Outstanding optical temperature sensitivity and dual-mode temperature-dependent photoluminescence in Ho3+-doped (K,Na)NbO3–SrTiO3 transparent ceramics

High optical temperature sensing properties based on rare-earth-doped (K,Na)NbO₃-based ferro-/piezoelectrics have attracted much attention due to their potential application in novel optoelectronic devices. Here, we fabricated Ho³⁺-doped (K_0.5Na_0.5)NbO₃–SrTiO₃ transparent ceramics by conventional pressureless sintering. Their microstructures, transmittances, up-conversion photoluminescence, and optical temperature sensing properties have been characterized in details. Because of the cubic-like phase, dense, and fine-grained structure as well as relaxor-like feature, the ceramics exhibit high transmittance (~70%) in the near-infrared region. Owing to Ho³⁺, green and red up-conversion emissions have been observed, which can be easily modulated by temperature. The ceramics have stable emission colors (<200°C) and superior temperature-modulating emission color-tunable performance (>200°C). Furthermore, the temperature sensing behavior based on the thermally coupled levels (⁵F₄, ⁵S₂) of Ho³⁺ has been analyzed by a fluorescence intensity ratio technique. The transparent ceramics possess outstanding optical temperature sensitivity (~0.0096/K at 550 K), higher than most rare-earth-doped materials (e.g., ceramics, glasses, and phosphors).