On the basis of shell-model and embedded-cluster simulations, the nature of holes trapped at acceptor defects in this material was characterized. First-principles calculations were based upon Hartree-Fock theory, Møller-Plesset perturbation theory and density functional theory. It was deduced that all hole states were of comparable energy. Most favorable were the delocalized state and the formation of Vk centers. However, there was no clear evidence in favor of the latter hole states. Overall, the results suggested that a delicate balance existed between the local electronic structure and defect-induced lattice distortions. It was noted that increasing acceptor ionicity would result in the formation of completely localized off-acceptor holes, whereas a reduced ionicity would lead to the delocalization of hole states. The formation of Vk centers was suggested to be most favorable within a narrow intermediate range. It was suggested that this explained why Vk centers were rarely observed in oxides.

H.Donnerberg: Materials Science Forum, 1997, 239-241, 341-4