Structural Intergrowth in δ‑Al2O3

Metastable polymorphs of Al2O3 accommodate a large amount of structural disorder that has proven difficult to quantify. In the case of δ-Al2O3, the nature of crystallographic disorder has recently started to emerge as a planar structural intergrowth. In this work, which uses aberration-corrected high-angle annular dark-field scanning transmission electron microscopy imaging and ab initio calculations, we reveal a new type of structural intergrowth in δ-Al2O3 and show that it is an important part of the complex/defected δ-Al2O3 microstructure. The newly identified intergrowth is different from the previously known type due to intergrowth direction and variant selection. In its most basic representation, the new intergrowth consists of three variants of δ2-Al2O3, δ3-Al2O3, and δ4-Al2O3. As a part of this work, we derive the crystallography and structural relationship between the variants and discuss the overall crystallography of δ-Al2O3 in the context of previous structural interpretations. Finally, it is shown that the individual δ-Al2O3 variants can be related to other metastable polymorphs that are known to evolve from γ-Al2O3.