Upconversion Luminescence and Thermosensitive Properties of NaGd(PO3)4:Yb3+/Er3+

High-sensitivity temperature measurement has attracted extensive attention in both fundamental studies and practical applications. This study develops a series of upconversion (UC) phosphors composed of NaGd(PO3)4 (NGP) doped with 20 at% Yb3+ and various concentration of Er3+ (0.5 at% as the optimal concentration) synthesized by high-temperature solid-state method. Comprehensive analysis using such as X-ray diffraction with Rietveld refinement verifies their crystal structure and evenly distributed lanthanide dopants. Under 980 nm laser excitation, the phosphors show characteristic Er3+ upconverison green and red emission bands, both arising from two-photon processes. Fluorescence intensity ratio (FIR) analysis based on the thermally coupled states demonstrates the thermal sensing ability feasibly in a wide temperature range of 200-600 K. The thermal sensitivity is relatively high with the maximum absolute thermal sensitivity Sa of 0.53% K-1 (523 K) and the maximum relative thermal sensitivity Sr of 2.60% K-1. The phosphors also exhibit high repeatability as thermal sensors reaching 97%. These findings postulate the potential of NGP:Yb/Er as a promising candidate in optical thermal sensing applications.