Persistent Luminescence in Halide Double Perovskites: Comparison between Bulk and Nanoscale Forms

Persistent luminescent (PeL) materials can store excitation energy and release it gradually as light emission, over minutes to weeks, after the excitation source is removed. This unique optical behaviour makes them intersting for applications such as bioimaging, safety indicators, and data storage. However, efficient PeL at the nanoscale remains challenging, since reducing particle size frequently leads to a strong reduction or complete quenching of the emission at room temperature (RT).1 In this work, Mn-activated halide double perovskites are explored to investigate how composition and size control PeL properties. A series of Cs₂(AgₓNa₁₋ₓ)InCl₆:Mn²⁺ compounds was synthesized in different morphologies, including single crystals as well as nanostructures such as nanocubes and nanoplates. Optical characterization, including photoluminescence, PeL, and thermally stimulated luminescence measurements in the 10–450 K range, reveals that even nanosamples can exhibit PeL at low temperature (15 K), despite its suppression at RT. These observations suggest that the disappearance of RT PeL upon size reduction may be related to a modification of trap depth distribution. Since PeL is governed by traps, often associated with local structural defects, a detailed structural study was performed. Synchrotron-based Pair Distribution Function analysis and Mn K-edge X-ray Absorption Near Edge Structure measurements were used to probe local structure and Mn oxidation state, respectively. The PDF results show that both Ag/Na composition and size induce measurable changes in local structure, while XANES data reveal a broader pre-edge region in bulk samples, consistent with a mixed Mn2+/Mn3+ species, while Mn3+ is not detectable in nanostructures, correlating with the PeL behaviour. Taken together, these results provide a perspective on how structural disorder, composition, and electronic configuration interplay to govern PeL in halide double perovskites at bulk and nano.