Defect structure in transition-metal monoxides

The electronic structure and stabilization energy of metal vacancy and vacancy-interstitial clusters are studied in transition-metal monoxides using the first-principles local-density theory. The discrete-variational method in the embedded-cluster scheme is used to obtain both the one-electron properties and cohesive energies. Our calculations predict greater stability for 2:1 (vacancy:interstitial) defect structure over simple vacancies. This is in agreement with experiments for MnO and the heavier 3d compounds, but for TiO and VO lattice vacancies at the metallic or oxygen sites are experimentally predominant. The energy for the single metal vacancy is calculated to be close to the 2:1 defect energy, and so cluster-size effects and computational limitations need to be considered further. For FeO and CoO, however, the 4:1 interstitial complex is found to be more stable than other simple defects, in accord with experiment. The probability of formation of 4:1 clusters and their aggregates in TiO and MnO is explored; the interstitial metal ion tends to be in a trivalent state as previously determined for FeO and CoO