Theoretical Study Of The Spin Hamiltonian Parameters For 3d~9 Ions In Low Symmetrical Crystal-fields

Many functional materials doped with 3d9(Cu2+) ions exhibit novel magnetic,catalytic, conductive, non-linear optical properties and self-assembly structural features and have aroused extensive attention. Normally, optical and magnetic properties of these materials depend sensitively on the immediate environments of transition-meal dopants(e.g., Cu2+), which can be effectively investigated by means of electron paramagnetic resonance(EPR) spectroscopy. For 3d9 ions, the important systems of transition-metal groups, EPR measurements have yielded abundant experimental data, which were described by the spin Hamiltonian parameters(anisotropic g factors and hyperfine structure constants). Unfortunately, previous theoretical analysis for the above results has some shortcomings, e.g., the conventional crystal-field model without the ligand orbital and spin-orbit coupling contributions, failing to connect EPR analysis with local structure in the vicinity of impurities.In order to remove the above imperfections, the improved formulae of the spin Hamiltonian parameters are constructed in this work for a 3d9 ion in tetragonally and rhombically(orthorhombically) elongtaed octaheda based on the cluster approach by including the ligand contributions. Meanwhile, the quantitative relationships among the relevant model parameters such as crystal-field parameters, normalization factors and the local structural information and observed optical spectral data are theoretically established in a uniform way. The above formulae are applied to the following 3d9 systems having low symmetries, and the experimental EPR results are satisfactorily interpreted.1) For the tetragonal Cu2+ centers in NaCl and AgCl, the theoretical spin Hamiltonian parameters based on the improved formulae are in good agreement with the observed values. It is found that the impurity centers show the relative elongations of about 0.15 and 0.08 ? for NaCl and AgCl, respectively, along the C4 axis due to the Jahn-Teller effect. Although the spin-orbit coupling coefficient of ligand Cl– is slightly smaller than that of central ion Cu2+, the ligand contributions cannot be ignored in the EPR analysis because of obvious covalency of the systems.2) For the orthorhombic Cu2+ in oxygenated and nonoxygenated BaCuO2+x, the[CuO6]10-groups suffer the relative elongations of 1% and 0.6% along c axis and theplanar relative bond length changes of 8.9% and 6.9% along a and b axes, respectively,owing to the Jahn-Teller effect. The above local orthorhombic distortions can acount for the experimentally measured axial and vertical g anisotropies. The investigations in this work would be helpful to understand the EPR behaviors of the parasitic phase BaCuO2+x and the influences on spectroscopic properties and related superconductivity of host R123 high-temperature superconductors.3) The experimental spin Hamiltonian parameters of rhombic Cu2+in[Cu(ipt)(dap)H2O]n?nH2O were reasonably explained. The measured near-axial anisotropic g factors and hyperfine structure constants can be attributed to the significantly elongated pentagonal pyramid [CuN2O3] group. However, the tiny vertical anisotropy of g factors is ascribed to the different planar ligands N and O, whose contributions along the perpendicular direction may largely cancel each other and result in the near- axial EPR signal within experimental error. The above analysis would be useful to the understandings of the local structures and properties for[Cu(ipt)(dap)H2O]n?nH2O and relevant systems.4) For Cu2+in [Cu(men)2(BF4)2], the suitable explanation of the anisotropic g factors is carried out based on the high-order perturbation formulae for rhombically elongated octahedral 3d9 clusters. The theoretical g factors and the anisotropies show good agreement with the experimental data. The hexacoordination [CuN4F2] group exhibits significant rhombic elongation distortion, responsible for the observed near – axial anisotropies of g factors. In view of significant covalency due to the short Cu2+-N3–bond and typical covalent ligand N3–, the ligand orbital and spin-orbit coupling contributions are important and should be considered in the computations.