Structures of uncharacterised polymorphs of gallium oxide from total neutron diffraction

Abstract

A structural investigation is reported of polymorphs of Ga(2)O(3) that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ-Ga(2)O(3) was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ-Ga(2)O(3) has a defect Fd3m spinel structure, while pair distribution function analysis shows that the short-range structure is better modelled with local F43m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga(5)O(7)(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ-Ga(2)O(3), before transformation into β-Ga(2)O(3). In contrast, the thermal decomposition of Ga(NO(3))(3)·9H(2)O first forms ε-Ga(2)O(3) and then β-Ga(2)O(3). Examination of in situ thermodiffraction data shows that ε-Ga(2)O(3) is always contaminated with β-Ga(2)O(3) and with this knowledge a model for its structure was deduced and refined--space group P6(3)mc 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 δ-Ga(2)O(3); at the early stages of thermal decomposition of Ga(NO(3))(3)·9H(2)O the first distinct phase formed is merely small particles of ε-Ga(2)O(3).