Mechanism of upconversion luminescence enhancement in Yb3+/Er3+ co-doped Y2O3 through Li+ incorporation

Abstract

Li⁺ doping is a well-known, simple, yet efficient strategy to optimize the properties of upconverting materials. Nonetheless, the position of Li⁺ in the lattice and the mechanism of upconversion enhancement are still controversial, especially in Yb³⁺/Er³⁺ co-doped Y₂O₃. This paper presents a comprehensive investigation of the above issues (i.e. the position occupied by Li⁺ in the lattice and the mechanism of luminescence enhancement, in terms of decreased defects) by studying (Y_0.78−XYb_0.20Er_0.02Li_X)₂O₃ powders. Neutron powder diffraction was employed for the first time in the literature to show that Li⁺ ions are accommodated in Y sites of YO₆ octahedra, confirmed also by the content of oxygen defects, which was increased with the increase of Li⁺ concentration. FT-IR showed that there was a small change in the amount and the type of the surface-absorbed groups with the increase in the Li⁺ content, thus not supporting the prevailing conclusion that the quenching groups are decreased by doping Li⁺. Positron annihilation lifetime (PLAS) experiments showed that the total defect concentration and the large defect clusters, which are considered as quenching centers, are decreased with increasing Li⁺-content, resulting in the enhancement of the emission intensity in Yb³⁺/Er³⁺ co-doped Y₂O₃.