Shao-Yi Wu

Studies of the EPR Parameters gi and Ai for Co2+ in the Octahedral Sites of Y3Al5O12 and Y3Ga5O12 Crystals

The perturbation formulas of EPR g factors g() g(perpendicular to) and hyperfine structure constants A() A(perpendicular to) for the trigonal octahedral 3d(7) cluster are derived considering not only the configuration interaction effect due to the admixture among ground and excited states, but also the covalency effect due to the admixture between the d electrons of the 3d(7) ion and the p electrons of ligands. In these formulas, the parameters related to both effects and the trigonal field parameters are estimated from the optical spectra and the structural parameters of the studied system. According to these formulas, the EPR parameters g(), g(perpendicular to), A() and A(perpendicular to) for Co2+ in the octahedral sites of Y3Al5O12 and Y3Ga5O12 garnets are satisfactorily explained from the optical spectra and the structure data. The role of the covalency effect is discussed.

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 of EPR parameters and microstructure of the rhombic Co2+-VAg centre in AgCl

The perturbation formulae of EPR g-factors g(i) (i = x, y, z) and hyperfine constants Ai for a 3d(7) (Co2+) ion at a rhombic octahedral site in a crystal are established from a cluster approach. In these formulae, the contributions from the covalency effect, the configuration interaction and the rhombic crystal field are considered and the parameters used in the calculation of EPR parameters. except the core polarization constant x in the calculation of Al, can be estimated from the optical spectra and the structure data of the studied system. From these formulae, the EPR parameters g(i) and A(i) for the rhombic Co2+-V-Ag centre in AgCl are reasonably explained by considering suitable rhombic distortion and so the microstructure of this rhombic centre is obtained.

Studies of the spin Hamiltonian parameters and the local structure for the trigonal Cr3 + centers I, III and IV in CsMgCl3

The spin Hamiltonian (SH) parameters (zero-field splitting D, anisotropic g factors g(parallel to) and g(perpendicular to)) and the local structure of the trigonal Cr(3+) centers I, III and IV in CsMgCl(3) are studied using the perturbation formulae for the SH parameters for a 3d(3) ion in trigonal symmetry based on the cluster approach. In these formulae, not only the contributions from the conventional crystal-field (CF) mechanism, but also those from the charge-transfer (CT) mechanism are taken into account. According to the studies, the metal-ligand bond angle related to the C(3)-axis is found to increase from beta(H)(approximate to 51.71 degrees) in the host to beta(approximate to 54.48 degrees) in the impurity center I (in the absence of local charge compensation). As for center III (or IV), the three Cl(-) ions closest to the compensator V(Mg) (or Li(+)) are expected to displace away from the compensator by an amount Delta X(III) approximate to 0.009 nm (Delta X(IV) approximate to 0.003 nm) due to the electrostatic repulsion. The local structure of the trigonal centers and the contributions from the CT mechanism are discussed.