Simultaneous Yb³⁺-Induced Phase Transition and Sensitized Luminescence in Er³⁺-Doped KNN-Based Lead-Free Ceramics for Optical Thermometry

Environment-friendly potassium sodium niobate-based ferroelectrics have been widely and deeply studied for their electrical properties, but there remains much potential for applications in other fields, one of which is in optical temperature sensing. Herein, a host (K,Na)Li0.04NbO3:0.5 mol% Er material with an orthogonal–tetragonal phase transition was constructed by Li+ regulation and then increased Yb2O3 codoping used to promote the occurrence of orthogonal to tetragonal and tetragonal to pseudocubic phase transitions. Interestingly, codoping with Yb2O3 as a sensitizer and phase structural changes regulated the resulting materials’ luminescent characteristics and greatly enhanced luminescence intensity and temperature sensing properties, compared with those of merely Er3+-doped samples. In addition, strong green and red light emissions were related to the large absorption cross-section of Yb3+ near 980 nm, which transferred more energy to Er3+. In particular, significantly enhanced red light, or red shift, was associated with energy back transfer at high Yb3+ concentrations. Furthermore, the optimal ceramic was (K, Na)Li0.04NbO3:0.5 mol % Er/2.5 mol % Yb (KNL0.04N:0.5 Er/2.5 Yb), and its maximum absolute and relative sensitivity of 0.0112 and 0.0173 K–1, respectively, which were higher than those of other reports. These results showed that KNL0.04N:0.5Er/2.5Yb lead-free ceramic possessed the potential to be an excellent optical temperature sensor with high intensity and sensitivity.