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 δ-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 δ-Al2O3 represents
a complex structural intergrowth from two main crystallographic variants,
which are identified as δ1-Al2O3 and δ2-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.