From Local Structure to Persistent Luminescence: A PDF and XANES Study of Mn-Doped Halide Double Perovskites

Persistent luminescence materials (PeLMs) can emit for long periods, ranging from minutes to several weeks, after the excitation source has been removed. This characteristic makes them promising for a wide range of applications such as emergency signage, road safety indicators, bioimaging, photocatalysis, optical data storage, and anti-counterfeiting technologies. In many of these contexts, the use of nanocrystalline PeLMs is necessary; however, reducing the particle size frequently leads to a significant decrease or complete loss of Persistent Luminescence (PeL) at room temperature.1 In this study, Mn-doped halide double perovskites were investigated by synthesizing a series of compositions of Cs2(AgxNa1-x)InCl6:Mn2+ in the form of single crystals (SC) as well as nanostructures, specifically nanocubes (NC) and nanoplates (NP). Because PeL is closely related to the presence of trapping centers, often associated with local structural defects, additional structural characterization was carried out at European Synchrotron using X-ray Powder Diffraction, Pair Distribution Function (PDF) analysis, and X-ray Absorption Near Edge Structure (XANES) spectroscopy at the Mn K-edge, corresponding to the emitting center. The experimental results reveal a pronounced dependence of PeL behavior on both particle size and chemical composition. By modifying the Ag/Na ratio and the dimension of the samples, local structural variations were detected through PDF analysis – Fig. 1a. Furthermore, XANES measurements show that the pre-edge feature is significantly broader in PeL bulk samples (such as SC x=0.8), suggesting the coexistence of Mn²⁺ and Mn³⁺ oxidation states – Fig. 1b. These results contribute to a deeper understanding of how both composition and particle size affect PeL, improving the performance of perovskite-based PeLMs.