Structures of Uncharacterised Polymorphs of Gallium Oxide from Total Neutron Diffraction

A structural investigation is reported of polymorphs of Ga2O3 that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of -Ga2O3 was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals -Ga2O3 has a defect Fd$\bar 3$m spinel structure, while pair distribution function analysis shows that the short-range structure is better modelled with local F$\bar 4$3m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga5O7(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, -Ga2O3, before transformation into -Ga2O3. In contrast, the thermal decomposition of Ga(NO3)39H2O first forms epsilon-Ga2O3 and then -Ga2O3. Examination of in situ thermodiffraction data shows that epsilon-Ga2O3 is always contaminated with -Ga2O3 and with this knowledge a model for its structure was deduced and refinedspace group P63mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close-packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured -Ga2O3; at the early stages of thermal decomposition of Ga(NO3)39H2O the first distinct phase formed is merely small particles of epsilon-Ga2O3.