Up-Converting K2Gd(PO4)(WO4):20%Yb3+,Ho3+ Phosphors for Temperature Sensing

Inorganic luminescent materials that can be excited with NIR radiation and emit in the visible spectrum have recently gained much scientific interest. Such materials can be utilized as anti-counterfeiting pigments, luminescent thermometers, bio-imaging agents, etc. In this work, we report the synthesis and optical properties of K2Gd(PO4)(WO4):Ho3+ and K2Gd(PO4)(WO4):20%Yb3+,Ho3+ powders. The single-phase samples were prepared by the solid-state reaction method, and the Ho3+ concentration was changed from 0.5% to 10% with respect to Gd3+. It is interesting to note that under 450 nm excitation, no concentration quenching was observed in K2Gd(PO4)(WO4):Ho3+ (at least up to 10% Ho3+) samples. However, adding 20% Yb3+ has caused a gradual decrease in Ho3+ emission intensity with an increase in its concentration. It turned out that this phenomenon is caused by the increasing probability of Ho3+ → Yb3+ energy transfer when Ho3+ content increases. K2Gd(PO4)(WO4):20%Yb3+,0.5%Ho3+ sample showed exceptionally high up-conversion (UC) emission stability in the 77–500 K range. The UC emission intensity reached a maximum at ca. 350 K, and the intensity at 500 K was around four times stronger than the intensity at 77 K. Moreover, the red/green emission ratio gradually increased with increasing temperature, which could be used for temperature sensing purposes.