Unveiling the structural determinants of Persistent Luminescence in Mn-Doped Halide Double Perovskites

Materials exhibiting persistent luminescence (PeL) represent a fascinating subclass of luminescent systems, distinguished by their ability to store excitation energy and release it gradually as light long after the excitation source has been removed. This delayed emission can last from minutes to weeks, making such materials appealing for technologies requiring autonomous and long-lasting luminescence, including safety signaling, bioimaging, photocatalysis, and information storage. Despite this potential, achieving efficient PeL at the nanoscale remains a critical challenge, as size reduction often leads to quenching of the PeL at room temperature. Here, Mn-activated halide double perovskites are a versatile platform to investigate PeL behavior. A compositional series of Cs2(AgxNa1-x)InCl6:Mn2+ was prepared in different forms, including single crystals (SC) and nanocrystals such as nanocubes (NC) and nanoplates (NP). This approach enables a systematic comparison of how size and composition influence both the PeL. We performed optical analyses, including photoluminescence, PeL, and thermally stimulated luminescence measurements (across 10 K–450 K), interestingly showing PeL at 15 K for even nanocrystals. We hypothesize that the PeL disappearance at room temperature from bulk to nanocrystals is ascribed to different position of the traps in the bandgap. Given that PeL relies on trapping states, typically linked to local structural defects, an in-depth structural investigation was carried out using synchrotron-based techniques: Pair Distribution Function (PDF) analysis, and X-ray Absorption Near Edge Structure (XANES) spectroscopy at the Mn K-edge. Variations in the Ag/Na ratio and size induce detectable changes in the local structure, as revealed by PDF analysis. In parallel, XANES results show a noticeable broadening of the pre-edge feature in PeL bulk samples, pointing to the coexistence of Mn2+ and Mn3+ species. These results provide important insight into the mechanisms governing PeL in halide double perovskites.