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

The synthesis of mixed-metal spinels based on substituted γ-Ga(2)O(3) 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 [Image: see text] (0.33)Ga(1.87)Cr(0.8)O(4) ([Image: see text] = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr(3+), which in turn implies a mixture of tetrahedral and octahedral Ga(3+), and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa(2)O(4) is shown to contain octahedral Mn(2+), along with some Mn(3+), 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(2+). 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 (57)Fe Mössbauer spectroscopy that shows the presence of Fe(2+) and Fe(3+) 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.