EPR AND ENDOR OF GD3+AT CUBIC SITES IN PBF2

The crystal-field parameter b4 and transferred-hyperfine- interaction (THFI) parameters for ligands and second-neighbour 19F nuclei have been measured for Gd3+ at cubic sites in PbF2. So also has b4 for Eu2+ at cubic sites in CdF 2. The values of b4 indicate, for Gd3+ that the ligand distance in PbF2 is less than in SrF2, and for Eu2+ that the ligand distance in CaF2 is less than in CdF2 in the opposite order to ligand distances in the undistorted host crystals. The ENDOR measurements of second neighbours show angular as well as radial displacements from the ionic positions in the undistorted host crystal. They also show that erroneous conclusions could be drawn from ENDOR measurements for only one direction of the external field, which may cast doubt on conclusions drawn from some earlier measurements of THFI parameters of second neighbours in fluorite crystals.