Determination of the Crystal Structure of Gamma-Alumina by Electron Diffraction and Electron Energy-Loss Spectroscopy

Abstract

Gamma-alumina (γ-Al2O3) is a metastable phase of Al2O3 with wide-ranging applications in fields such as catalysis due to its high surface area-to-volume ratio and surface properties. Yet despite heavy interest in and study of γ-Al2O3, its true crystal structure is still debated. γ-Al2O3 is traditionally described as having a spinel structure, but with cation vacancies added to maintain the correct stoichiometry. However, many different spinel and nonspinel structures for γ-Al2O3 have since been proposed. This is a problem for theoretical simulations of systems containing γ-Al2O3, since a model of γ-Al2O3 must be selected from the many models described in the literature. At present, the choice of model is often based on computational considerations rather than the accuracy of the model to experiment. Thus, clarification is needed on the accuracy of the existing models, and if possible, what the true structure of γ-Al2O3 is. A major contributing factor to the uncertainty surrounding the structure of γ-Al2O3 is the heterogeneity of commercially available γ-Al2O3. To mitigate this, we synthesized near-single-crystal (SC) γ-Al2O3 thin films by controlled oxidation of single-crystal NiAl (110) and characterized these films using TEM, selected-area electron diffraction (SAED), and electron energy-loss spectroscopy (EELS). Through correlating these experimental measurements with simulated electron diffraction and EELS, we gauged the accuracy of the most commonly cited γ-Al2O3 models. We considered the following γ-Al2O3 models: Smrcok cubic spinel [1], Paglia tetragonal nonspinel [2], Digne monoclinic nonspinel [3], and the Pinto monoclinic spinel [4]. From the near-SC γ-Al2O3 ~80 nm thin film, a cross-sectional TEM sample was prepared using focused ion beam (FIB). A polycrystalline γ-Al2O3 thin film was also synthesized, by extending the growth duration, and transferred to a carbon-coated TEM grid. SAED was acquired from both samples, producing spot patterns from the near-SC film and ring patterns (e.g., Figure 1a) from the polycrystalline film. The ring pattern was converted to the line profile by azimuthally averaging the intensity of the rings and is plotted in Figure 1b along with the equivalent simulated diffraction profiles for each model. Comparison of peak positions shows that the Digne model, arguably the most commonly used model in theoretical studies (by citation numbers), is the least accurate model. Furthermore, comparison of peak intensities suggests that the spinel models are more accurate than the Paglia nonspinel model. We are now working to determine the distribution of Al cations in γ-Al2O3 based on our data. While SAED provides information on the unit cell dimensions and atomic distribution in crystallographic sites, EELS provides complimentary information about chemical state (and thus binding) by probing the electronic structure. Al L2,3 and O K edge core-loss EELS spectra were acquired from the SC γ-Al2O3 film