Nanocrystalline transition-metal gallium oxide spinels from acetylacetonate precursors via solvothermal synthesis

Abstract

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 Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed thus formed, with composition □0.33Ga1.87Cr0.8O4 (□ = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr3+, which in turn implies a mixture of tetrahedral and octahedral Ga3+, and the material is stable on to at least 850 °C. An analogous manganese material with average chemical composition MnGa2O4 is shown to contain octahedral Mn2+, along with some Mn3+, but a different inversion to materials reported by conventional solid-state synthesis in the literature, which are are known to have a significant proportion of tetrahedral Mn2+. 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 57Fe Mössbauer spectroscopy that shows the presence of Fe2+ and Fe3+ 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.