Papers by Keyword: Defect Structures

Abstract: The spin Hamiltonian parameters (SHPs, g factors and hyperfine structure constants) defect structure for LiMgPO₄ doped with 0.1% Co²⁺ at 4.2 K are theoretically investigated from the perturbation formulas of the SHPs for a rhombically distorted octahedral 3d⁷ cluster. The impurity Co²⁺ on host Mg²⁺ site is found to suffer the larger axial relative compression ratio ρ (≈ 0.76%) and the planar angular variation Δφ (≈ 6.64^°) related to the host oxygen octahedron due to size mismatch. The calculated SHPs based on the above defect structure show good agreement with the measured results. Present study can be helpful to the preparation and characterization for the local structures for transition-metal impurities in lithium-magnesium phosphate, which may is helpful to search for the phosphor materials with better dosimetric characteristics.

Abstract: The spin Hamiltonian parameters (g factors g||, g and the hyperfine structure constants A||, A) for the Cu²⁺ centers in alkali barium borate glasses were theoretically studied based on the high-order perturbation formulas of these parameters for a 3d⁹ ion in a tetragonally elongated octahedron. From the calculations, the ligand octahedra around Cu²⁺ are suggested to suffer about 9.4%, 10.7%, and 11.1% relative elongation along C4 axis for the alkali barium borate glasses (Li-Ba-B, Na-Ba-B and K-Ba-B, respectively), the results are in good agreement with the observed values. In addition, the negative signs for A|| and A of the studied Cu²⁺ centers were also suggested in the discussion.

Abstract: The isotropic g factor and hyperfine structure constant for Ir⁴⁺ in CdO are theoretically studied from the perturbation formulas of these parameters for an octahedral 5d⁵ cluster based on the cluster approach. The calculated results show good agreement with the experimental data. The ligand orbital contributions should be taken into account due to significant covalency of the system with high impurity valence state even in the oxide.

Abstract: The defect structure for Ni3+ in ZnO crystal is theoretically investigated using the perturbation formulas of the spin Hamiltonian parameters for a 3d7 ion in trigonally distorted tetrahedra. In view of the significant covalency of the system due to the high valence state of Ni3+, the ligand orbital and spin-orbit coupling contributions are taken into account in a uniform way based on the cluster approach. The impurity Ni3+ is found not to occupy the ideal Zn2+ site in ZnO but to undergo the small axial displacement of about 0.044 Ǻ away from the oxygen triangle along the C3 axis. The theoretical spin Hamiltonian parameters based on the above impurity displacement show good agreement with the experimental data. The defect structure of this impurity center is compared with that for the similar Fe3+ in ZnO.

Abstract: The EPR parameters and the local structure for Co2+ in ZnO are deduced from the perturbation formulas of these parameters for a 3d7 ion in a trigonally distorted tetrahedron. The ligand orbital and spin-orbit coupling contributions are taken into account uniformly from the cluster approach in view of the covalency of the system. The impurity V3+ is found not to locate exactly on the Zn2+ site but to experience a small displacement of 0.04 Ǻ away from the ligand triangle, along the C3 axis. The theoretical EPR parameters based upon the above impurity displacement are in good agreement with the observed values.

Abstract: The electron paramagnetic resonance parameters (g-factors and the hyperfine structure constants) for the Cu2+ center in lanthanum magnesium nitrate (LMN) are theoretically studied from the conventional perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. The studied complex is found to exhibit the slight tetragonal elongation (characterized by the relative elongation ratio ρ  4%) due to the Jahn-Teller effect, which may entirely conceal the original trigonal distortion of the host Mg2+ site in LMN. The conventional formulas containing only the metal orbital and spin-orbit coupling contributions are proved to be valid for the Cu2+ center in view of the weak covalency and ligand spin-orbit coupling interactions. This defect is also compared with the similar Cu2+ center of the Jahn-Teller nature on the octahedral interstitial site in the CdSe nanocrystals.

Abstract: The axial displacements for Co2+ and Ni3+ in Al2O3 are theoretically investigated starting from the perturbation formulas of the EPR parameters for a 3d7 ion with high spin (S=3/2) and low spin (S=1/2) in trigonal symmetry, respectively. Based upon these studies, the Co2+ is found to shift towards the center of the oxygen octahedron by an amount
ZCo (≈ 0.03 Å) along the C3 axis, while the Ni3+ may suffer another axial displacement
ZNi (≈ −0.38 Å) away from the center of the octahedron. The calculated EPR parameters based upon the above axial displacements show good agreement with the observed values. The differences in the EPR parameters and the axial displacements for the two 3d7 ions are discussed.