Wu Shao-Yi

Studies of electron paramagnetic resonance parameters and defect structures for Ti2+ and V3+ ions in CdS crystals

By applying the high-order perturbation formulas based on the cluster approach for the EPR parameters of 3d2 ions in trigonal symmetry, the zero-field splitting D, g factors gparallel, gperpendicular, and hyperfine structure constants Aparallel, Aperpendicular for Ti2+ and V3+ ions in CdS crystals are studied. From the studies, the defect structures of these paramagnetic impurity centers are obtained and the EPR parameters are also explained reasonably.

Investigations of the g factors and hyperfine structure parameters for Er3+ ion in zircon-type compounds

The electron paramagnetic resonance (EPR) g factors g(parallel), g(perpendicular) and hyperfine structure parameters A(parallel), A(perpendicular) of the tetragonal Er3+ centers in zircon-type compounds YXO4 (X = As, P, V), ScVO4 and RSiO4 (R = Zr, Hf, Th) are calculated from the perturbation formulas of EPR parameters for 4f11 ion in tetragonal symmetry. In these formulas, the second-order perturbation contributions are included in addition to the first-order perturbation contributions considered in the previous papers. The crystal-field parameters used in the calculations are obtained by analyzing the optical spectral data from the superposition model. Although the superposition model intrinsic parameters An(R0) used in this paper for Er3+ in various zircon-type compounds are not as scattered as those in the previous paper, the calculated results of both the optical spectra and EPR parameters show better agreement than those in the previous paper with the observed values, suggesting that the above calculation method and parameters are more reasonable. The contributions of the second-order perturbation terms to EPR parameters are also discussed.

Electron paramagnetic resonance parameters of V2+ ions in both Cd2+ sites of CsCdCl3 crystal

From the perturbation formulas based on a two-spin-orbit-parameter model, the electron paramagnetic resonance (EPR) zero-field splitting (D), g-factors (g //, g perpendicular) and hyperfine structure constants (A //, A perpendicular) for V2+ in Cd2+ (I) and Cd2+ (II) sites of CsCdCl3 crystal at room and liquid nitrogen temperatures are calculated. From the calculations, the signs of zero-field splittings and hyperfine structure constants are determined and so all of the EPR parameters are explained reasonably on the basis of the structure data of lattice.

Local structure and local compressibilities around Co2+ impurity in ZnSiF6.6H2O crystal determined from electron paramagnetic resonance data.

The perturbation formulas of g-factors gparallel, gperpendicular and hyperfine structure constants Aparallel, Aperpendicular for 3d7 ion in trigonal octahedral crystal field are established on the basis of a cluster approach. These formulas consist of the contributions from configuration interaction and covalency effect and the parameters related to both effects can be estimated from the optical spectra and the structural data of the system under study. According to these formulas, the local trigonal distortion angle beta at pressure P = 0 and the local compressibilities d ln beta/dP in two pressure regions around Co2+ impurity in ZnSiF6.6H2O crystal are estimated by fitting the calculated electron paramagnetic resonance parameters gi, Ai and their pressure coefficients to the observed values. The results show that these local values are different from those of the host crystal because of the influence of impurity.

Spin Hamiltonian parameters and local structures for CO2+ ions in calcite-type trigonal carbonates MCO3 (M = Co, Cd and Ca)

From the perturbation formulas based on the cluster approach for 3d(7) ion in trigonal octahedral sites of crystals, the spin Hamiltonian parameters (g factors g(parallel to), g(perpendicular to) and hyperfine structure constants A(parallel to), A(perpendicular to)) for Co2+ in calcite-type MCO3 (M = Co, Cd, Ca) crystals are calculated. In the calculations, for CoCO3 crystal, the structural data of pure crystal are used, whereas for the Co2+-doped CdCO3 and CaCO3 crystals, the local structural data caused by the impurity-induced local lattice relaxation are considered. The calculated results show good agreement with the observed values obtained from electron paramagnetic and antiferromagnetic resonance experiments. The difficulty shown in the previous paper related to the g(perpendicular to) value of CoCO3 obtained from the change of the g factor as a function of the crystallographic data (i.e., the unit cell parameters) a(0) and alpha(0) of pure MCO3 crystals is removed

Investigations of the g factors and hyperfine structure constants for tetragonal and cubic Co2+ centers in AgCl crystals

In this paper, the calculation formulas based on a cluster approach for the EPR g factors g(parallel to), g(perpendicular to), and hyperfine structure constants A(parallel to), A(perpendicular to) of 3d(7) ions in tetragonal octahedral crystal fields are established. In these formulas, the parameters related to configuration interaction and covalency effects can be obtained from the optical spectra and structural parameters of the studied system. Only one adjustable parameter, the core polarization constant kappa, is used for the calculation of A(parallel to) and A(perpendicular to). From these formulas, the EPR parameters g(parallel to), g(perpendicular to), A(parallel to), and A(perpendicular to) for tetragonal Co2+-V-Ag center in AgCl crystal are satisfactorily explained by considering the suitable tetragonal distortion Delta R (i.e., the displacement of the intervening Cl- ion towards Co2+. ion along the C-4-axis). The local geometry of the Co2+-V-Ag center is therefore obtained. The EPR parameters g and A for cubic Co2+ center in AgCl are also calculated from the above formulas for the case where the distortion Delta R = 0. The results are consistent with the observed values

High-order perturbation formulas of the zero-field splitting for 3d5 ion in tetragonal symmetry

The third- and fourth-order perturbation formulas based on the dominant spin-orbit coupling mechanism for the zero-field splitting D of 3d(5) ion in tetragonal symmetry have been established from the strong-field scheme. From the formulas, the spin-lattice coupling coefficient G(11) (which is related to the zero-field splitting D due to the slight tetragonal distortion under stress) of MgO:Mn2+ and the zero-field splitting D of the tetragonal Mn2+ -V-O center in SrTiO3 are calculated. The results suggest that in the cases of 3d(5) clusters with small tetragonal distortion, the lowest (third)-order perturbation formula is applicable; however, in the cases of large tetragonal distortion, the higher (fourth)-order contribution is comparable with the third-order one and should be taken into account. The displacement of Mn2+ in the Mn2+ -V-O center and hence the microstructures of this centre in SrTiO3 are also obtained. The relaxation pattern is consistent with that of the isoelectronic Fe3+-V-O center in KNbO3 crystal obtained from both the shell-model simulations and the embedded-cluster calculations

Theoretical investigations of the zero-field splitting and g factors for CdS : Ti2+ and CdSe : Ti2+ crystals

The zero-field splitting D and g factors g(parallel to) and g(perpendicular to) for Ti2+ ions in CdS and CdSe crystals are calculated from the high-order perturbation formulas of EPR parameters based on the two-spin-orbit (SO) coupling parameter model. In this model, the contributions to EPR parameters from both the SO coupling parameter of the central 3d(2) ion and that of the ligand are considered. The calculated results show good agreement with the observed values. Comparing these results with those calculated from the conventional one-SO-parameter model where the contributions only from the SO coupling parameter of the 3d(2) ion are considered, it suggests that the calculations of EPR parameters D. g(parallel to) and g(perpendicular to) for 3d(2) ions in tetrahedrally coordinated semiconductors should prefer the two-SO-parameter model over the one-SO-parameter model

Local tilting angles τ for Fe+ in Cd2+ site and Fe3+ in Si4 + site of CdSiP2 semiconductor

The EPR parameters (g factors, g(parallel), g(perdendicular) and zero-field splitting D) for Fe+ in Cd2+ site and Fe3+ in Si4+ site of CdSiP2 semiconductor are calculated from the distinct high-order perturbation formulas. From the calculations, the local tetragonal distortions and hence the local tilting angles tau (which are different from the corresponding host values) for both paramagnetic centers are estimated. The results are discussed.

Theoretical studies of the g-shift for Cr4+ ions in GaN crystal from crystal-field and charge-transfer mechanisms

The assignment of the 1.193 eV zero-phonon line of the 3d(2) ion in GaN crystal to the internal luminescence of Cr4+ on the Ga3+ site is analysed. Based on this assignment, the g-shift Ag (=g - g(s)) of the groundstate of this luminescence is calculated by considering not only the conventional contribution due to the crystal-field mechanism, but also the contribution due to charge-transfer mechanism. The calculated result shows good agreement with the observed value, suggesting that the assignment is reasonable. The calculated Ag due to the charge-transfer mechanism is opposite in sign and 62% in magnitude, compared with that due to the crystal-field mechanism, and so it cannot be neglected. It appears that for the 3d(2) (or 3d(n)) ion with a high valence state in crystals (particularly, in the case of the ligand having small optical electronegativity), the reasonable explanation of the g-shift should consider the contributions from both the crystal-field and charge-transfer mechanisms.