A new metastable alumina polymorph with monoclinic symmetry

Abstract

Polymorphic phase transitions between alumina structures based on fcc packing of oxygen have been studied by electron diffraction and highresolution electron microscopy. A new metastable alumina polymorph with monoclinic symmetry was identified in alumina obtained by both plasma spraying and thermal oxidation of aluminium. The structure of this phase was described by the P 2 / c space group and a unit cell containing 32 formula units 1 with lattice parameters a = 8.45 A, b = 16 A, c = 12.64 A and beta = 115o. It is suggested that the new phase is evolved from cubic gamma-Al O by ordering of 2 3 aluminium cations on interstitial sites of the fcc packed oxygen sublattice. The transformation proceeds through an orthorhombic distortion of the cubic anion structure. Crystallographic domain and interdomain boundaries in lambda-Al O were 2 3 identified and related to the symmetry changes which accompany gamma lambda phase transition.