Wen-Chen Zheng

LOCAL ROTATION ANGLE IN THE STRUCTURAL PHASE-TRANSITION FOR THE MN2+ ION IN A CSCACL3 CRYSTAL

In this paper, the spin-lattice coupling coefficient G(11) for Mn2+ in the cubic phase of a CsCaCl3 crystal has been calculated from the three microscopic mechanisms which contribute to zero-field splitting. Based on this, the local rotation angle phi(T) for Mn2+ in the tetragonal phase of a CsCaCl3 crystal is estimated from the EPR zero-held splitting b(2)(0)(T). The reasonableness of the local rotation angle is discussed.

Theoretical investigations of the zero-field splitting and gyromagnetic factors for the tetragonal V2+ clusters in MCl (M=Na,K,Rb) crystals

In this paper, the perturbation formulas for zero-held spliting D and gyromagnetic factors g(parallel to) and g(perpendicular to) for d(3) ions in tetragonal symmetry are established by using a two spin-orbit coupling parameter model, in which not only the contribution due to the spin-orbit coupling of the central d(3) ions but also that of the ligands are included. Based on these formulas, the zero-field splitting D, the g-shifts Delta g(parallel to)(=g(parallel to)-g(s)) and Delta g(perpendicular to)(=g(perpendicular to)-g(s)) are calculated for the tetragonal V2+ clusters in MCl crystals from the spectral parameters in the previous papers obtained from the experimental data. All the calculated results show good agreement with the observed values. By comparing these calculated results with those based on one spin-orbit coupling parameter model, where only the contribution due to the spin-orbit coupling of d(3) ions is taken into account, one can find that the spin-orbit coupling coefficient of ligand is much greater than that of the central d(n) ions; the contributions to the EPR parameters from the spin-orbit coupling of ligands cannot be neglected.

LOCAL TRIGONAL DISTORTIONS FOR THE SUBSTITUTING CR3+ IONS IN GARNET CRYSTALS

The local trigonal distortions beta - beta(0) for the substituting Cr3+ ions in garnet (YAlG, LuAlG and YGG) crystals are determined from a more reasonable method based on the studies of the zero-field splitting D, the first excited state splitting Delta((2)E) and the anisotropy of the g factor (characterized by Delta g = g(parallel to) - g(perpendicular to)). The results show that the local octahedral environments of the substituting Cr3+ ions are more regular than those of the host ions. This confirms the previous assumption for the local environment of Cr3+ in YGG based on the studies of only part of these spectral parameters.

Relative magnitude between the axial and equatorial impurity-ligand distances for RbCaF3:Ni2+ crystal in the structural phase transition

In this paper, by studying the local release factor k of the structural phase transition for RbCaF3:Ni2+ crystal from the temperature dependence of electron paramagnetic resonance spectra, the relative magnitude between the axial and equatorial impurity-ligand distances is found to be different from that between the axial and equatorial cation-ligand distances in the host RbCaF3 crystal.

Investigations of the zero-field splittings and g factors for the rhombic Cr3+–Li+ and Cr3+–Na+ defect centers in Cs2CdCl4 crystals

Abstract The high-order perturbation formulas for the zero-field splitting parameters D and E , and g factor g x , g y , g z for 3d 3 ions in rhombic symmetry are derived by using a two spin–orbit coupling parameter model, in which both the contributions from the spin–orbit coupling of central d 3 ion and that of the ligands are considered. From these formulas, spin Hamiltonian parameters for the rhombic defect centers Cr 3+ –Li + and Cr 3+ –Na + in Cs 2 CdCl 4 crystals are calculated by considering the suitable defect-induced lattice distortions. The results agree well with the observed values. The contributions to spin Hamiltonian parameters from the spin–orbit coupling of ligands are discussed.

Explanations of the optical spectra and g factor for V3+ ions in ZnX (X = S, Se, Te) crystals

This paper presents a cluster approach for the calculations of the absorption band positions and the g factor of d(2) tetrahedral cluster. In the approach, the different modifications of e and t(2) orbitals of d electrons and the contribution from the spin-orbit coupling of ligands to the EPR g factor due to the mixing of d(2) ions with the ligands are considered. The calculated band positions and g factor for ZnX : V3+ (X = S, Se, Te) crystals show good agreement with the observed values by using only two adjustable parameters. The contribution from the spin-orbit coupling of ligands to the g factor is discussed

Superposition-model investigations of the zero-field splittings and their stress and temperature dependence for Mn2+ in guanidinium aluminum sulfate hexahydrate

The zero-field splittings of sites I and II for Mn2+ in guanidinium aluminum sulfate hexahydrate (GASH) are calculated from the superposition model by using the normal intrinsic parameter b2(R0) obtained from Mn2+ ions in many oxygen octahedra. The results are in good agreement with the experimental findings. Based on this, the local compressibilities and local thermal expansion coefficients in the vicinity of the impurity ion Mn2+ have been studied from the stress and temperature dependence of zero-field splitting and are found to be half of the host ones. The reasonableness of these local values is discussed.

A study of the temperature dependence of g factor for MgO:V2+ crystal

The temperature dependence of EPR g factor for MgO:V2+ crystal has been studied theoretically by considering both the static contribution due to the thermal expansion of crystal and the vibrational contributions due to the electron-phonon interaction of the acoustic and optical branches. The static contribution is calculated from the high-order perturbation formula of g factor and the thermal expansion coefficients alpha(T) at various temperatures. The vibrational contribution of acoustic phonons is obtained by using a Debye model for the lattice vibrations and that of optical phonons is calculated by use of a single frequency model. The calculated results show that for the g factor at various temperatures, the static contribution is dominant, but for the temperature dependence of g factor, i.e., (dg/dT)(p) or Delta g(T) = g(T) - g(0), the vibrational contributions from the acoustic and optical phonons are comparable with the static contribution due to thermal expansion. It appears that a reasonable theoretical explanation for the temperature dependence of g factor should take all of these contributions into account.

Local compressibility around the impurity in nickel-doped CaO crystal

In this paper, by using the bond length dependence of Racah parameter B obtained from the high pressure spectroscopy of MgO:Ni2+ crystal, the local compressibility around the impurity ion in nickel-doped CaO crystal has been determined from the pressure-induced spectral shifts. The result is greater than that in the pure CaO crystal. The reasonableness of this local value is discussed.

Studies of the spin-Hamiltonian parameters and defect structures for the tetragonal and cubic Yb(3+) centers in AgCl crystal

The spin-Hamiltonian parameters, g factors and hyperfine structure constants, for the tetragonal and cubic Yb(3+) centers in AgCl crystal are calculated from the complete diagonalization (of energy matrix) method. The calculations are based on the defect models that the tetragonal Yb(3+) center is formed by the substitutional Yb(3+) associated with two nearest Ag(+) vacancy (V(Ag)) along <001> and <001¯> axes (i.e., C(4) axis) owing to charge compensation and in cubic Yb(3+) center the two compensators V(Ag) are remote. From the calculations, the spin-Hamiltonian parameters of both Yb(3+) centers in AgCl are explained reasonably, the signs of hyperfine structure constants A(i)((171)Yb(3+)) and A(i)((173)Yb(3+)) are suggested, the above defect models of both Yb(3+) centers are confirmed and their defect structures are determined.