Resonance Properties ofEr3+in Thorium Oxide

An electron-spin-resonance study at 35 GHz of ${\mathrm{Er}}^{3+}$ in Th${\mathrm{O}}_2$ has distinguished three types of erbium sites, one having cubic symmetry and the other two having $g$ values with axial symmetry about a [111] crystallographic axis. Pertubation calculations based on a point-charge model suggest that the axial $g$ values result from the presence of an interstitial charge, in one case positive and in the other case negative, located in an oxygen cube of the Th${\mathrm{O}}_2$ lattice, adjacent to the site of the ${\mathrm{Er}}^{3+}$ along the [111] axis.