Cubic to monoclinic phase transformations in alumina

Cubic to monoclinic phase transformations in alumina were studied by high-resolution electron microscopy of anodic films annealed at 1200 degrees C. In addition to the known monoclinic theta-Al2O3 structure a new metastable alumina polymorph was identified also with apparent monoclinic symmetry. This new phase appears to evolve from gamma-alumina by cation ordering. The stoichiometric structure of this polymorph is consistent with the C2/m space group and unit cell containing 40 ions (8Al(2)O(3)). Reasonably good fit between experimental and computer-simulated lattice images was obtained for the suggested structural model. The transformations of gamma-Al2O3 to both monoclinic alumina structures were assumed to be non-reconstructive and the possible transformation paths were analyzed in terms of symmetry group/subgroup relationships. (C) 1997 Acta Metallurgica Inc.