Order and disorder around Cr3+ in chromium doped persistent luminescent AB2O4 spinels

Abstract

The X-ray absorption near edge structure (XANES) spectroscopy technique is used to better understand the charging and decharging processes of the persistent luminescence in the Cr³⁺doped AB₂O₄ spinels (A = Zn, Mg and B = Ga and Al) with low photon energy excitation by visible light. Cr K edge XANES spectra have been simulated for different near neighbour environments around the Cr³⁺ recombination centres and compared with the experimental curve. In the Cr³⁺:ZnGa₂O₄ compound, the Cr³⁺ local structure corresponds mostly to that of a normal spinel (∼70%), while the rest comprises of a distorted octahedral environment arising from cationic site inversion and a contribution from chromium clustering. This local structure is considerably different in Cr³⁺:MgGa₂O₄ and Cr³⁺:ZnAl₂O₄, where, for both cases, chromium clustering represents the main contribution. The strong correlation between the intensity of persistent luminescence and the percentage of Cr in clusters leads us to infer that the presence of Cr clusters is responsible for the decrease of the intensity of the visible light induced persistent luminescence in the Cr³⁺ doped AB₂O₄ spinels.