Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences

The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close‐packed oxygen sublattice (either face‐centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face‐centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.