The ligand hyperfine interaction with rare earth ions. III. 19F NMR single-crystal study of ErF 3

Abstract

The 19F NMR of ErF 3 and LiErF 4 powders and a single crystal of ErF 3 are reported. For ErF 3 the shift tensor is determined for the two different fluoride ion sites in the crystal lattice and the orientation of the principal axes in each site is found. The temperature dependence (200–400 K) of the isotropic and traceless shift tensors is separately determined for each site. The traceless tensor agrees with that calculated from a dipolar model within 10% and its temperature dependence is identical with that reported for the paramagnetic susceptibility of ErF 3. The isotropic shifts for the two sites have different temperature dependences, showing that theories of the isotropic shift which assume one hyperfine parameter for all states of the J manifold cannot be applied to this system. Analysis of the shifts using recent theories shows that (1) the shift contributions of more than one nearest-neighbor rate earth ion are additive, and (2) both the covalent and polarization mechanisms of spin transfer must make comparable contributions to account for the results. An analysis of hyperfine parameters reported for the isoelectronic Ho 2+ in CaF 2arrives at a similar conclusion about the contribution of covalent and polarization mechanisms for that ion. It is shown that useful information about the shift tensor can be obtained from powder spectra if there is only one fluoride ion site, as is the case for LiErF 4, but not from the rare earth trifluorides, which have two different sites in the unit cell.