Ji-Zi Lin

Studies of the local structure and the g factors for the orthorhombic Ti3+ center in YAP

The local structure and the g factors for the orthorhombic Ti3+ center in YAP are theoretically studied from the perturbation formulas of the g factors gx, gy and gz for a 3d1 ion in orthorhombic symmetry based on the cluster approach. The observed large anisotropy of the g factors may be attributed to the significant low-symmetrical (tetragonal and orthorhombic) distortion near the Al3+ site occupied by the impurity Ti3+. The calculated g factors in this work show reasonable agreement with the experimental data.

Investigations on the impurity displacements and the g factors of the two tetragonal Ni3+ centres in SrTiO3

The impurity displacements and the g factors g(parallel to) and g(perpendicular to) of the two tetragonal Ni3+ centres (I and II) in SrTiO3 are theoretically investigated by using the perturbation formulae of the g factors for a 3d(7) ion with low spin (S = 1/2) in tetragonally distorted octahedra. In these formulae, the contributions to the g factors from the tetragonal distortion, the ligand spin - orbit coupling coefficient, and the ligand orbitals, which were usually neglected in previous studies (Muller K A et al 1969 Phys. Rev. B 186 361; Lacroix R et al 1964 Helv. Phys. Acta 37 627), are taken into account based on the cluster approach. Centre I ( or II) was ascribed to the substitutional Ni3+ at the Ti4+ site, associated with the nearest neighbour oxygen vacancy V-O (or the near neutral V-O -(- 2e)) along the C-4 axis as a compensator. The impurity Ni3+ is found to shift away from ( or towards) the compensator by about 0.033 nm ( or 0.010 nm) along the C-4 axis, respectively. As a result, the ligand octahedron around the impurity is elongated ( compressed) in centre I ( II), corresponding to the negative ( positive) anisotropy Delta g (= g(parallel to) - g(perpendicular to)). The theoretical g factors based on the above impurity displacements show good agreement with the observed values.

Theoretical studies on the EPR parameters of the interstitial V4+ in rutile

The electron paramagnetic resonance (EPR) parameters g-factors g i (i=x, y, z) and the hyperfine structure constants A i for the interstitial V4+ in rutile are theoretically studied from the perturbation formulas of these parameters for a 3d1 ion in rhombically distorted octahedra. On the basis of the studies, the local axial distortion angle Δα′ in the impurity center is found to be about 2° smaller than the host value, characterized as stretching and contraction of the parallel and perpendicular bonding lengths by about 0.28 and 0.14 Å,respectively. This results in the less compressed ligand octahedron because of the Jahn–Teller effect and space effect arising from occupation of the impurity V4+ at the interstitial site. The theoretical EPR parameters based on the above local structural parameters of this work are in better agreement with the experimental data than those of the previous studies in the absence of the local angular distortion and the ligand orbital contributions. The two experimental optical absorption bands are also reasonably analyzed.

Theoretical investigations of the spin hamiltonian parameters for the tetragonal Cu2+ centers in NH4X (X = Cl, Br, I)

The spin Hamiltonian (SH) parameters g factors g parallel to, g perpendicular to and the hyperfine structure constants A parallel to and A perpendicular to for the tetragonal Cu2+ centers (i.e., [CUX4(H2O)(2)] clusters) in NH4X (X = Cl, Br, I) are theoretically investigated by means of the diagonalization procedure of complete (10 x 10) energy matrix (DPCEM) for a 3d(9) ion in tetragonal symmetry. The crystal-field parameters in the energy matrix are determined from the superposition model. The calculated SH parameters are in reasonable agreement with the experimental data.

Investigation of the Spin Hamiltonian Parameters of a Tetragonal VO2+ Center in (NH4)2SbCl5

The spin Hamiltonian parameters g‖ and g⊥ and the hyperfine structure constants A‖ and A⊥ of a tetragonal VO2+ center in (NH4)2SbCl5 are investigated, using the perturbation formulas for a 3d1 ion in tetragonally compressed octahedra. In these formulas, the contributions to the spin Hamiltonian parameters from the s- and p-orbitals as well as the spin-orbit coupling coefficient of the Cl− ligand are taken into account, based on the cluster approach. According to these studies, compression of the ligand octahedra results from the strong axial crystal-fields due to the short V4+-O2− bond in the [VOCl4]2− cluster. The obtained spin Hamiltonian parameters agree well with the experimental data and need fewer adjustable parameters than the previous studies. The covalency of the studied system is also discussed.

Investigations of the spin Hamiltonian parameters and defect structure for the rhombic Gd3+ center in calcium oxide

The spin Hamiltonian (SH) parameters (zero-field splittings , and ) and the defect structure for the rhombic Gd3+ center in CaO are theoretically investigated by the superposition model. Meanwhile, the g-factor is also quantitatively studied from the approximation formula on the basis of an admixture of the ground 8S7/2 and the excited 6L7/2 (L = P, D, F, G) states via the spin–orbit coupling interactions. The defect structure of this center can be described by the impurity Gd3+ occupying the host octahedral Ca2+ site, associated with one nearest neighbouring cation vacancy V Ca in the [1 1 0] direction. On the basis of these studies, the impurity Gd3+ is found to shift towards the V Ca along the [1 1 0] axis by about 0.582 Å, whereas the oxygen ligands closest to the V Ca suffer a displacement towards the latter by about 0.211 Å. The calculated SH parameters based on the earlier analyses show reasonable agreement with the observed values.