The Electron Paramagnetic Resonance technique has been used to study alkaline earth fluoride crystals doped with both hydride and gadolinium (or erbium) ions, and the spin Hamiltonian parameters have been determined for the various kinds of charge-compensated rare earth ion site which occur. In particular, two sites of tetragonal symmetry with the structures RE³⁺-FI⁻, RE³⁺-HI⁻ have been examined, and small shifts in the EPR spectra of the latter site have been measured when a deuteride or tritiide ion replaces the hydride ion. A crystal lattice model of point charges and point dipoles at distorted lattice sites predicts a value of the crystal field parameter B₀² for the tetraqonal sites which is only one half of that estimated from the observed spectra, but the model successfully accounts for the larger value of B₀² for the RE³⁺-HI⁻ site compared with the RE³⁺-FI⁻ site on the basis of the larger polarisability of the H⁻ ion. Isotope shifts are interpreted by the electron phonon interaction between the 4f electrons of the rare earth ion and the localised mode of vibration of the light anion. The magnitudes of the shifts, calculated on a point charge/point dipole model, are in good agreement with experiment.

The reorientation of the tetragonal Gd³⁺-HI⁻ sites has been examined by EPR line broadening and dielectric loss techniques. No distinct dielectric loss peak corresponding to this site was observed, and it is proposed that it cannot be distinguished from that for Gd³⁺-FI⁻ sites. An interstitialcy model for the reorientation has been investigated and is found to be consistent with this explanation and with the observation of a metastable Gd³⁺-HS⁻-FI⁻ site in u.v. irradiated calcium fluoride.