A new metastable alumina polymorph with monoclinic symmetry

摘要

Polymorphic phase transitions between alumina structures based on fcc packing of oxygen have been studied by electron diffraction and high-resolution 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 P2_1/c space group and a unit cell containing 32 formula units with lattice parameters a = 8.45 A, b = 16 A, c = 12.64 A and β = 115°. It is suggested that the new phase is evolved from cubic γ-Al_2O_3 by ordering of 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 λ-Al_2O_3 were identified and related to the symmetry changes which accompany γ → λ phase transition.