Phonon-AssistedThermally Enhanced Up-conversion FluorescenceLifetime Thermometry and Visualized Temperature Warning in Yb³⁺–Ho³⁺-Codoped Lu₂Mo₃O₁₂

Fluorescence thermometers have been plagued by the problem of thermal quenching (TQ) for a long time when operating at higher temperatures, which seriously hindered their performances in practical application. Ho³⁺–Yb³⁺-codoped Lu₂Mo₃O₁₂ phosphors were synthesized for designing TQ-immune fluorescence thermometers. Temperature-sensitive upconversion (UC) fluorescence lifetime (FL) and fluorescence color were modulated by utilizing appropriate phonon-assisted energy transfer (PAET) from Yb³⁺ (²F_5/2) to Ho³⁺ (⁵I₅) based on Lu₂Mo₃O₁₂ with negative thermal expansion (NTE) properties. Under a 980 nm laser excitation, the FL of the ⁵F₄ → ⁵I₈ (Ho³⁺) transition extended from 35.65 to 52.94 μs (280 to 480 K) and the corresponding fluorescence color changed from green to red in Lu₂Mo₃O₁₂: Ho³⁺, Yb³⁺. Accordingly, a dual-mode self-calibration fluorescence thermometer based on Lu₂Mo₃O₁₂: Ho³⁺, Yb³⁺ was constructed through FL technology (⁵F₄ → ⁵I₈ transition) and Commission Internationale de Eclairage chromaticity coordinate ratio (CIER) technology. Ultimately, the feasibility of the thermometer was verified to have high stability and temperature sensitivity (δT = 0.9 K, CIER), indicating that it is a prospective candidate for visualized temperature warning and sensing.