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

• Autor(a) principal: Daniel Sean Cook
• Data de Publicação: 2019
• Outros Autores: Reza J. Kashtiban, Klaus Wilhelm Heinrich Krambrock, Geraldo Magela de Lima, Humberto Osório Stumpf, Luciano Roni Silva Lara, José Domingos Ardisson, Richard I. Walton
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Institucional da UFMG
• Texto Completo: https://doi.org/10.3390/ma12050838; http://hdl.handle.net/1843/47196; https://orcid.org/0000-0002-3871-1647; https://orcid.org/0000-0002-7562-0285
• topic: Ferrite; Crystallisation; Magnetism; XANES; Mössbauer spectroscopy; EPR; Ferrita; Cristalização; Magnetismo

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 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 annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa₂O₄ is shown to contain octahedral Mn2+, along with some Mn3+, 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 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.