It was recalled that the vacancy in this material was a fundamental defect which had been studied theoretically and experimentally for 4 decades. It was noted that, although early theories had been extremely successful in explaining the nature of the ground state of the neutral defect, and the expected Jahn-Teller distortion, several aspects had not been satisfactorily addressed; such as many-electron effects and configuration interactions. The latter determined not only the order of electronic levels in the vacancy, but also the GR1 optical transition; which could not be expressed in terms of 1-electron levels alone. Recent experimental data on the various charge states and excited states of the vacancy were reviewed here in order to identify a simple empirical model for the defect. It was shown that the stability of the states, and their photoconductivity, could be reproduced. It was predicted that other states of the neutral vacancy, as observed by means of electron paramagnetic resonance, were very close to the ground state, and might lead to another high-energy optical transition.

A.Mainwood, A.M.Stoneham: Journal of Physics - Condensed Matter, 1997, 9[11], 2453-64