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 be en previously determined experimentally. The model is based on effective ch arges, located around the middle of the rare earth ion-ligand distance, whi ch are assumed to be proportional to the magnitude of the overlap, rho, bet ween the rare earth ion and ligand wavefunctions. Good agreement between th eoretical and experimental crystal field parameters is found for most of th e 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 ag reement with the general assumption that the f orbitals have a small part i n the chemical bonding. For the two examples involving the 5f elements, the overlaps with the ligand have, curiously, the same magnitude as in the cas e of the rare earth elements. As expected, the 3d element wavefunctions sho w a stronger mixing with those of the ligands (0.10 < rho < 0.25).