Optical temperature-sensing properties of trivalent europium with high sensitivities in a wide temperature range

To develop new luminescent materials for optical thermometer, the Eu³⁺-activated BaY₂ZnO₅ (BYZ) phosphors were designed. Upon 394 nm excitation, several groups of ⁵D_0-2→⁷F_J (J = 0-4) transitions are observed, and the dominant emission is located at 625 nm. The temperature-dependent emission spectra reveal that the emission peaks are weakened with different rates, depending on the excited states of Eu³⁺. The transient decay kinetics, studied at various temperatures, are in agreement with the emission spectral features. The optical temperature-sensing performance is evaluated with two strategies. For the thermally-coupled (TC) levels of Eu³⁺, the fluorescence intensity ratio (FIR) of the 536 and 593 nm emissions follows the Boltzmann distribution, and the sensor sensitivities rise with increasing temperature. For the non-TC levels of ⁵D₀ and ⁵D₂, the piecewise functions between the FIRs and absolute temperature are utilized, and the highest absolute and relative sensitivities in the BYZ:7%Eu³⁺ phosphor are obtained to be 0.674 and 2.19% K^-1 at 533 K, respectively. Thus, the developed samples can show higher sensitivities at higher temperature.