Upconversion luminescence of Ba₃La(PO₄)₃:Yb³⁺-Er³⁺/Tm³⁺ phosphors for optimal temperature sensing

To explore new upconversion (UC) luminescent materials for optical temperature sensing, a series of Ba₃La_1-x(PO₄)₃:0.1Yb³⁺,xEr³⁺ (abbreviated as BLP:0.1Yb³⁺, xEr³⁺, 0.01≤x≤0.03) and Ba₃La_1-y(PO₄)₃:0.1Yb³⁺,yTm³⁺ (abbreviated as BLP:0.1Yb³⁺, yTm³⁺, 0.005≤y≤0.015) phosphors was designed. For Yb³⁺-Er³⁺ 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 Er³⁺ 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 H_11/22-⁴I_15/2 and S_3/24-⁴I_15/2 emissions of Er³⁺ increases gradually due to the thermalization of populations at the H_11/22 and S_3/24 levels. With the temperature increased, the absolute sensitivity (S_A) shows a continuous increase and reaches about 4.38×10^-3  K^-1 at 498 K, but the relative sensitivity (S_R) decreases gradually. For Yb³⁺-Tm³⁺ codoped BLP, the emission intensity of Tm³⁺ shows a gradual decrease with increasing Tm³⁺ concentration. By studying the temperature dependence of FIR for the (F₂3,F₃3)-H₆3 and H₄3-³H₆ emissions, it has been found the sample exhibits a higher S_R value but a lower S_A value relative to the Yb³⁺-Er³⁺ codoped BLP.