Theoretical investigations of the local distortion and spectral properties for VO2+ in SiO2 Glass

The local distortions and the spin Hamiltonian parameters g factors g∥, g⊥ and the hyperfine structure constants A∥ and A⊥ for isolated vanadyl ions VO2+ doped in SiO2 glass at 700°C are theoretically investigated from the perturbation formulas of these parameters for a 3d1 ion in tetragonally compressed octahedra. In these formulas, the relationships between local structure of VO2+ ions center and the tetragonal crystal field parameters are established. As a result, the distortion of the ligand octahedron is attributed to the strong axial crystal-fields associated with the short V4+–O2− bond due to the strong V=O bonding in the silica matrix. The theoretical spin Hamiltonian parameters obtained in this work show reasonable agreement with the experimental data.

INVESTIGATIONS ON THE EPR g FACTORS FOR THE SIX-COORDIANTED Cu2+(1) SITE IN YBa2Cu3O6+x

The EPR g factors g // and g ⊥ for the six-coordinated Cu 2+(1) site in YBa 2 Cu 3 O 6+x are theoretically investigated from the perturbation formulas for a 3 d 9 ion in tetragonally elongated octahedra. The related tetragonal crystal-field parameters Ds and Dt are determined from the superposition model and the structural data for the Cu 2+(1) site, by considering the effective elongation of the oxygen octahedron due to the Jahn-Teller effect of the Cu 2+ cluster. Based on the calculations, the effective elongation (≈ 15%) of the ligand octahedron is obtained for the signals at 100K. As for the EPR signals at room temperature, the elongation declines to merely 4%, corresponding to the smaller anisotropy Δ g (= g // - g ⊥). The theoretical results for the g factors are in good agreement with the observed values, and the local tetragonal distortion due to the Jahn-Teller effect is discussed.