Studies on the Local Angular Distortion and Spin Hamiltonian Parameters for the Trigonal Co2+ Center in MgCL2

The local angular distortion and spin Hamiltonian parameters (g factors g(parallel to), g(perpendicular to) and the hyperfine structure constants) for the trigonal Co2+ center in MgCL2 are theoretically studied by diagonalizing the 6 x 6 energy matrix of ground 471 state for a trigonally distorted octahedral 3d(7) cluster. Based on the cluster approach, the contributions from the admixtures of various J (= 1/2,3/2,5/2) states and the ligand orbital and spin orbit coupling interactions are taken into account in a uniform way. The local impurity ligand bond angle in the Co2+ center is found to be about 3.44 degrees larger than the host metal ligand bond angle in the pure crystal due to substitution of smaller Mg2+ by bigger Co2+, inducing a further compressed ligand octahedron. The calculated spin Hamiltonian parameters using the above local angular distortion are in good agreement with the experimental data. The present studies on the local structure and the spin Hamiltonian parameters for Co2+ in MgCL2 are tentatively extended to a more general case by comparing the relevant impurity behaviours for Co2+ in various trigonal environments.