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The local structure and the electron paramagnetic resonance (EPR) g factors g(parallel to), g(perpendicular to) and the hyperfine structure constants A(parallel to) and A (perpendicular to) for the trigonal Co2+ center in K4CdCl6 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 beta (approximate to53.54degrees) related to the C-3-axis in the Co2+ impurity center is smaller than the corresponding metal-ligand bonding angle beta(H) (approximate to 54.90degrees) in the host K4CdCl6. From the difference Deltabeta between the bonding angle beta (or beta(H)) and the corresponding angle beta(0) (approximate to54.74degrees) in cubic symmetry, the chlorine octahedron changes from compression (Deltabeta(H) = beta(H) - beta(0) > 0) in the host to elongation (Deltabeta = beta - beta(0) < 0) in the impurity center. The calculated EPR parameters, especially the anisotropies of the g and A factors for K4CdCl6: Co2+ based on the angle beta show good agreement with the observed data. The theoretical studies on the local structure for Co2+ in K4CdCl6 would be helpful to understand the structure properties for the equivalent octahedral transition-metal M site in the isostructural A(3)M'MO6 compounds. (C) 2003 Elsevier B.V. All rights reserved.