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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Optical Materials

Background:

• Chromium-doped inorganic materials are crucial for near-infrared optical applications.
• The precise distribution and valence states of multivalent chromium (Cr) ions in these materials are often unclear.
• Understanding ion distribution is key to optimizing material properties.

Purpose of the Study:

• To synthesize and characterize Li(Sc1-xInx)O2:Cr phosphors with multivalent Cr ions.
• To investigate the distribution and valence state changes of Cr ions before and after washing.
• To elucidate the electron-lattice interactions influencing optical properties.

Main Methods:

• High-resolution synchrotron X-ray diffraction (XRD) for phase analysis.
• Cr K-edge X-ray absorption spectroscopy (XAS) for valence state determination.
• X-ray fluorescence (XRF) and X-ray excited optical luminescence (XEOL) for ion distribution.
• Photoluminescence (PL) and decay curve analysis for optical properties.

Main Results:

• Two phases were identified in Li(Sc1-xInx)O2, confirmed by XRD and Raman spectra.
• As-prepared samples predominantly contained Cr6+ ions, which transformed to dominant Cr3+ after water washing.
• Cr6+ ions were found to aggregate, while Cr3+ ions exhibited even distribution.
• Electron-lattice interactions were resolved, explaining spectral asymmetries.

Conclusions:

• This work clarifies the distribution of low-concentration multivalent ions in solid-state materials.
• Washing provides a method to control Cr ion distribution and valence state.
• The findings enhance the understanding of Cr-doped phosphors for optical applications.