Influence of Mn oxidation in persistent luminescence halide double perovskites

Our study focuses on persistent luminescence (PeL) halide double perovskites (HDPs), both bulk and nano. PeL materials, or glow-in-the-dark materials, can temporarily store the excitation energy in metastable traps and release it as photons after a certain time. Given the same stoichiometry (Cs₂Na₁₋ₓAgₓInCl₆:Mn²⁺), at room temperature (RT) bulk shows PeL, but nano does not. The mechanism behind this behaviour is still unknown. To understand what causes the PeL annihilation at nano size at RT, we performed thermally stimulated luminescence measurements across a broad temperature range (10 K – 450 K) and PeL decay measurements (at RT and 15 K) after X-ray charge. These measurements enabled us to determine at which temperatures the materials exhibit PeL. At 15 K, even nano materials have PeL. So, to find out what causes the PeL behaviour change with T, we analysed electronic structure with T, using X-ray Absorption Near Edge Structure (XANES) spectroscopy at the K-edge of Mn (the emissive centre) at three different temperatures: 15 K (nano and bulk have PeL), 290 K (bulk has PeL) and 430 K (no PeL). The XANES analysis suggests that Mn is present in the bulk sample in both 2+ and 3+ oxidation states, but in the nano sample only 2+, so the PeL arising could be associated with the presence of the 3+ oxidation state.