Structure of δ-Alumina: Toward the Atomic Level Understanding of Transition Alumina Phases

Transition Al2O3 derived from thermal decomposition of AlOOH Boehmite have complex structures and to a large extent remain poorly understood. Here, we report a detailed atomic level analysis of beta-Al2O3 for the first time using a combination of high-angle annular dark field electron microscopy imaging, X-ray diffraction refinement, and density functional theory calculations. We show that the structure of delta-Al2O3 represents a complex structural intergrowth from two main crystallographic variants, which are identified as delta-Al2O3 and delta-Al2O3. The two main variants are fully structurally described, and in addition, we also derive their energy of formation. On the basis of comparison with other relevant transition Al2O3 phases, it is shown how energetic degeneracy leads to the structural disorder and complex intergrowths among several transition Al2O3. The results of the work have important implications for understanding thermodynamic stability and transformation processes in transition alumina.