The authors have investigated in detail the two models of the V- centre, where a hole is trapped at a cation vacancy. These are the model of Bartram, Swenberg and Fourn (1965), where optical transitions occur within an O- ion, and the model of Schirmer, Koidl and Reik (1974), which involves a hole hopping from one oxygen ion to another. The calculations show (i) that the hole should be self-trapped on a single oxygen, as observed, (ii) that the predictions of ground-state ionization energy and of elastic and electrical dipole moments (apart from some uncertainties in local-field corrections) agree adequately with measurements, (iii) that the transitions within an O- ion appear to account for observed structure at around 1.5 eV, both in energy and oscillator strength, and (iv) that the observed 2.3 eV band corresponds to the transition suggested by Schirmer et al. Calculations are also given for some centres related to the V- centre, V0, V(Al)- and (Na)0, again predicting optical absorption in good agreement with experiment
