Electronic Structure of a Near Σ11 a‐axis Tilt Grain Boundary in α‐A12O3

Abstract

The electronic structure and interatomic bonding of a near Σ11 α‐axis tilt grain boundary in α‐A12O3 are determined using first‐principles calculations based on a model structure constructed by Kenway. This Σ11 boundary, which has a low grain boundary energy, has been the focus of various previous studies, including high‐resolution transmission microscopy (HRTEM) and pair potential based structural studies. The relaxed structure model, which contains 72 Al atoms and 108 O atoms, is periodic in two dimensions and reproduces the grain boundary structure obtained by HRTEM. To accurately identify the changes arising in the boundary, parallel calculations on the bulk supercell models containing the same number of Al and O atoms with and without surfaces were also carried out. This grain boundary does not have deep levels in the band gap, and its defect states appear mainly near the O‐2p derived valence band maximum and the Al‐3s derived conduction band minimum. The calculated partial densities of states for Al in the conduction band region are in good agreement with recent measurements on the near‐edge energy loss spectrum and the valence electron energy loss spectrum. By means of Mulliken population analysis, the effective charges, the bond order, and the partial density of states of the grain boundary atoms are identified and elucidated. In the region of the grain boundary, there is increased charge transfer from Al to O, arising mainly from the lower coordination number of the grain boundary atoms and therefore the reduction in the covalent bond formation. In addition, it is found that as the Al‐O bond lengths decrease, the Al—O charge transfer increases. For this Σ11 boundary, the structural features most associated with grain boundary electronic structure changes are undercoordination and shortened bond lengths.