Theoretical investigations of the local structure and the EPR parameters for the trigonal Co 2+ center in K 4CdCl 6
The local structure and the electron paramagnetic resonance (EPR) g factors g||, g⊥ and the hyperfine structure constants A|| and A⊥ for the trigonal Co 2+ center in K 4CdCl 6 are theoretically investigated by using the perturbation formulas of the EPR parameters for a 3d 7 ion in trigonal symmetry based on the cluster approach. In these formulas, the contributions to the EPR parameters arising from the admixture of different energy levels, covalency effect and low symmetry distortion are included. The parameters related to the above effects are obtained from the optical spectral data and the local geometrical relationship of the studied system. By analyzing the EPR spectra, we find that the impurity-ligand bonding angle β (≈53.54°) related to the C3-axis in the Co 2+ impurity center is smaller than the corresponding metal-ligand bonding angle βH (≈54.90°) in the host K 4CdCl 6. From the difference ∆ β between the bonding angle β (or βH) and the corresponding angle β0 (≈54.74°) in cubic symmetry, the chlorine octahedron changes from compression (∆ βH= βH- β0>0) in the host to elongation (∆ β= β- β0<0) in the impurity center. The calculated EPR parameters, especially the anisotropies of the g and A factors for K 4CdCl 6:Co 2+ based on the angle β show good agreement with the observed data. The theoretical studies on the local structure for Co 2+ in K 4CdCl 6 would be helpful to understand the structure properties for the equivalent octahedral transition-metal M site in the isostructural A 3M'MO 6 compounds.