Zheng Wen-Chen

Studies of the defect structures and g factors for two trigonal Ti3+ centers in LaMgAl11O19 : Ti3+ crystal

The defect models of two trigonal Ti3+ centers in the LaMgAl11O19:Ti3+ (LMA:Ti3+) crystal are resuggested as center 1 at the 2a site and center 3 at the 4f site. By using the crystal field parameters Dq based on the suggestion, the g factors g(parallel to) and g(perpendicular to) for both Ti3+ centers are calculated from two microscopic spin-Hamiltonian theory methods, the complete diagonalization (of the energy matrix) method (CDM) and the perturbation theory method (PTM), based on the two spin-orbit (SO) parameter model (where both the contribution to spin-Hamiltonian parameters from the SO coupling parameter of the central 3d(n) ion and that of ligand ions are included). The calculated results from CMD and PTM are not only close to each other, but are also in agreement with the observed values. This suggests that (i) the present defect models of the two Ti3+ centers in the LMA:Ti3+ crystal are suitable and (ii) both CDM and PTM methods are effective to the explanations of spin-Hamiltonian parameters of 3d(1) ions in crystals. The defect structures of both Ti3+ centers (which are quite different from the corresponding structures in the host LMA crystal, in particular, for center 1, the oxygen octahedron changes from the trigonal elongation in the host LMA crystal to the trigonal compression in the Ti3+ center) are also estimated from the calculations. The results are discussed. (c) 2007 American Institute of Physics.

Spin-Hamiltonian parameters and local structures for Co4+ and Ir4+ impurity centers in the tetragonal phase of SrTiO3.

The calculated results in the recent paper about the spin-Hamiltonian (SH) parameters (g factors g (//), g perpendicular and hyperfine structure constants A(//), A perpendicular) and the local structures of Co4+ and Ir4+ impurity centers in the tetragonal phase of SrTiO3 are doubtful because there are several mistakes in the calculations. So, we restudy the SH parameters and local structures by using the correct methods and parameters. From the studies, for Co4+ and Ir4+ in SrTiO3, the SH parameters are explained rationally, the signs of hyperfine structure constants A(//), A perpendicular are obtained and the suitable and more detailed local structures are given. The results are discussed.

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.

Defect structure and spin-Hamiltonian parameters for the CuCl64− cluster in the tetragonal RbCdCl3:Cu2+ crystal

The tetragonal distortion (characterized by R( parallel)-R( perpendicular) where R( parallel) and R( perpendicular) denote the metal-ligand distances parallel with and perpendicular to the tetragonal axis, respectively) for CuCl(6)(4-) cluster in the tetragonal RbCdCl(3):Cu(2+) crystal is studied by calculating its spin-Hamiltonian (SH) parameters (g factors g( parallel), g( perpendicular) and hyperfine structure constants A( parallel), A( perpendicular)). The calculations are performed by using the complete high-order perturbation formulas for 3d(9) ions in tetragonal symmetry based on a two-mechanism model, in which both the widely-used crystal-field (CF) mechanism and the charge-transfer (CT) mechanism (which is omitted in CF theory) are considered. From the calculations, the SH parameters are reasonably explained and the tetragonal distortion R( parallel)-R( perpendicular) approximately 0.14A is obtained. The distortion is much larger than those caused by the cubic-tetragonal transition phase, but is in the same order as those found in many similar CuCl(6)(4-) Jahn-Teller systems. So, the tetragonal distortion (or defect structure) for the CuCl(6)(4-) cluster in RbCdCl(3):Cu(2+) is due mainly to the Jahn-Teller effect. The relative importance of CT mechanism (characterized by Q(CT)/Q(CF), where Q=Deltag(i) or A(i)((2)), i= || or perpendicular are about 12% and 9% for Q=Deltag(i) and A(i)((2)), respectively. It appears that for the exact calculations of SH parameters of CuCl(6)(4-) clusters in crystals, the contributions due to both CF and CT mechanisms should be taken into account.

Investigations of the optical and EPR spectra for VO2+ in LiKSO4 crystals

The optical spectra and EPR spectra (characterized by the spin-Hamiltonian parameters g(//), g(perpendicular), A(//) and A(perpendicular)) for the molecular ion VO2+ in LiKSO4 crystals are calculated from two microscopic theory methods, one of which is the complete diagonalization (of energy matrix) method (CDM) and the other is the perturbation theory method (PTM). The calculated three optical absorption bands and four spin-Hamiltonian parameters from the two methods are not only close to each other, but also in reasonable agreement with the experimental values. It appears that both theoretical methods are effective in the explanation of optical and EPR spectra for 3d1 ions in crystals. The negative signs of hyperfine structure constants A(//) and A(perpendicular) for VO2+ in LiKSO4 crystals are also suggested from the calculations.

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.

EPR parameters and defect structure of the tetrahedral Ti3+ defect center in beryl crystal

The EPR parameters (g factors g i and hyperfine structure constants A i , where i=x, y, z) of Ti3+ ion at the tetrahedral Si4+ site of beryl crystals are calculated within the rhombic symmetry approximation from the high-order perturbation formulas based on the two-spin-orbit (SO)-parameter model. In these formulas both the contribution due to the SO coupling parameter of the central 3d1 ion and that of ligand ions are considered. From the calculations, the defect structure of the Ti3+ defect center in beryl crystal is estimated and the EPR parameters g x , g y , g z and A y are reasonably explained. The values of the parameters A x and A z (which were not reported) are suggested and remain to be checked by the further experimental studies.

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