Investigations of the electron paramagnetic resonance parameters and defect structures for Ni2+ ions in CdX2 (X=Cl, Br) crystals

The high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters (g factors g∥,g⊥ and zero-field splitting D) for 3d8 ions in trigonal octahedral clusters are established. These formulas contain the contributions not only from the crystal-field (CF) mechanism, but also from the charge-transfer (CT) mechanism (which is not considered in the widely used CF theory). From these formulas, the EPR parameters and the impurity-induced defect structures for Ni2+ ions in CdX2 (X=Cl, Br) crystals are studied. The calculated EPR parameters are coincident with the experimental values, and the defect structure of Ni2+ impurity center obtained from the calculation is different from the corresponding structure in the host crystal. The sign of QCT (Q=Δg∥, Δg⊥, or D) due to CT mechanism agrees with that of the corresponding QCF due to CF mechanism and the relative importance of CT mechanism (characterized by QCT/QCF) increases with increasing covalence of 3d8 clusters and hence with raising atomic number of ligand X. So, in the explanations of the EPR parameters of 3d8 (or other 3dn) ions in crystals with the heavy-element ligand ion (e.g., Br−), the calculated formulas based on the two-mechanism (CF and CT mechanisms) model are preferable to those based on only the CF mechanism in the CF theory.