Ligand electron-nuclear double resonance of T1 trigonal Gd3+ centers in CaF2 with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T1 trigonal 157Gd3+ centers in the CaF2 compound. It is experimentally found that the nearest environment of an impurity center contains only one 19F ion. Anions in the other coordination shells are identical to those in the pure CaF2 crystal. However, 19F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for 19F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF2 crystal are determined. It is demonstrated that the unusual isotropic LHFI constant As > 0 for Gd3+ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed, provided the structural model of the nearest environment of impurities in the T1 centers is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for 157Gd3+ centers.