Yue-Xia Hu

Spin Hamiltonian parameters and local structures for tetragonal and orthorhombic Ir2+ centers in AgCl

The perturbation formulae of the spin Hamiltonian parameters (the anisotropic g factors, hyperfine structure constants and superhyperfine parameters) are established for a 5d7 ion in an orthorhombically elongated octahedron based on the cluster approach. These formulae are applied to the theoretical studies of the EPR spectra and the local structures for the tetragonal and orthorhombic Ir2+ centers in AgCl. For the tetragonal Ir2+ center, the uncompensated substitutional [IrCl6]4 cluster is found to experience a relative elongation of about 0.08 Å along the C 4 axis due to the Jahn–Teller effect. For the orthorhombic center, the ligand octahedron also suffers Jahn–Teller elongation (by about 0.08 Å) along the [001] (or Z) axis. Meanwhile, the ligand Cl intervening in the impurity Ir2+ and the next nearest neighbor silver vacancy VAg along the [100] (or X) axis may undergo an inward displacement of 0.004 Å towards the center of the octahedron due to electrostatic repulsion of the VAg. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with experimental data for both centers.

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.

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 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.

Investigations on the EPR Parameters for the Square Planar Cu(2+) Centers in K(2)PdX(4) (X = Cl, Br)

The EPR parameters g factors g||, g⊥ and the hyperfine structure constants A|| and A⊥ for the square planar Cu2+ centers in K2-PdX4 (X = Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under tetragonally elongated octahedra. In these formulas, not only the contributions from the conventional crystal-field (CF) mechanism, but also those from the charge-transfer (CT) mechanism are taken into account. The related molecular orbital coefficients are uniformly determined from the cluster approach, and the tetragonal field parameters Ds and Dt are obtained from the superposition model and the local structures of the systems. Based on only one adjustable parameter, the present results are in reasonable agreement with the observed values. Importance of the charge-transfer contributions is more significant for ligand Br than that for Cl due to the stronger covalency and much larger spin-orbit coupling coefficient of the former.

Investigations of the local structures and spin Hamiltonian parameters for the trigonal Rh4+ and Ir4+ centers in rhombohedral BaTiO3.

The local structures and spin Hamiltonian parameters (g factors and the hyperfine structure constants) of the Rh(4+)(4d(5)) and Ir(4+)(5d(5)) centers in rhombohedral BaTiO(3) are theoretically investigated from the formulas of these parameters for a nd(5) (n=4 and 5) ion with low spin (S=1/2) in a trigonally distorted octahedron. From the calculations, the impurity ions are found not to occupy exactly the host Ti(4+) site in BaTiO(3) but to suffer a slight inward shift ( approximately 0.13A) towards the center of the oxygen octahedron along the C(3) axis, yielding much smaller trigonal distortion as compared with that of the host Ti(4+) site. The theoretical spin Hamiltonian parameters based on the above impurity axial shifts are in good agreement with the observed values.

Theoretical investigations of the EPR parameters and local structure for Cu2+in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu2+ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO4]6− cluster. From the calculations, the impurity Cu2+ is suggested not to occupy exactly the ideal Be2+ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C3 axis. The theoretical EPR parameters show good agreement with the experimental data.

THEORETICAL STUDIES OF THE SPIN HAMILTONIAN PARAMETERS AND LOCAL STRUCTURE FOR THE TETRAGONAL Rh2+ CENTER IN RHOMBOHEDRAL BaTiO3

The spin Hamiltonian parameters and local structure for the tetragonal Rh(2+) center in rhombohedral BaTiO(3) are theoretically studied from the perturbation formulas of these parameters for a 4d(7) ion with low spin (S = 1/2) in tetragonally elongated octahedra. This center is ascribed to substitutional Rh(2+) at the Ti(4+) site in BaTiO(3). The [RhO(6)](10-) cluster suffers relative elongation (characterized by the elongation parameter rho approximate to 0.9%) along the [100] axis due to the Jahn-Teller effect. The tetragonal elongation can entirely depress the slight trigonal distortion of the original Ti(4+) site in rhombohedral BaTiO(3). The calculated spin Hamiltonian parameters based on the above Jahn-Teller elongation show good agreement with the experimental results.

INVESTIGATIONS ON THE g FACTORS AND THE SUPERHYPERFINE PARAMETERS FOR THE TETRAGONAL Pd(3+) CENTERS IN AgX (X = Cl, Br)

The g factors and the superhyperfine parameters for the tetragonal Pd(3+) centers in AgX (X = Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 4d(7) ion in tetragonally elongated octahedra In the calculations, the related molecular or coefficients and the unpaired spin densities of the ligands are determined quantitatively from the cluster approach in a uniform way. Both centers originate from substitutional Pd(3+) on Ag(+) site in AgX, with no charge compensator (silver vacancy) nearby. The [PdX(6)](3-) clusters suffer a relative elongation of 0.01 angstrom (or 0.06 angstrom) along [1 0 0] (or C(4)) axis for X = Cl (or Br) due to the Jahn-Teller effect The calculated g factors and the superhyperfine parameters based on the above Jahn-Teller elongations show good agreement with the experimental data.

Investigations of the defect structures and the EPR parameters for the substitutional Mo(5+) centers in rutile type crystals

The defect structures and the electron paramagnetic resonance parameters for the substitutional Mo5+ centers in rutile type SnO2, TiO2 and GeO2 crystals are theoretically investigated from the perturbation formulas of these parameters for a 4d1 ion in rhombically compressed octahedra. The [MoO6]7− clusters suffer the Jahn–Teller effect and transform the ligand octahedra from original elongation on host tetravalent sites to compression in the impurity centers, with additional smaller rhombic (perpendicular) distortions when compared with those in the hosts. The defect structures and the importance of the ligand contributions are discussed.