STRUCTURE REFINEMENT OF GAMMA ALUMINA - REVISITED. Bee K. Gan, Brian H. O'Connor, Department of Applied Physics, Curtin University of Technology, GPO Box U1987, Perth6001 ,Western Australia

The extensive literature on gamma alumina has established that the structure approximates the spinel, AB2O4 form, with either a cubic or tetragonally distorted cell. Various authors have proposed different models for the locations and site-occupancies of the aluminium ions. From the literature, it is clear that the structure of gamma alumina have not been established unequivocally. It was therefore seen to be of interest in this study to re-examine the gamma alumina structure.

Deuterated gibbsite specimens were used as starting materials to study the transformation from gibbsite to boehmite, which then transformed to gamma alumina. Gibbsite was selected because of its availibility as one of the products produced from bauxite by the Bayer process by Alcoa of Australia Limited, Western Australia.

The present neutron diffraction data and Rietveld refinement results have shown that two different polymorphs of gamma alumina have been found to co-exist, which has resolved the long-standing dispute over the nature of the structure, with either the cubic or tetragonal models. In this study, two conclusions have been note: (i) the boehmite to gamma alumina transformation occurs at approximately 500 C, resulting in the formation of a dominant tetragonal phase and a smaller fraction of tghe cubic phase and (ii) the proportion of the tetragonal phase increases at the expense of the tetragonal phase over the range 500-900 C, before transforming to higher temperature transition aluminas.