Visual optical thermometry with large color shifting based on a novel negative thermal expansion up-conversion...

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.