Upconversion luminescence and thermosensitive properties of NaGd(PO₃)₄:Yb³⁺/Er³

High-sensitivity optical temperature measurement has attracted extensive attention in both fundamental studies and practical applications. In this study, a series of upconversion (UC) luminescence phosphors composed of NaGd(PO₃)₄ (NGP) doped with 20 at% Yb³⁺ and various concentrations of Er³⁺ (0.5 at% as the optimal concentration) was synthesized by high-temperature solid-state method. And their crystal structure and the distribution of lanthanide dopants were analyzed using X-ray diffraction with Rietveld refinement verifies. Under 980 nm laser excitation, the obtained phosphors show the characteristic Er³⁺ upconversion green and red emission bands through two-photon processes. The fluorescence intensity ratio (FIR) based on the thermal coupled states demonstrates the thermal sensing ability in a wide temperature range of 200-573 K. The thermal sensitivity is relatively high with the maximum absolute thermal sensitivity S of 0.53 % K^-1 (523 K) and the maximum relative thermal sensitivity S of 2.60 % K^-1. The phosphor NGP:Yb/Er also exhibits high repeatability as the thermal sensors reach 97 %. These findings postulate the potential of NGP:Yb/Er as a promising candidate in optical thermal sensing applications.