PNNL

Abstract

Metastable polymorphs of Al2O3 accommodate a large amount of structural disorder that has proven difficult to quantify. In the case of delta-Al2O3, the nature of crystallographic disorder has recently started to emerge as a planar structural intergrowth. In this work, which uses aberration corrected HAADF STEM imaging and ab initio calculations, we reveal a new type of structural intergrowth in delta-Al2O3, and show that it is an important part of the complex/defected delta-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 d2-Al2O3, d3 Al2O3 and d4-Al2O3. As a part of this work, we derive the crystallography and structural relationship between the variants, and we discuss the overall crystallography of d-Al2O3 in the context of previous structural interpretations. Finally, it is shown that the individual ? Al2O3 variants can be related to other metastable polymorphs that are known to evolve from gamma-Al2O3.