Li-Hua Wei

Investigations on the local structure and the EPR parameters for Cu(2+)-doped GaN

The local structure and the EPR parameters (g factors and the hyperfine structure constants) for Cu(2+) in GaN are theoretically studied from the perturbation formulas of these parameters for a 3d(9) ion in trigonally distorted tetrahedra. In these formulas, the ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach, in view of the strong covalency effect of the system. Based on the studies, the impurity Cu(2+) is found not to occupy exactly the host Ga3+ site but to suffer a slight displacement (approximate to 0.004 angstrom) towards the ligand triangle along C(3) axis due to charge and size mismatching substitution. The theoretical EPR parameters show good agreement with the experimental data. The validity of the impurity displacement is also discussed.

Studies of the spin Hamiltonian parameters and local structure for ZnO:Cu2+

The spin Hamiltonian parameters (the g factors and the hyperfine structure constants) and local structure for ZnO:Cu2+ are theoretically studied from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach due to the significant covalency of the [CuO4](6-) cluster. According to the investigations, the impurity Cu2+ is suggested not to locate on the ideal Zn2+ site in ZnO but to undergo a slight outward displacement (approximately 0.01 angstroms) away from the ligand triangle along C3 axis. The calculated spin Hamiltonian parameters are in good agreement with the observed values. The validity of the above impurity displacement is also discussed.

Studies on the spin Hamiltonian parameters of vitamin B12r

The spin Hamiltonian parameters g factors g(i) (i=x, y, z) and the hyperfine structure constants A(i) of vitamin B(12r) have been theoretically studied from the perturbation formulas of these parameters for a Co(2+)(3d(7)) ion with low spin (S=1/2) in rhombically distorted octahedra. The related crystal-field parameters are determined from the point-charge-dipole model and the local structure around Co(2+) in vitamin B(12r). The theoretical spin Hamiltonian parameters are in good agreement with the experimental data.

Studies of the g factors and the local structure for Ni3+in LaAl0.9Ni0.1O3, La0.75Y0.25Al0.99Ni0.01O3 and YAl0.9Ni0.1O3

The electron paramagnetic resonance g factors and the local structure for Ni3+ in LaAl0.9Ni0.1O3 (LAN), La0.75Y0.25Al0.99Ni0.01O3 (LYAN) and YAl0.9Ni0.1O3 (YAN) are theoretically studied from the perturbation formulas of the g factors for a 3d7 ion of low spin (S = 1/2) in tetragonally elongated octahedra. In these formulas, the contributions to the g factors from the tetragonal distortion, characterized by the tetragonal field parameters Ds and Dt are taken into account. According to the calculations, the ligand octahedra around Ni3+ are suggested to suffer 2% relative elongation along the [001] (or C4) axis due to the Jahn-Teller effect.

Studies on the local structure and the spin Hamiltonian parameters for the trigonal Mn2+ center in Bi4Ge3O12

The local structure and the spin Hamiltonian parameters (the zero-field splitting D, the g factors g parallel to, g perpendicular to and the hyperfine structure constants A// and A perpendicular to) for the trigonal Mn2+ center in Bi4Ge3O12 are theoretically studied from the perturbation formulas of these parameters for a 3d(5) ion in trigonal symmetry. The impurity Mn2+ replacing host Bi3+ is not found to occupy the exact Bi3+ site but to suffer a large off-center displacement by about 0.36 angstrom towards the center of the oxygen octahedron along the C-3-axis due to the size and charge mismatching substitution. The calculated spin Hamiltonian parameters based on the above displacement show good agreement with the observed values. The results and the mechanism of the impurity displacement are discussed.

Investigations of the spin Hamiltonian parameters and the local structures of the substitutional V(4+) centres in rutile-type MO(2) (M = Sn, Ti, Ge)

The spin Hamiltonian parameters and the local structures of the substitutional V4+ centres in rutiletype MO2 (M = Sn, Ti and Ge) are theoretically investigated from the perturbation formulas of these parameters for a 3d1 ion in a rhombically compressed octahedron. The oxygen octahedra around V4+ are found to transform from the original elongation on the host M4+ site to compression in the impurity centres due to the Jahn-Teller effect. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with the experimental data.

Studies on the EPR parameters for the interstitial tetragonal Co2+ center in NH4I

The electron paramagnetic resonance (EPR) parameters (g factors g //, g ⊥ and the hyperfine structure constants) for the interstitial Co2+ center in NH4I are theoretically investigated by the diagonalization of the 6×6 energy matrix of the 4T1 ground state for a 3d7 ion under tetragonal symmetry. The contributions from the admixtures of various J (=1/2, 3/2, 5/2) states and the ligand orbital and spin–orbit coupling interactions, which were normally ignored in previous works, are taken into account in the present treatments. The contributions to the EPR parameters are discussed, and the discrepancies between the calculated hyperfine structure constants and the experimental data are also analyzed.

Theoretical investigations on the spin Hamiltonian parameters and local structures for the trigonal Ni2+ centers in CsMgX3 (X=Cl, Br, I)

The spin Hamiltonian parameters (zero-field splitting and the anisotropic g factors) and the local structures for the trigonal Ni2+ centers in CsMgX3 (X=Cl, Br, I) are theoretically investigated from the perturbation formulas of these parameters for a 3d8 ion in trigonally distorted octahedra, by including the ligand s-orbital contributions. Based on the studies, the local impurity-ligand bond angles β related to the C 3 axis in the Ni2+ centers are found to be about 2° larger than the corresponding angles, βH, in the hosts, due to the size mismatching substitution of Mg2+ by Ni2+. The theoretical results based on the inclusion of the ligand s-orbital contributions show an improvement when compared with those in the absence of the above contributions, especially for the ligand I−.

THEORETICAL STUDIES OF THE LOCAL STRUCTURES AND SPIN HAMILTONIAN PARAMETERS FOR THE RHOMBIC Ni(2+) CENTERS IN AgX (X = Cl, Br)

The local structures and the spin Hamiltonian parameters (zero-field splittings D and E and the anisotropic g factors gx, gy and gz) for the rhombic Ni2+ centers in AgX (X = Cl, Br) are theoretically studied from the perturbation formulas of these parameters for a 3d8 ion under rhombically distorted octahedra. In these formulas, the ligand p- and s-orbital and spin-orbit coupling contributions are included on the basis of the cluster approach. The rhombic centers are attributed to the impurity Ni2+ associated with one nearest-neighbor silver vacancy VAg along the [110] direction as charge compensation. Based on the calculations, Ni2+ is found to suffer an off-center displacement 0.092 A (or 0.335 A) for AgCl (or AgBr) towards the VAg along the [110] axis, while the ligands closest to the VAg undergo a small shift 0.065 A (or 0.006 A) away from (or towards) the VAg. The theoretical spin Hamiltonian parameters based on the above defect structures show good agreement with the experimental data.