Emission colours of bistable photochromic compounds: Ce-doped alkaline (Rb, K, Na)–indium fluorides

Abstract

The structural and spectroscopic properties of three Ce³⁺-doped A₃InF₆ compounds (with A⁺ = Rb⁺, K⁺, Na⁺) are investigated. Containing indium, they all exhibit 2Ce³⁺ + In³⁺ ⇔ In⁺ + 2Ce⁴⁺ redox processes induced by UV irradiation between Ce³⁺ and In³⁺ cations. The photochromo-luminescent behaviour and associated kinetics, with the Ce³⁺ and In⁺ ion emission quenching through the redox equation, respectively, are investigated. Our studies highlight that different mechanisms (surface vs. bulk) need be considered to explain the decrease in the blue (Ce³⁺) and orange (In⁺) emission curves. Moreover, the comparison of Ce³⁺-doped Rb₂KInF₆, K₂NaInF₆ elpasolites and K₃InF₆ cryolite spectral distributions shows that changing the nature of the alkali cation makes colour tuning possible. The shift of the Ce³⁺ and In⁺ colour emissions between the three as-prepared compounds is mainly due to the variation in the ionic-covalent nature of the chemical bonds around the active cation sites. The final calculated optical difference between the two bistable states varies from 0.31 to 0.48 depending on the alkali subnetwork.