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

To develop new luminescent materials for optical thermometer, the Eu-activated BaYZnO (BYZ) phosphors were designed. Upon 394 nm excitation, several groups of D→F (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 Eu. 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 Eu, 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 D and D, the piecewise functions between the FIRs and absolute temperature are utilized, and the highest absolute and relative sensitivities in the BYZ:7%Eu phosphor are obtained to be 0.674 and 2.19% K at 533 K, respectively. Thus, the developed samples can show higher sensitivities at higher temperature.