The crystallography of transition Al2O3 has been extensively studied in the past, because of the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al2O3 and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at least two different end-members. By correlating imaging and spectroscopy results with density functional theory (DFT) calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely related to δ-Al2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.
By correlating imaging and spectroscopy results during in situ heating with DFT calculations, the authors propose a model for θ-Al2O3 as a disorder intergrowth of two crystallographic variants at the unit cell level.