Electron Spin Resonance from a Vacancy-Pair Center in Magnesium Oxide

We report both experimental and theoretical investigations of a vacancy-pair center, the ${F_c}^{+}$ center, in the alkaline-earth oxides. For MgO, CaO, and SrO the $g$ tensor of the ESR spectrum is axially symmetric about a $〈100〉$ crystal axis. Measurements in MgO confirm that the most important hyperfine interaction occurs with the single nearest-neighbor ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}$ nuclide, $I_{\gamma }$, situated along the tetragonal axis of the defect. For this interaction we determine experimentally that $A_{\gamma }=17.5\mathrm{}$G and $B_{\gamma }=0.7\mathrm{}$G. The interaction with four equivalent nearest-neighbor ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}$ nuclides, $I_{\alpha }$, situated perpendicular to the defect axis is much weaker. Our calculations, which assume a simple dielectric continuum model in which an electron is bound to the finite field of an electric dipole, yield binding energies and hyperfine coupling constants which are in good agreement with experimental results for MgO.