The atomic structure of $\gamma$-alumina, a metastable oxide central to applications ranging from heterogeneous catalysis to energy conversion and environmental remediation, has remained elusive for decades. This persistent uncertainty arises from the high concentration of aluminum vacancies and the difficulty of determining precise cation occupancy within the oxygen sublattice. Here, we address this challenge by combining aberration-corrected scanning transmission electron microscopy combined with ultrahigh-sensitivity energy-dispersive spectroscopy to investigate $\gamma {\text{-Al}}_2{\text{O}}_3$ formed during high-temperature oxidation of NiAl alloys. By integrating atomic-resolution imaging, elemental mapping, diffraction analysis, and density functional theory calculations, we identify a spinel-type $\gamma {\text{-Al}}_2{\text{O}}_3$ structure featuring a cubic close-packed oxygen framework, in which Al cations exclusively occupy octahedral interstitial sites. This site-selective occupancy leads to ordered distribution of aluminum vacancies, resolving a long-standing structural ambiguity regarding cation distribution in $\gamma {\text{-Al}}_2{\text{O}}_3$. Comparison with previous studies of hydroxide-derived $\gamma {\text{-Al}}_2{\text{O}}_3$ further reveals that the local structure of this metastable oxide is highly sensitive to synthesis pathway, precursor chemistry, and kinetic equilibration history. These results provide direct atomic-level insight into defect ordering and cation topology in $\gamma {\text{-Al}}_2{\text{O}}_3$, with implications for ion transport, catalytic activity, and thermal stability in alumina-based materials.

Close

PrevNext

View figure in article

• Crystal structure
• Defects
• Scanning transmission electron microscopy