T.E.M. study of Al2O3metastable phases

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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 (Al2.66□0.33O4) is consistent with the observed periodicities. The “δ” and “θ” forms are considered as variants of this structure.