Defect structure of Co2+ center in rhombohedral BaTiO3

The spin-Hamiltonian parameters (g factor g∥, g⊥ and hyperfine structure constants A∥, A⊥) of Co2+ ion in rhombohedral BaTiO3 crystal are calculated from the perturbation formulas based on the cluster approach for the spin-Hamiltonian parameters of 3d7 ion in trigonal octahedral site. The calculations are related to the trigonal field parameters and hence to the defect structure of Co2+ center. From the calculations, it is found that in order to reach a good fit between calculations and experiments, the off-center displacement of Co2+ ion in oxygen octahedron of BaTiO3 is much smaller than that of the host Ti4+ ion it replaces. This point is similar to the case of Fe3+ ion in BaTiO3 obtained in the previous studies by analyzing EPR zero-field splitting from the superposition model.