Theoretical studies on the local structure and spin Hamiltonian parameters for the orthorhombic Cu2+ center in LiNbO3

The local structure and spin Hamiltonian parameters (the g factors and the hyperfine structure constants) for the orthorhombic Cu²⁺ center in LiNbO₃ are theoretically studied from the perturbation formulas of these parameters for a 3d⁹ ion in an orthorhombically elongated octahedron. This center is ascribed to Cu²⁺ occupying the Nb⁵⁺ site in LiNbO₃, associated with one nearest neighbour oxygen vacancy V_O along the Z axis. The planar bond lengths are found to suffer the relative variation of about 0.16 Å by compressing and stretching the Cu²⁺–O^2? bonds along the X and Y axes, respectively, due to the Jahn–Teller effect and the charge mismatching substitution of Nb⁵⁺ by Cu²⁺. Meanwhile, the effectively positive V_O can make the central Cu²⁺ displace away from the V_O along the Z axis by about 0.3 Å. The theoretical spin Hamiltonian parameters based on the above local distortions show good agreement with the experimental data.