Zero-Field Splitting in 3d5 Ions

The origin of the zero‐field splittings of the orbitally nondegenerate ground states of the transition metal ions (3d³, 3d⁵, and 3d⁸ solutes) has been studied on the assumption that these splittings are due to the combined action of an electric field gradient and the spin—spin interaction. A relation between the splitting parameters D and E of the conventional spin Hamiltonian and the field gradients $q_{\mathrm{|\; |}}$ and ${\mathrm{\eta q}}_{\mathrm{|\; |}}$ , respectively, has been deduced on the above basis using hydrogenic wavefunctions and d→d, d→g, and d→s excitations. A reasonably good agreement with the splitting has been obtained for Mn²⁺ in corundum but an apparent disagreement has been found for Fe³⁺ in Al₂O₃, considering the known values of the field gradients in these cases. For Cr³⁺ (d³ solute) and Ni²⁺ (d⁸ solute) in corundum, the major sources of splitting seem to be the spin—orbit interaction and the distortion of the ground state wavefunctions due to covalent π bonding and not the spin—spin interaction. The cause of the apparent disagreement for Fe³⁺ in Al₂O₃ is discussed. A definite understanding of the cause of the zero‐field splittings in the transition metal ions depends on more accurate knowledge of the field gradients acting on these ions in different crystalline environments.