Comparison of Electron-Nuclear Double Resonance Spectra of ${\mathrm{Ce}}^{3+}$-${\mathrm{H}}^{-}$ and ${\mathrm{Ce}}^{3+}$-${\mathrm{F}}^{-}$ in Calcium Fluoride

From electron-nuclear double resonance spectra we deduce that the tetragonal EPR spectra of rare-earth ions arise from ${\mathrm{F}}^{-}$ or ${\mathrm{H}}^{-}$ ions located interstitially in the adjacent cube. The hyperfine tensor is similar in both cases. The anisotropy and the isotropic part of the hyperfine interaction must result mainly from the ${\mathrm{Ce}}^{3+}$ configurations. Simple linear combination of atomic orbitals theory would yield very different values for ${\mathrm{H}}^{-}$ and ${\mathrm{F}}^{-}$ ions. Admixture from three-electron configurations or from the odd crystal field may explain the results.