The ligand hyperfine interaction with rare earth ions. I. A revised covalent model

It is shown that a reformulation of the covalent model can explain the 19F hyperfine interactions observed for Yb3+ and Tm2+ in cubic and tetragonal sites of alkaline earth fluorides. The same model does not explain data for Ce3+, Gd3+, or Eu2+ indicating that the spin polarization model is the dominant mechanism in the first part of the rare earth series but that the covalent model becomes dominant in the latter half of the series. The analysis shows that the distortion from cubic symmetry in the tetragonal site is not as severe as an earlier NMR analysis suggested but is still large. It further suggests that the interstitial ion is closer to the rare earth ion than most observers have assumed.