Resonance properties of Er3+ in thorium oxide

An electron-spin-resonance study at 35 GHz of Er³⁺ in ThO₂ 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 ThO₂ lattice, adjacent to the site of the Er³⁺ along the [111] axis.