Synthesis and Er³⁺ upconversion emission characteristics of calcined Er₂O₃-MgO-Nb₂O₅ phosphors for temperature sensing

A number of Er₂O₃+MgO+Nb₂O₅ phosphors with different molar ratios were calcined. The crystalline phases contained in these phosphors are determined by X-ray diffraction. The results show that two solid-state reactions, namely Er₂O₃+Nb₂O₅ =2ErNbO₄ and 4MgO+Nb₂O₅ =Mg₄Nb₂O₉, take place, with the former tending to occur faster than the latter. The phases in the phosphors may contain two or three phases among ErNbO₄, MgO, Er₂O₃ and Mg₄Nb₂O₉, depending on the Nb₂O₅ content. The Er³⁺ spectroscopic features of these phosphors were investigated, providing evidence for the Er³⁺-related phases determined from the X-ray diffraction data. The upconversion mechanism study shows that either cooperative energy-transfer or excited-state absorption, or both, play a role in the upconversion emission, and that each fluorescence band is dominated by the emission of the ErNbO4 phase. Further study of thermal effects shows that the upconversion intensity decreases with increasing temperature, with the 560 nm emission revealing stronger temperature sensitivity than the 530 and 670 nm emissions. Mg doping adjusts the ErNbO₄ phase content and hence determines the fluorescent intensity. On the other hand, the Mg presence increases the multi-phonon relaxation probability and, hence, the temperature sensitivity of upconversion intensity. Without the Mg doping, the 560 nm upconversion emission intensity reduces by 40% with the temperature increase of only 57 °C. The Mg presence leads to further reduction by as much as 65%, showing that the phosphors are promising candidates for temperature sensing.