Investigations of the local structures and the EPR g factors for V4+ and Cr4+ in Bi4Ge3O12 crystals

The local structures and the electron paramagnetic resonance (EPR) g factors g∥ and g⊥ for the substitutional V4+ and Cr4+ at the Ge4+ sites in Bi4Ge3O12 are theoretically investigated from the perturbation formulas of the g factors for 3d1 and 3d2 ions under tetragonally elongated tetrahedra. In the calculations, the ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach in view of the covalency of the systems. The local impurity-ligand bond angles β related to the C4 axis in the V4+ and Cr4+ centers are found to be about 5° and 6°, respectively, lower than the host angle βH in the pure crystal. So the ligand tetrahedra transform from original compression at the host Ge4+ site into elongation in the impurity centers due to the size mismatching substitution of the smaller Ge4+ by the larger impurities. Meanwhile, the Jahn-Teller effect also brings forward some contribution to the local structure of the V4+ center. The calculated g factors are in good agreement with the experimental data. The local structures of both centers are discussed.