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Neutron scattering and specific-heat studies of the crystal fields in the rare-earth monophosphides are reported. It is found that the fourth-order crystal-field parameters for the phosphides and for other rare-earth pnictides fall on a universal curve which is close to that predicted by an effective-point-charge model for the light rare earths but deviates markedly for the heavy rare earths.
Birgeneau et al. (1973) studied this question.