Nanocrystalline Transition-Metal Gallium Oxide Spinels from Acetylacetonate Precursors via Solvothermal Synthesis

The synthesis of mixed-metal spinels based on substituted γ-Ga₂O₃ is reported using metal acetylacetonate precursors in solvothermal reactions with alcohols as solvents at 240 °C. New oxides of Cr, Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen by high-resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed oxide is thus formed, with composition Ga Cr O₄ ( = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr , which in turn implies a mixture of tetrahedral and octahedral Ga , and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa₂O₄ is shown to contain octahedral Mn , along with some Mn , but a different inversion factor to materials reported by conventional solid-state synthesis in the literature, which are known to have a significant proportion of tetrahedral Mn . In the case of iron, higher amounts of the transition metal can be included to give an Fe:Ga ratio of 1:1. Elemental mapping using energy dispersive X-ray spectroscopy on the TEM, however, reveals inhomogeneity in the distribution of the two metals. This is consistent with variable temperature Fe Mössbauer spectroscopy that shows the presence of Fe and Fe in more than one phase in the sample. Variable temperature magnetisation and electron paramagnetic resonance (EPR) indicate the presence of superparamagnetism at room temperature in the iron-gallium oxides.