Unraveling the Origin of Structural Disorder in High Temperature Transition Al₂O₃: Structure of θ-Al₂O₃

The crystallography of transition Al 2 O 3 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 Al 2 O 3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al 2 O 3 and provide a structural description for θ-Al 2 O 3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al 2 O 3 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-Al 2 O 3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga 2 O 3, and the other on a monoclinic phase that is closely related to δ-Al 2 O 3 . The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al 2 O 3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.