Compositional dependence of photoluminescence (PL) of ZnGa 2O 4 : Li +; Li + ion incorporated as LiGa 5O 8, LiGaO 2, and Li 2O

Abstract

Compositional dependency of luminescence enhancement of the self-activated ZnGa 2O 4 : Li + has been speculated in terms of cation ordering, structural change or strain, and vacancy concentration. The [ xZnGa 2O 4+(1− x)LiGa 5O 8] phosphors form solid solutions in the whole x range (0< x<1) and their photoluminescence (PL) and excitation spectra are similar to pure ZnGa 2O 4 with the enhanced intensity. The enhanced luminescence of [ xZnGa 2O 4+(1− x)LiGa 5O 8] phosphors is ascribed to the cation disorder in the mixed spinel structure. There appear two PL excitation bands at 254 and 300 nm for both [ xZnGa 2O 4+(1− x)LiGaO 2] and [ xZnGa 2O 4+(1− x)Li 2O] phosphors. The PL spectrum of these phosphors excited at 254 nm shows a blue emission band maximum at 430 nm, which corresponds to the self-activated luminescence center of pure ZnGa 2O 4. Another PL band maximum at 490 nm appears with the excitation at 300 nm. This additional luminescence center is speculated as the structurally strained one of spinel ZnGa 2O 4 by wurtzite LiGaO 2 in the lattices. The luminescence intensity of [ xZnGa 2O 4+(1− x)Li 2O] phosphor is the highest among Li + ion doped ZnGa 2O 4 phosphors. The composition of [ xZnGa 2O 4+(1− x)Li 2O] phosphor is considered as Ga 3+ and O 2− deficient [ xZnGa 2O 4+(1− x)LiGaO 2]. The highest intensity is speculated to result from the high concentration of oxygen vacancies in the lattices. It is evident that the intensity of self-activated PL of ZnGa 2O 4 : Li + is enhanced more by either vacancies or structural strain than by normal/inverse cation disorder in the spinel structure.