Structure of theF+center in magnesium oxide

Electron-nuclear double resonance has been used to determine nuclear hyperfine interactions for ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}$ nuclei near an $F^{+}$ center in MgO. Isotropic, anisotropic, and quadrupole couplings have been measured for nuclei in the third and fifth shells surrounding the vacancy, including nonaxial terms for the low-symmetry fifth-shell site. The quadrupole terms are sensitive to contributions from lattice relaxations around the defect, and a rigid-ion model is used to extract these relaxations. We find an outward motion of 6% of the near-neighbor separation for the first ionic shell, and smaller outward motions for shells three, four, and five. The net volume of formation for the defect is approximately 4 ${\mathrm{cm}}^3$/mole. The values are shown to be relatively insensitive to the limitations of the model. A relatively large isotropic interaction with fifth-shell nuclei can be quantitatively explained by including indirect overlaps via the second-neighbor oxygens. Analysis of the anisotropic dipolar terms gives direct confirmation of these large second-neighbor overlaps, and suggests some 20% of the total charge is localized in oxygen $p_{\sigma }$ lobes outside the second-neighbor shell.