Covalent effects in rare-earth crystal-field splittings

Covalent contributions to the optical splitting of rare-earth ions are discussed. Tm2+ in CaF2, the system studied by Bleaney, Axe, and Burns, and others, was chosen because it is an outstanding example of the breakdown of the traditional electrostatic theory when applied to rare-earth ions in cubic environments. Upon separating out overlap from covalent contributions, overlap effects are found to dominate. The role of the closed 5p shell in covalent mixing is shown to be as important as, or more important than, that due to the 4f shell alone.