Hidden Hexavalent Chromium Ions with Subtle Structural Evolution in Near-Infrared Phosphors.

Cr-doped inorganic materials are pivotal in developing near-infrared optical materials; however, multivalent Cr ions and their respective distribution in the materials remain ambiguous. Herein, a series of Li(ScIn)O:Cr phosphors containing both Cr/Cr ions are prepared. High-resolution synchrotron X-ray diffraction (XRD) reveals two similar phases in Li(ScIn)O. Raman spectra further confirm distinct scattering patterns for the two end-member compositions, corroborating the findings from the synchrotron XRD analysis. Cr -edge X-ray absorption near-edge structure and extended X-ray absorption fine structure demonstrate that most Cr ions in the as-prepared samples are Cr, while Cr becomes dominant after washing with water. Moreover, the source and distribution of Cr and Cr ions in the as-prepared and washed samples are revealed through X-ray fluorescence and X-ray excited optical luminescence techniques, which indicate that Cr ions aggregate within the sample, while Cr ions are evenly distributed. Photoluminescence, decay curves, and line shape analyses are implemented to resolve the electron-lattice interactions, and the corresponding mechanisms are provided to explain the asymmetry between photoluminescence and photoluminescence excitation spectra. Overall, this study provides valuable insights into the distribution of low-concentration multivalence ions in solid-state materials and offers a deeper understanding of the approaches to precisely resolve the subtle changes in the crystal structure.