Electronic structure and chemical bondings of MgCr2-xO4

Cr-vacancies are known to be abundant in MgCr2O4 which have been thought to contribute to p-type conduction. In order to understand the electronic mec hanism behind the formation of Cr-vacancies and spatial distribution of hol es associated with the Cr-vacancy, first principles molecular orbital calcu lations of MgCr2O4 and its isostructural compound, MgAl2O4, have been syste matically performed using model clusters. The holes are found to be almost confined to an Al-vacancy in MgAl2O4. The spatial distribution of holes is wider in the case of a Cr vacancy in MgCr2O4. However, the major part of th e hole-charge is localized on the neighboring oxygen ions, not on Cr-ions. Covalent bonding between Cr and O is markedly reinforced by the formation o f Cr-vacancies in MgCr2O4. At the same time, the decrement of the ionic bon ding is much smaller also in MgCr2O4. These two factors are expected to pla y important roles for the relative stability of Cr-vacancies in MgCr2O4 as compared with MgAl2O4.