Studies of the g factors and the local structure of the orthorhombic Ni center in KTaO _3 crystal

摘要

The local structure and the g factor (g _x, g _y, and g _z) of the Ni center in KTaO _3 are theoretically studied using the perturbation formulas of the g factors for a 3d ~9 ion in orthorhombically elongated octahedra. The orthorhombic field parameters are determined from the superposition model and the local geometry of the system. In view of the covalency, the contributions from the ligand orbital and spin-orbit coupling interactions are taken into account from the cluster approach. In the calculations, the orthorhombic center is attributed to Ni occupying the host Ta ~5 site, associated with the nearest-neighboring oxygen vacancy V _O along the c-axis. Furthermore, the planar Ni-O ~2- bonds are found to experience the relative variation ΔR (≈0.076 ?) along the a- and b-axis, respectively, due to the Jahn-Teller effect and the size mismatching substitution of Ta ~5 by Ni. Meanwhile, the effectively positive V _O can make the central Ni displace away from V _O along the c-axis by about 0.20 ?. The calculated g factors based on the above local distortions show good agreement with the experimental data.