Structural complexity of γ-Al2O3: The nature of vacancy ordering and the structure of complex antiphase boundaries

The structure of γ-Al2O3 remains undetermined despite decades of research. This is due to the high degree of disorder, which poses significant challenges for structural analysis using conventional crystallographic approaches. Herein, we study the structure of γ-Al2O3 with scanning transmission electron microscopy (STEM) and ab-initio calculations and show that the structure can be interpreted as a domain microstructure of defective spinel interconnected via non-spinel segments that form the basis of antiphase boundaries (APBs). The spinel domains have a distinctive preference for vacancy ordering, which can be rationalized in terms of a structure with a stacking disorder. Tetragonal P41212 or monoclinic P21 models, all based on an identical motif but with a different stacking sequence, can be considered as representative ordered forms. Individual spinel domains are interconnected via distinct non-spinel bonding environment of δ-Al2O3, which we assign as complex APBs. The most common cAPB consists of a single delta motif with thickness of just 0.6 nm on (001), with the resulting displacement a/4 [101]. Remarkably, the cAPBs are shown to energetically stabilize the spinel domains of γ-Al2O3 explaining their high abundance. We demonstrate how the tetragonal distortions naturally arise in this intricate microstructure and place the proposed model in the context of phase transformations to high-temperature transition aluminas.