Compositional dependence of photoluminescence (PL) of ZnGa2O4:Li+;: Li+ ion incorporated as LiGa5O8, LiGaO2, and Li2O

Compositional dependency of luminescence enhancement of the self-activated ZnGa2O4:Li+ has been speculated in terms of cation ordering, structural change or strain, and vacancy concentration. The [xZnGa(2)O(4) + (1 - x)LiGa5O8] phosphors form solid solutions in the whole x range (0 <x <1) and their photoluminescence (PL) and excitation spectra are similar to pure ZnGa2O4 with the enhanced intensity. The enhanced luminescence of [xZnGa(2)O(4) + (1 - x)LiGa5O8] 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(2)O(4) + (1 - x)LiGaO2] and [xZnGa(2)O(4) + (1 - x)Li2O] 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 ZnGa2O4. 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 ZnGa2O4 by wurtzite LiGaO2 in the lattices. The luminescence intensity of [xZnGa(2)O(4) + (1 - x)Li2O] phosphor is the highest among Li+ ion doped ZnGa2O4 phosphors. The composition of [xZnGa(2)O(4) + (1 - x)Li2O] phosphor is considered as Ga3+ and O2- deficient [xZnGa(2)O(4) + (1 - x)LiGaO2]. 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 ZnGa2O4:Li+ is enhanced more by either vacancies or structural strain than by normal/inverse cation disorder in the spinel structure. (C) 2001 Elsevier Science B.V. All rights reserved.