The ground state of two-hole centres in oxides

The nature of the ground state of the V centre in ionic oxides is discussed, where two holes are trapped by a cation vacancy. Experiment appears to indicate an S=1 ground state for BeO and Al 2 O 3 and possibly ZnO, with S=0 in MgO and CaO. Simple theoretical arguments suggest S=0. It is shown that a weak mixture of (O 0 O 2- ) configurations into the dominant (O - O - ) configuration can explain the observations. Quantitative calculations are given for MgO, and suggest the singlet state is lowest by about 30 cm -1 .