Determination of the vacancy distribution over Al cation sites in -Al₂O₃

Although gamma-alumina (gamma-Al2O3) is an extensively used material with wide-ranging applications due to its inherently high surface area and acidity, its atomic structure is still not fully understood. gamma-Al2O3 is described as having a spinel-like structure, where the O sublattice has a face-centered cubic (FCC) arrangement and Al cations are placed in the spinel tetrahedral and octahedral interstitial sites. Achieving the correct stoichiometry of Al2O3, however, requires the introduction of Al vacancies into some of the interstitial sites. Despite the importance of accurately describing the structure of gamma-Al2O3, the distribution of vacancies between tetrahedral and octahedral sites remains unclear, in part because of the usually poor crystalline quality of gamma-Al2O3 that has often been used in previous studies. To determine the actual cation distribution in gamma-Al2O3, single-crystalline gamma-Al2O3 was investigated using a correlative approach of experimental and simulated selected-area electron diffraction (SAED) and high-resolution electron energy-loss spectroscopy (EELS). Comparison of the reflection intensities in single-crystal SAED to simulated SAED from models with varied vacancy distributions revealed that vacancies exist primarily on tetrahedral sites, contrary to the placement of vacancies on octahedral sites proposed in several common models. Comparison of EELS spectra-acquired with the highest energy resolution reported so far for gamma-Al2O3-with ab initio multiple scattering EELS simulations confirmed the distribution of vacancies on tetrahedral sites. These results enable more accurate modeling of gamma-Al2O3 to better predict its properties in existing and future applications.