Reversible photoionization process in luminescent Ce³⁺doped elpasolite-type fluoroindates

Ce3+ luminescence was investigated in elpasolite fluoroindates A2BInF6 (A = K, Rb; B = Na, K). All A2BInF6:Ce3+ have been synthesized by solid state reactions from stoichiometric mixtures in sealed gold tubes at 700°C. Transparent colourless Rb2KInF6:Ce3+ crystals were grown by Bridgman method in 10% Rh-Pt crucibles sealed under dry argon atmosphere. At room temperature Rb2KInF6:Ce3+ presents a blue-green broad-band emission when excited with a 315 nm UV radiation. This emission can be ascribed to the Ce3+ ions in In3+ site. Two other emissions in the blue and UV can be assumed to originate from Ce3+ ions located in the 6 coordinated potassium and 12 coordinated rubidium sites. Under a steady 315 nm UV excitation the main blue-green Ce3+ emission decreases progressively, giving rise to a new emitting centre which exhibits a red emission under 255 nm excitation. This phenomenon is optically reversible and moreover the original state can be thermally regenerated. An hypothesis based on the ln(III)-Ce(III) redox couple is proposed to explain this behaviour: photoionization of Ce3+ leads to Ce4+ with electron transfer on In3+ acceptors. This red luminescence is ascribed to the formation of In+ ions.