Relationship between phenomenological crystal field parameters and the crystal structure: The simple overlap model

The simple overlap model is used to calculate crystal held parameters from the crystal structure. It is applied to 25 rare earth and some actinide and 3d transition element compounds for which crystal field parameters have been previously determined experimentally. The model is based on effective charges, located around the middle of the rare earth ion-ligand distance, which are assumed to be proportional to the magnitude of the overlap, rho, between the rare earth ion and ligand wavefunctions. Good agreement between theoretical and experimental crystal field parameters is found for most of the cases studied. The best predictions are obtained when the overlap varies slightly from one rare earth compound to another (9.05 < rho < 0.08), in agreement with the general assumption that the f orbitals have a small part in the chemical bonding. For the two examples involving the 5f elements, the overlaps with the ligand have, curiously, the same magnitude as in the case of the rare earth elements. As expected, the 3d element wavefunctions show a stronger mixing with those of the ligands (0.10 < rho < 0.25).