XANES reveals Mn2+ and Mn3+ coexistence in Persistent Luminescence Mn-doped halide double perovskites

Persistent luminescence materials (PeLMs) have attracted significant attention due to their ability to store excitation energy and release it gradually as light over extended periods (from minutes to several weeks) after the excitation source is removed. These materials are widely investigated for applications like emergency signals, road markings, bioimaging, photocatalysis, optical storage and anti-counterfeiting technologies. For most of these applications, the use of PeLMs nanocrystals is preferable. However, the downscaling of PeLMs often has a detrimental effect on the persistent luminescence (PeL) observable at room temperature. In this work, we investigated Mn-doped halide double perovskites in single crystals (SCs) and nanocrystals forms through X-ray Absorption Near Edge Spectroscopy (XANES). The study was specifically designed to elucidate the oxidation state distribution of Mn and its dependence on crystal size and its influence on PeL. In particular, we focused on two compositions: Cs₂NaInCl₆:Mn (hereafter Ag0) and Cs2Na0.2Ag0.8InCl6:Mn (Ag0.8). These stoichiometries allow to explore the influence of partial substitution of Na⁺ with Ag⁺ on the local electronic structure of Mn and, consequently, on macroscopic PeL-related properties. In both compositions (Ag0 and Ag0.8), XANES revealed the coexistence of Mn2+ and Mn3+ species for SCs and only Mn2+ for nanocrystals (both nanocubes – NC and nanoplates – NP), highlighting a mixed-valence scenario that is common in other PeLMs, like SrAl2O4: Eu2+, Dy3+. These results contribute to the broader understanding of how compositional modifications can be exploited to optimize PeL properties in emerging perovskite-based materials and beyond.