Characterization of Structural Disorder in 纬-Ga2O3

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• 作者：Helen. Y. Playford ; Alex C. Hannon ; Matthew G. Tucker ; Daniel M. Dawson ; Sharon E. Ashbrook ; Reza J. Kastiban ; Jeremy Sloan ; Richard I. Walton
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：July 24, 2014
• 年：2014
• 卷：118
• 期：29
• 页码：16188-16198
• 全文大小：697K
• ISSN：1932-7455

文摘

Solvothermal oxidation of metallic gallium in monoethanolamine for 72 h at 240 掳C yields a crystalline sample of 纬-Ga₂O₃ (30 nm crystallites). While Rietveld refinement (cubic spinel structure, Fd3̅m; a = 8.23760(9) 脜) 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鈥揋a contacts produced if neighboring ideal spinel and nonspinel sites are simultaneously occupied. To create an atomistic model to reconcile this situation, a 6 脳 6 脳 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 尾-Ga₂O₃. ⁷¹Ga solid-state NMR results reveal a octahedral:tetrahedral Ga ratio that is consistent with the model obtained from RMC. Nanocrystalline samples of 纬-Ga₂O₃ are produced by either a short solvothermal reaction (240 掳C for 11 h in diethanolamine; 15 nm crystallites) or by precipitation from an ethanolic solution of gallium nitrate (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鈥揙 distance with decreasing particle size, consistent with ⁷¹Ga 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.