Crystal Field Theory in the Rare Earths

The paramagnetic properties (in particular the resonance data) of the rare earth ethyl sulfates are considered in the light of a theory in which each ion is assumed to be in a static electric field of $C_{3h}$ symmetry. It is found that this is consistent with the observations in cerium ethyl sulfate, and it is possible to solve that problem to find the magnitude of the interaction of this field with an ion. It is then shown that similar fields give good agreement with the observations in other salts.