Hua-Ming Zhang

Studies on the defect structure for Cu(2+) in CdSe nanocrystals

The defect structure for Cu(2+) in CdSe nanocrystals is theoretically studied by analyzing the spin Hamiltonian parameters of this impurity center. This center is ascribed to Cu(2+) occupying the octahedral interstitial site, rather than the tetrahedral substitutional Cd(2+) site proposed by previous work. The Cu(2+) center exhibits slight tetragonal elongation distortion (characterized by the elongation parameter rho approximately 0.03) due to the Jahn-Teller effect. The theoretical spin Hamiltonian parameters and optical transition show good agreement with the experimental data. The above unusual defect structure (occupation and symmetry) for Cu(2+) in CdSe nanocrystals is discussed, as compared with the conventional trigonally distorted tetrahedral Cu(2+) centers in bulk II-VI semiconductors.

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.

Investigations on the EPR g factors for the six-coordianted Cu2+(1) site in YBa2Cu3O6+x

The EPR g factors g(//) and g(perpendicular to) for the six-coordinated Cu2+(1) site in YBa2Cu3O6+x are theoretically investigated from the perturbation formulas for a 3d(9) ion in tetragonally elongated octahedra. The related tetragonal crystal-field parameters Ds and Dt are determined from the superposition model and the structural data for the Cu2+(1) site, by considering the effective elongation of the oxygen octahedron due to the Jahn-Teller effect of the Cu2+ cluster. Based on the calculations, the effective elongation (= 15%) of the ligand octahedron is obtained for the signals at 100K. As for the EPR signals at room temperature, the elongation declines to merely 4%, corresponding to the smaller anisotropy Delta g (=g(//) - g(perpendicular to)). The theoretical results for the g factors are in good agreement with the observed values, and the local tetragonal distortion due to the Jahn-Teller effect is discussed.

INVESTIGATIONS OF THE SPIN HAMILTONIAN PARAMETERS AND THE LOCAL STRUCTURE FOR THE RHOMBIC Cu(2+) CENTER IN RUTILE

The spin Hamiltonian parameters (the anisotropic g factors and the hyperfine structure constants) and the local structure for the rhombic Cu(2+) center in rutile (TiO(2)) are theoretically investigated using the formulas of these parameters for a 3d(9) ion in rhombically elongated octahedra. From the studies, the planar impurity-ligand bond angle is found to be about 5.8 degrees larger than that for the host Ti(4+) site due to the Jahn-Teller effect via bending the planar Cu(2+)-O(2-) bonds, which yields much smaller rhombic distortion in the impurity center. The theoretical spin Hamiltonian parameters based on the above local angular distortion show good agreement with the experimental data, and the improvement of the calculation results are also achieved as compared with those of the previous works.

Studies of the local structure and the g factors for the orthorhombic Ti3+ center in YAP

The local structure and the g factors for the orthorhombic Ti3+ center in YAP are theoretically studied from the perturbation formulas of the g factors gx, gy and gz for a 3d1 ion in orthorhombic symmetry based on the cluster approach. The observed large anisotropy of the g factors may be attributed to the significant low-symmetrical (tetragonal and orthorhombic) distortion near the Al3+ site occupied by the impurity Ti3+. The calculated g factors in this work show reasonable agreement with the experimental data.

Theoretical studies of the spin Hamiltonian parameters for the tetragonal Rh2+ centers in MgO and CaO

The spin Hamiltonian parameters (the anisotropic g factors, the hyperfine structure constants and the ligand superhyperfine parameters) for the tetragonal Rh(2+) centers in MgO and CaO are theoretically studied from the perturbation formulae of these parameters for a 4d(7) ion in tetragonally elongated octahedra. In these formulae, the related molecular orbital coefficients as well as the ligand unpaired spin densities are determined quantitatively from the cluster approach in a uniform way. The above centers are attributed to Rh(2+) occupying divalent cation sites in MgO (and CaO), associated with a relative elongation of 0.5% ( and 1.4%) along the [001] (or C(4)) axis due to the Jahn-Teller effect. The calculated spin Hamiltonian parameters based on the Jahn-Teller elongations are in good agreement with the observed values. The local structures of the impurity centers are 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 on the EPR parameters of the interstitial V4+ in rutile

The electron paramagnetic resonance (EPR) parameters g-factors g i (i=x, y, z) and the hyperfine structure constants A i for the interstitial V4+ in rutile are theoretically studied from the perturbation formulas of these parameters for a 3d1 ion in rhombically distorted octahedra. On the basis of the studies, the local axial distortion angle Δα′ in the impurity center is found to be about 2° smaller than the host value, characterized as stretching and contraction of the parallel and perpendicular bonding lengths by about 0.28 and 0.14 Å,respectively. This results in the less compressed ligand octahedron because of the Jahn–Teller effect and space effect arising from occupation of the impurity V4+ at the interstitial site. The theoretical EPR parameters based on the above local structural parameters of this work are in better agreement with the experimental data than those of the previous studies in the absence of the local angular distortion and the ligand orbital contributions. The two experimental optical absorption bands are also reasonably analyzed.

Investigations on the local angular distortions and the spin-Hamiltonian parameters for Ni+ in sulfides

The local angular distortions and the spin-Hamiltonian parameters (the g factors and the hyperfine parameters) for Ni(+) in ABS(2) (A=Cu, Ag; B=Al, Ga) ternary sulfides are theoretically investigated from the perturbation formulas of these parameters for 3d(9) ions in a tetragonally distorted tetrahedron. In view of the strong covalency of such systems, the ligand orbital and spin-orbit coupling contributions are taken into account using the cluster approach. The local impurity-ligand bond angles in the Ni(+) centers are found to be about 1.4-4.5 degrees smaller than those of the host monovalent A sites in the pure crystals, due to size mismatching substitution. As a result, the ligand tetrahedra exhibit slight elongation in CuBS(2):Ni(+) and slight compression in AgGaS(2):Ni(+). The calculated spin-Hamiltonian parameters, optical transitions and the relative intensity ratios show reasonable agreement with the experimental data.

Theoretical investigations of the spin hamiltonian parameters for the tetragonal Cu2+ centers in NH4X (X = Cl, Br, I)

The spin Hamiltonian (SH) parameters g factors g parallel to, g perpendicular to and the hyperfine structure constants A parallel to and A perpendicular to for the tetragonal Cu2+ centers (i.e., [CUX4(H2O)(2)] clusters) in NH4X (X = Cl, Br, I) are theoretically investigated by means of the diagonalization procedure of complete (10 x 10) energy matrix (DPCEM) for a 3d(9) ion in tetragonal symmetry. The crystal-field parameters in the energy matrix are determined from the superposition model. The calculated SH parameters are in reasonable agreement with the experimental data.