Dual‐Mode Optical Temperature Sensing of Mg 0.75 Al 2.16 O 4 : Cr 3 + /Er 3 + Transparent Ceramic With Spatial Separation Structure

ABSTRACT Designing the energy transfer process between activator ions is an effective strategy for achieving multimodal, high‐sensitivity fluorescent temperature sensing. Herein, a dual‐mode fluorescent temperature sensor was developed based on the Mg 0.75 Al 2.16 O 4 : 0.1at%Cr 3+ /0.03at%Er 3+ transparent ceramic. Spatial separation of Cr 3+ /Er 3+ ions was achieved by utilizing both the lattice and grain boundary environments within the transparent ceramic. Within this unique structure, efficient energy transfer from Er 3+ to Cr 3+ was established. As a result, Cr 3+ emitted near‐infrared upconversion fluorescence peaking at 797 nm ( 4 T 2 level) under 980 nm laser excitation. The downconversion fluorescence lifetime of Cr 3+ and the upconversion fluorescence intensity ratio of Cr 3+ /Er 3+ can serve as temperature sensing signals. Within the range of 300–573 K, the maximum relative sensitivity reached 4.2% K −1 . This work demonstrates the potential for designing functional ionic spatial structures within transparent ceramics.