effects of enhanced Yb3+ to Ho3+ energy transfer and the non-radiative relaxation of Ho3+. Using fluorescence intensity ratio (FIR) and Commission Internationale de l'Éclairage chromaticity coordinate ratio (CIER) technology, excellent temperature sensitivity and low temperature uncertainty are achieved. In addition, benefiting from the broad range thermochromic behavior of the phosphor, its emission color can be reversibly switched during heating and cooling cycles. Finally, a naked-eye readable temperature-sensing platform is constructed, and the temperature evolution of the ceramic heating element is visually demonstrated.