Upconversion luminescence of BaLa(PO):Yb-Er/Tm phosphors for optimal temperature sensing.

To explore new upconversion (UC) luminescent materials for optical temperature sensing, a series of BaLa(PO):0.1Yb,xEr (abbreviated as BLP:0.1Yb, xEr, 0.01≤x≤0.03) and BaLa(PO):0.1Yb,yTm (abbreviated as BLP:0.1Yb, yTm, 0.005≤y≤0.015) phosphors was designed. For Yb-Er codoped BLP, the UC emission spectra exhibit a gradual enhancement with increasing pump power, and two-photon process for the green and red emissions of Er has been confirmed by studying the dependence of UC emission intensities on excitation power. When the temperature rises, the fluorescence intensity ratio (FIR) for the H2-I and S4-I emissions of Er increases gradually due to the thermalization of populations at the H2 and S4 levels. With the temperature increased, the absolute sensitivity (S) shows a continuous increase and reaches about 4.38×10  K at 498 K, but the relative sensitivity (S) decreases gradually. For Yb-Tm codoped BLP, the emission intensity of Tm shows a gradual decrease with increasing Tm concentration. By studying the temperature dependence of FIR for the (F3,F3)-H3 and H3-H emissions, it has been found the sample exhibits a higher S value but a lower S value relative to the Yb-Er codoped BLP.