The ligand hyperfine interaction with rare earth ions. IV. 19F NMR study of single crystals of TbF 3 and HoF 3

Abstract

The 19F NMR of single crystals of TbF 3 and HoF 3 is reported. For both crystals the shift tensor is determined at room temperature for the two different fluoride ion sites in the orthorhombic crystal lattice. The shift tensor for Type I sites in TbF 3 is also determined at 548 K. The temperature dependence of both the isotropic and traceless shift tensors is separately determined for each site in the temperature interval of 175–370 K for HoF 3 and 175–548 K for TbF 3. Comparison of the experimental traceless shift tensor of TbF 3, and ErF 3 with that calculated from a point dipole model, using experimental susceptibility data from single crystals, shows systematic differences that cannot be attributed to spin transfer mechanisms or defects in the point dipole model. The differences are most probably due to errors in the crystal structure used to calculate the point dipole contributions. It is shown that the pseudo-contact term contributes only 10% of the observed isotropic shift and that the isotropic shift exhibits remarkable changes as we go across the periodic table from TbF 3 to ErF 3. 19F resonances from one site in TbF 3 are observed to broaden and disappear in the interval 420–490 K owing to some type of diffusional motion whose activation energy is estimated to be 40–60 kJ/mole.