Influence of phase transitions on green fluorescence intensity ratio in Er³⁺ doped K_5.0Na_0.5NbO₃ ceramic

The fluorescence intensity ratio (FIR) method is a non-contact temperature (T) measurement technique based on thermally coupled levels of rare earth ions in a doped host. Green fluorescence originating from ²H_11/2 and ⁴S_3/2 states of Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) ceramic are studied in the temperature range of 300 K to 720 K. The fluorescence intensities change dramatically around phase transition points where the crystal symmetry changes, inducing deviation of the FIR from Boltzmann's law. The temperature determined by the FIR method deviates from thermocouple measurements by 7 K at the orthorhombic to tetragonal phase transition (T_O-T) point and 13 K at the Curie point (T_C). This finding gives guidance for developing fluorescent T sensors with ferroelectrics and may also provide a fluorescent method to detect phase transitions in ferroelectric materials.