The structure of δ-alumina evolved from the melt and the γ → δ transformation

Abstract

Electron diffraction patterns are used to analyze the structure of a metastable alumina (δ) found in powders <10 μm in diameter produced by atomization of molten Al₂O₃. The δ phase is believed to form directly from the liquid and also by solid state transformation of the cubic spinel γ. Both phases are based on an f.c.c. packing of anions with the interstitial cations having a higher degree of order in the δ phase. The γ → δ transformation begins with the ordering of tetrahedral cations in domains ~1-2 nm in size. The intensity distribution in reciprocal space indicates that the transformation is continuous from the spots of the disordered spinel, through diffuse scattering and finally the appearance of discrete superlattice reflections of the δ-phase. The δ is orthorhombic with crystal axes parallel to those of the parent spinel and unit cell dimensions that are simple multiples of the lattice parameter of γ. Two variants of the structure were observed with lattice parameters a_δ = 2a_γ, b_δ = 1.5a_γ, c_δ = either a_γ or 2a_γ. The allowed reflections indicate a space group of P2₁2₁2₁ for the structure with c_δ = a_γ, which is the more commonly observed.