The electronic structure and interatomic bonding of a near-11 a-axis tilt grain boundary were determined by using first-principles calculations. This boundary, which had a low grain-boundary energy, had already been studied by using high-resolution transmission microscopy and pair-potential methods. The relaxed structural model, which contained 72 Al atoms and 108 O atoms, was periodic in 2 dimensions and reproduced the grain boundary structure as revealed by high-resolution transmission electron microscopy. In order to identify accurately the changes which arose in the boundary, parallel calculations were also made of bulk super-cell models which involved the same number of Al and O atoms; with and without surfaces. This grain boundary did not have deep levels in the band-gap, and its defect states appeared mainly near to the O-2p derived valence band maximum and the Al-3s conduction band minimum. The calculated partial densities of states for Al in the conduction band region were in good agreement with measurements of the near-edge energy loss spectrum and the valence-electron energy-loss spectrum. It was found that, in the vicinity of the grain boundary, there was an increased charge transfer, from Al to O, that arose mainly from the lower coordination number of the grain boundary atoms and the reduction in covalent bond formation. It was also found that, as the Al-O bond lengths decreased, Al-O charge transfer increased. For this 11 boundary, the structural features that were mainly associated with grain boundary electronic structure changes were under-coordination and shortened bond-lengths.

S.D.Mo, W.Y.Ching, R.H.French: Journal of the American Ceramic Society, 1996, 79[3], 627-33