Optical Interpretation of a Second-Order Phase Transition Induced by Thermal-Driven Li+ Migration via Configurational Entropy in CaTiO3:Li+,Yb3+,Er3+

Thermal-induced ion migration might be accompanied by a phase transition, which is common for ion conductors but rarely studied for optical materials in the optoelectronic devices. Herein, CaTiO3:Li+,Yb3+,Er3+ is chosen as a model to optically interpret a second-order phase transition induced by thermal-driven Li+ migration. The intensity pairwise ratios of the emission peaks of Er3+ versus temperature show a continuous but not derivable behavior with a break point at similar to 125 degrees C (the ignition point of Li+ motion), which is analogous with the behaviors of thermal properties and the deduced configurational entropy. The gap between the phase transition and Er3+ luminescence variation can be bridged by the number variation of microstates, referring to the chemical environment around Er3+ ions. This research gives an optical insight into the second-order phase transition induced by thermalactivated ion migration, which may help to optimize the optical materials with the concern of thermal properties.