The ligand hyperfine interaction with rare earth ions. II. Calculation of the paramagnetic NMR shift

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Abstract
The isotropic and anisotropic 19F NMR shifts for Yb3+ doped into alkaline earth fluorides is calculated using a covalent model for the spin transfer. The good agreement between theory and experiment is strong evidence for the covalent model being the correct model in the case of Yb3+. The calculations reveal that the main contribution to the isotropic shift does not come from the Fermi contact interaction, as has been generally assumed, but rather from a second order Zeeman term due to a spin–orbit interaction between the nuclear spin and the orbital motion of unpaired electrons in the ligand p orbitals. It has been found that the isotropic shift of Yb3+ can be reasonably estimated by calculating the high temperature limit of the shift (kT≫ crystal field splittings) even though kT is about one‐third the crystal field in the sytem studied. A general method of calculating the NMR shift in the high temperature limit is developed which is readily applicable to multispin ions and is applied to the Er3+ system.