STRUCTURES AND TRANSFORMATION MECHANISMS OF THE ETA, GAMMA AND THETA TRANSITION ALUMINAS

The defect crystal structures of eta-, gamma- and theta-alumina obtained from dehydroxylation of well crystallized bayerite and boehmite have been derived from the analysis of their X-ray powder diffraction patterns and from the Rietveld refinement of their neutron powder diffraction patterns. Profile analysis of the various reflection zones in these defect spinel structures shows different coherent domain sizes which can be associated with the tetrahedral and octahedral aluminium and the oxygen sublattices. These observations have been used to define the nature of the crystal structures, and to give insight into the transformation mechanisms. The very large surface energies of these phases are evident in the observation of a nearly three-coordinated surface Al atom in the eta phase, and are the reason for the stability of the defect spinel structures of the transition aluminas. The reduction of surface area and ordering of the tetrahedral Al sublattice which occurs on heating causes the spinel framework to collapse so that the structure, which exhibits tetragonal character at the early stage of the transition, settles into monoclinic theta-alumina displacively at the later stage, and eventually transforms to hexagonal corundum reconstructively. Thus theta-alumina should be considered the ultimate rather than the intermediate structural form into which the transition aluminas could evolve on the way to corundum. The overall crystal structure of the transition aluminas should therefore be viewed intrinsically as spinel deformed rather than as tetragonally deformed. Crystal data at room temperature: eta-alumina, cubic, Fd3m, a = 7.914 (2) angstrom, R(B) = 6.24, R(p) = 6.50, R(w) = 8.43%; gamma-alumina, cubic, Fd3m, a = 7.911 (2) angstrom, R(B) = 10.53, R(p) = 7.61, R(w) = 10.25%; theta-alumina, monoclinic, C2/m, a = 11.854 (5), b = 2.904 (1), c = 5.622 angstrom, beta = 103.83 (7)-degrees, R(B) = 15.02, R(p) = 9.37, R(w) = 11.93%.