Electron-nuclear-double-resonance investigations of[Li]0and[Na]0centers in MgO, CaO, and SrO

Electron-nuclear-double-resonance (ENDOR) measurements of the ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ and ${\mathrm{}\left[\mathrm{Na}\right]\mathrm{}}^0$ centers in the alkaline-earth oxides, MgO, CaO, and SrO, are reported. These centers, formed by the trapping of holes generated by ionizing radiation at 77 K, are not stable at room temperature. The investigation established that the symmetry of these trapped-hole centers is exactly axial along a direction and that the monovalent alkali impurity ion occupies a substitutional cation site. The spin-Hamiltonian parameters obtained show that the isotropic and anisotropic portions of the magnetic hyperfine tensor have opposite signs for all six systems. Interpretation of the latter as pure dipolar implies that there is outward relaxation of the axial oxygens upon the introduction of the smaller lithium ion but negligible relaxation in the case of the larger sodium ion. Conclusive evidence for hopping of the positive hole between the six nearest oxygen neighbors at temperatures as low as 1.5 K was seen in the ENDOR spectra. The electric field gradients appear to be correlated with the ion positions in a manner which can be reasonably understood on the basis of a simple point charge and dipole model.