Magnesium-25 Hyperfine Confirmation of the Localized-Ground-State Model of theV−Center in MgO

The localized-hole model of the $V^{-}$ center in the alkaline earth oxides is represented as a positive hole trapped on one oxygen atom adjacent to a positive-ion vacancy (in the ground state and on the time scale of an ESR experiment). Upon trapping the hole, the oxygen atom is presumed to relax away from the vacancy, thus lowering the symmetry from octahedral to tetragonal. The validity of this model has recently been challenged on the basis that a proper description of the ground state is a molecular orbital representing a linear combination of states with the hole on either of the two oxygen atoms opposite one another about the vacancy. We present the analysis of some ESR spectra in which we observed the resolved hyperfine splitting due to a single ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}_{}^{2+}{}_{}{}^{}$ ion adjacent to the trapped hole and on the axis of the defect. From the natural abundance of the ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}$ isotope one can calculate the expected hyperfine line intensities relative to that of the central line arising from $V^{-}$ centers having only ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ or ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$ atoms in the axial positions, for the localized and for the delocalized models. The measured relative intensities agree with those calculated for the localized model. An analysis of the axial spectrum indicates the presence of an unusually intense forbidden line at $X$ band. This becomes relatively more intense at the $\mathrm{Ku}$ band, as expected. Shifts of line position and intensity relative to the lines of an allowed hyperfine sextet agree with those predicted. The values of the hyperfine parameters are found to be $A=+2.84\mathrm{}$ MHz and $B=-0.86\mathrm{}$ MHz. The quadrupole parameter is found to be +0.18 MHz. The sign of $A$ is anomalous in that a contact interaction is expected to give an isotropic parameter with the sign of the nuclear magnetic moment. This may result from exchange polarization. The magnitude of $B$ is interpreted as indicating a compact unpaired charge distribution. The $V^{-}$ centers studied here were generated by fast-neutron irradiation, and there is no confusion with impurity-associated hole centers; the former have a long-term stability in MgO, while the latter decay rapidly to negligible levels at room temperature.