K _0.5Na _0.5NbO ₃-based multifunctional transparent ceramics for ultrahigh sensitive optical thermometry

K_0.5Na_0.5NbO₃ (KNN)-based ceramic, with high Curie temperature, good ferroelectric performance, shows a promising potential in function materials. In this study, the incorporation of CaTiO₃ (CT) into KNN-based ceramics resulted in a high density, sub-micrometer grain size, and a pseudo-cubic phase, which collectively contributed to an impressive infrared transmittance of 78% at 1336 nm, as well as enhanced relaxor ferroelectric properties. A slim <em>P-E</em> loop and well-defined polar nanoregions (PNRs) can be clearly observed in the samples when the CT content exceeds 0.175. Moreover, we demonstrate that ultrahigh sensitivity in temperature sensing can be achieved using Pr³⁺-doped KNN-CT, based on the fluorescence intensity ratio (FIR) of the ¹D₂-³H₄ emission to the ³P₀-³H₅ emission. The FIR increases by as much as 75-fold when the temperature rises from room temperature to 500 K, resulting in an exceptional absolute sensitivity (<em>S_a</em>= 0.139 K^-1) and relative sensitivity (<em>S_r</em>= 2.69% K^-1). This exceptional FIR performance is believed to be closely linked to the temperature-sensitive PNRs in KNN-CT, highlighting its significant potential for applications in optical temperature sensors.