Spin Hamiltonian parameters and defect structure for the X-ray-induced NbLi4+ center in LiNbO3 crystal

The spin Hamiltonian (SH) parameters (g factors gx, gy, gz and hyperfine structure constants Ax, Ay, Az) of the low-symmetry NbLi4+ antisite defect center in the X-ray-irradiated congruent undoped LiNbO3 crystal are calculated from the high-order perturbation formulas of SH parameters for d1 ions. The calculated results are in reasonable agreement with the experimental values. Some information related to the defect structure of the NbLi4+ center (characterized mainly by the NbLi4+ displacement Δz from the ideal Li+ site along C3 axis and the displacement Δx of O2− ions in the oxygen triangle between NbLi4+ and NbNb5+) is obtained from the calculations. These displacements are consistent with the expectation based on the electrostatic interaction and electron cloud overlap. The results are discussed.