Reversible Structural and Colorimetric Transitions in LuMnGaO4 upon Oxygen Uptake and Release

Abstract

We investigate the synthesis, structural analysis, and characterization of LuMnGaO₄, an AB₂O₄-type compound, and focus on its reversible oxygen uptake and distinct color change upon oxidation. The reduced phase, LuMnGaO₄, synthesized via solid-state reactions, crystallizes in the R-3m space group. Heating this metal oxide in an oxygen-rich environment induces a phase transition to the oxidized phase, LuMnGaO_4.5, which adopts the P-3 space group. Through neutron diffraction studies, we elucidate the structural transition upon oxygen uptake, while in-situ synchrotron X-ray powder diffraction confirms a low temperature (~ T = 250 °C) transition. The oxidation process induces a significant color shift from greenish-grey to black, and we quantify this electronic transition by UV-visible spectroscopy. Electron diffraction and synchrotron X-ray data further reveal structural modulations in the form of superlattice reflections and diffuse scattering associated with oxygen disorder in the oxidized phase. The diffraction data show that the reversible oxygen uptake and release occurs in the bulk phase at relatively low temperatures, suggesting applications in oxygen transport technologies. Furthermore, the distinct color change highlights its potential as a bifunctional material for oxygen carriers and colorimetric oxygen sensors. This work provides a foundation for future exploration of the structural and electronic evolution of AB₂O₄-type compounds during oxygen uptake and release.