Electric field effect in ligand electron-nuclear double resonance of cubic centers in the CaF2:Gd3+ crystal

The linear electric field effect in ENDOR spectra for fluorine ion nuclei in the first coordination sphere of the Gd3+ impurity ions in the fluorite lattice is investigated at the temperature of liquid helium in an electric field of intensity up to 600 kV/cm. The parameters of the effective hamiltonian of the superhyperfine interaction in the electric field are derived from the orientation dependence of the pseudo-Stark splitting of the ENDOR lines. The contributions of various mechanisms of the electric field effect are determined by employing the data in the literature on variation of the ENDOR spectra on uniaxial and hydrostatic compression and also the results of a microscopic calculation of the local structure of the impurity lattice. It is shown that electric polarization of the ions plays the decisive role in the variation of the isotropic superhyperfine interaction constant