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(Sc _{1- x} In _x )O ₂ :Cr phosphors containing both Cr ³⁺ /Cr ⁶⁺ ions are prepared. High-resolution synchrotron X-ray diffraction (XRD) reveals two similar phases in Li(Sc _{1 -x} In _x )O ₂ . Raman spectra further confirm distinct scattering patterns for the two end-member compositions, corroborating the findings from the synchrotron XRD analysis. Cr K -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 ⁶⁺ , while Cr ³⁺ becomes dominant after washing with water. Moreover, the source and distribution of Cr ³⁺ and Cr ⁶⁺ 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 ⁶⁺ ions aggregate within the sample, while Cr ³⁺ 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.