Characterization of Structural Disorder in γ-Ga2O3

Solvothermal oxidation of metallic gallium in monoethanolamine for 72 h at 240 degrees C yields a crystalline sample of gamma-Ga2O3 (similar to 30 nm crystallites). While Rietveld refinement (cubic spinel structure, Fd (3) over barm; a = 8.23760(9) angstrom) reveals that Ga occupies two pairs of octahedral and tetrahedral sites (ideal spinel and nonspinel), it provides no information about their local distribution, which cannot be statistical owing to the short Ga-Ga contacts produced if neighboring ideal spinel and nonspinel sites are simultaneously occupied. To create an atomistic model to reconcile this situation, a 6 x 6 x 6 supercell of the crystal structure is constructed and refined against neutron total scattering data using a reverse Monte Carlo (RMC) approach. This accounts well for the local as well as long-range structure and reveals significant local distortion in the octahedral sites that resembles the structure of thermodynamically stable beta-Ga2O3. Ga-71 solid-state NMR results reveal a octahedral:tetrahedral Ga ratio that is consistent with the model obtained from RMC. Nanocrystalline samples of gamma-Ga2O3 are produced by either a short solvotherrnal reaction (240 degrees C for 11 h in diethanolamine; similar to 15 nm crystallites) or by precipitation from an ethanolic solution of gallium nitrate (similar to 5 nm crystallites). For these samples, the Bragg scattering profile is broadened by their smaller crystallite size, consistent with transmission electron microscopy results, and analysis of the relative Bragg peak intensities provides evidence that a greater proportion of tetrahedral versus octahedral sites are filled. In contrast, neutron total scattering shows the same average Ga-O distance with decreasing particle size, consistent with Ga-71 solid-state NMR results that indicate that all samples contain the same overall proportion of octahedral:tetrahedral Ga. It is postulated that increased occupation of tetrahedral sites within the smaller crystallites is balanced by an increased proportion of octahedral surface Ga sites, owing to termination by bound solvent or hydroxide.