T.E.M. study of Al2O3metastable phases

Abstract

Plasma spraying was employed to obtain rapidly solidified dense metastable alumina samples. They have been studied after being sprayed and in various annealed states by transmission electron microscopy and X-ray powder diffraction. The so-called “γ” phase has been imaged by T.E.M. and exhibits a more or less ordered domain structure with quasi-periodic 1/4 antiphase boundaries in the {100} planes of the defective spinel lattice. Heating “γ” between 850 and 1050°C leads to more ordered intermediate phases. They are shown to appear through a two-dimensional antiphase periodic boundaries mechanism. Aluminum vacant sites are likely located along the antiphase planes and their concentration (Al_2.66□_0.33O₄) is consistent with the observed periodicities. The “δ” and “θ” forms are considered as variants of this structure.