Studies of the EPR g factors and the local structure for the trigonal Nd3+ center in LiNbO3:Nd3+ crystal

In this paper, the perturbation formulas of the EPR g factors g∥ and g⊥ for a 4f3 ion in trigonal symmetry are established. In these formulas, the contributions to the g factors from the second-order perturbation terms, the admixtures of various states and covalency effect are taken into account. The above formulas are applied to the trigonal Nd3+ center in LiNbO3, with the related crystal-field parameters obtained from the local structural parameters of the impurity center. Based on the studies, one can find that the impurity Nd3+ may not occupy the ideal Li+ site, but undergo an off-center displacement ΔZ (≈−0.28 Å) away from the center of the oxygen octahedron along the C3 axis. The above displacement ΔZ is comparable with that (≈−0.40 Å) obtained from Rutherford backscattering spectrometry (RBS) experiment. In addition, the calculated g factors (and also the anisotropy Δg (= g⊥− g∥)) based on the ΔZ show reasonable agreement with the observed values. The contributions to the g factors and the local structure of this impurity center are also discussed.