Polymorphic phase transitions, between alumina structures that were based upon face-centered cubic packings of O, were studied by means of electron diffraction and high-resolution electron microscopy. A new metastable alumina polymorph, with monoclinic symmetry, was identified in alumina samples which were obtained by plasma spraying and thermal oxidation of Al. The structure of this phase belonged to the P21/c space group, with a unit cell that contained 32 formula units and had the lattice parameters: a = 0.845, b = 1.6, c = 1.264nm, β = 115º. It was suggested that the new phase evolved from cubic γ-Al2O3 via the ordering of Al cations on the interstitial sites of the face-centered cubic O sub-lattice. The transformation occurred via an orthorhombic distortion of the cubic anion structure. Crystallographic domain and inter-domain boundaries in λ- Al2O3 were identified and were related to symmetry changes which accompanied the phase transition.

I.Levin, D.G.Brandon: Philosophical Magazine Letters, 1998, 77[2], 117-24