【作者】 张华明；

【导师】 邬劭轶；

【作者基本信息】 电子科技大学 ， 凝聚态物理， 2011， 博士

【摘要】 不少功能材料的性能很大程度上取决于掺杂其中的过渡离子的电子结构和杂质缺陷行为,而电子顺磁共振（EPR）谱是研究掺过渡离子晶体和络合物光学和磁学以及局部结构性质的有效手段。3d⁹离子是过渡族中具有代表性和非常重要的体系,它在理想立方对称下只有一个基态和一个激发态,因其较简单的能级结构而备受关注。对于掺杂3d⁹离子的材料,已有大量EPR实验研究的报道,其实验结果通常用自旋哈密顿参量（各向异性g因子、超精细结构常数和超超精细结构参量等）描述。但是对上述实验结果的理论解释却不太令人满意,主要表现在i)前人工作大多基于简单的二阶微扰公式,且只考虑了中心离子旋轨耦合系数的贡献,而忽略了配体轨道和旋轨耦合作用的影响;ii)由于未能建立自旋哈密顿参量与杂质局部结构的关系,依靠引入较多调节参量来描述低对称畸变;iii)通常直接拟合两个超超精细结构参量实验值来获得未配对自旋密度f_s和f_σ,而未能建立它们与轨道混合系数和体系共价性等的定量关系。为了克服上述不足,本工作基于配位场理论,利用四角伸长和斜方（或正交）伸长（或压缩）八面体以及四角四面体中3d⁹(Ni⁺、Cu²⁺)离子自旋哈密顿参量高阶微扰公式,对一些前人未曾处理或满意解释的3d⁹体系进行了系统深入的理论分析,合理地解释了它们的EPR实验结果,并获得了杂质中心的局部结构信息。1)研究了PrBa₂Cu₃O_6+x和Pr_0.5Er_0.5Ba₂Cu₃O_6+x中Cu²⁺离子中心以及RbCaF₃中无电荷补偿Ni⁺中心I和轴线上分别出现一个和两个F-^离子空位的中心II和III的EPR谱和局部结构。对PrBa₂Cu₃O_6+x和Pr_0.5Er_0.5Ba₂Cu₃O_6+x体系,发现Jahn-Teller效应导致Cu-O键长沿C₄轴分别伸长约0.05（A|°）和0.01（A|°）。针对RbCaF₃中的三类Ni⁺中心,在离子簇模型基础上考虑了配体轨道和旋轨耦合作用的贡献,建立了四角场参量、分子轨道系数和未配对自旋密度等与杂质局部结构或光谱数据的关系。计算表明,Jahn-Teller效应引起中心I的杂质-配体键长沿C₄轴伸长5%;超超精细结构参量的研究表明,中心I、II和III的未配对自旋密度分别为f_s（≈0.28%、0.30%和0.31%）和f_σ（≈1.55%、2.39%和2.68%）。2)针对斜方（或正交）伸长（或压缩）八面体中3d⁹离子的自旋哈密顿参量,利用其高阶微扰公式研究了相关体系的EPR谱和局部结构性质。i)合理解释了Y₂BaCuO₅中正交伸长八面体下Cu²⁺中心的g因子g_x、g_y和g_z,发现Jahn-Teller效应引起配体八面体沿c轴伸长约0.05（A|°）,同时沿a和b轴方向的平面键长相对变化约为0.1（A|°）。ii)满意地解释了TiO2:Cu²⁺的各向异性g因子和超精细结构常数。由于Jahn-Teller效应,平面杂质-配体键将发生弯曲,导致键角比母体值增大约5.8°,从而显著减小了体系的斜方畸变。此外,还满意地解释了该杂质中心的光谱实验数据。iii)建立了正交压缩八面体中3d⁹离子自旋哈密顿参量的高阶微扰公式,据此分析了钨酸盐AWO₄（A= Zn、Cd和Mg）中Cu²⁺的EPR谱和局部结构性质。发现Cu²⁺替代母体A²⁺后,由于Jahn-Teller效应和杂质-母体离子尺寸失配,杂质中心的配体八面体平面键长相对母体时略有变化,即CdWO₄、ZnWO₄和MgWO₄中的杂质局部正交畸变可用平面杂质-配体键长相对差值0.096、0.021和0.028（A|°）表述。3)在离子簇模型基础上建立了四角畸变四面体中3d⁹离子自旋哈密顿参量的微扰公式,并将其中一些重要参数（如四角场参量、分子轨道系数等）与杂质局部结构和光谱数据相联系。将该公式应用于黄铜矿型ABS2硫化物中的四角Ni⁺中心,合理地解释了EPR实验结果。杂质Ni⁺取代母体A⁺离子后,由于尺寸失配,杂质-配体局部键角将有所改变。计算表明,CuAlS₂、CuGaS₂和AgGaS₂的局部键角变化分别为-1.73°、-1.44°和-4.54°。此外,由于体系具有明显的共价性,配体轨道和旋轨耦合作用的贡献不能忽略。4)基于离子簇模型,建立了四角伸长八面体中4d7离子g因子和超超精细结构参量的高阶微扰公式,其中相关的分子轨道系数和未配对自旋密度由离子簇模型统一得到,并将该公式应用于AgX（X=Cl, Br）中的四角Pd³⁺中心。研究表明,Pd³⁺替代母体Ag⁺后,Jahn-Teller效应使[PdX₆]^3–基团沿C₄轴分别伸长0.01和0.06（A|°）。有趣的是,四角伸长八面体中4d⁷离子的EPR行为与四角压缩八面体中3d⁹离子的情形非常类似,例如都表现出²A_1g基态和g_s（≈2.0023）≤g//≤g⊥的各向异性行为。同时,基于统一的理论公式和较少调节参量的超超精细结构参量计算值与实验符合较好,并克服了前人直接拟合超超精细结构参量实验值获得未配对自旋密度的不足。更多还原

【Abstract】 Properties of various functional materials largely depend on the electronic structures and impurity （or defect） behaviours of the doped transition-metal ions in the hosts. Electron paramagnetic resonance （EPR） is a powerful tool to investigate optical, magnetic and local structure properties of crystals and complexes containing transition-metal ions. 3d⁹ ions belong to the typical and important systems of the transition-metal groups and attract extensive interests of researchers due to the relatively simpler energy structure with only one ground state and one excited state under ideal cubic symmetry. Abundant EPR experimental data have been reported for many materials doped with 3d⁹ ions, which were usually described as the spin Hamiltonian parameters （anisotropic g factors and the hyperfine structure constants）. Up to now, however, theoretical explanations to these experimental results seem not satisfactory yet. i) The previous studies were often based on the simple second-order g formulas by considering only the central ion orbital and spin-orbit coupling contributions, while the influences of the ligand orbital and spin-orbit coupling interactions were not taken into account. ii) The previous treatments did not establish the theoretical relationships between EPR spectra and local structures of the systems but introduced various adjustable parameters to describe the low symmetrical distortions. iii) In the previous studies of the superhyperfine parameters, the unpaired spin densities were normally estimated by fitting the two experimental superhyperfine parameters, failing to connect the unpaired spin densities with the orbital admixture coefficients or covalency of the systems.In order to overcome the above shortcomings, the high order perturbation formulas of the spin Hamiltonian parameters based on the ligand-field theory for 3d⁹ ions in tetragonally elongated and rhombically （or orthorhombically） elongated （or compressed） octahedra and tetragonally distorted tetrahedra are adopted to the studies of some 3d⁹ (e.g., Cu²⁺, Ni⁺) centers, which have not been theoretically treated or unsatisfactorily investigated. These EPR experimental results are reasonably interpreted, and information of the local structures of the impurity centers are also acquired.1) The EPR spectra and the local structures are theoretically investigated for the Cu²⁺ centers in PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x as well as the uncompensated Ni⁺ center I and centers II and III with one and two nearest neighbour F– vacancy in RbCaF3. As for PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x, the Cu-O bonds suffer elongations of 0.05 and 0.01（A|°）, respectively, along the C₄ axis due to the Jahn-Teller effect. As regards the three Ni⁺ centers in RbCaF₃, the quantitative relationships of the tetragonal field parameters, the molecular orbital coefficients and the unpaired spin densities with the impurity local structures or experimental optical spectral data are established on the basis of the cluster approach including ligand orbital and spin-orbit coupling contributions. It is found that the impurity-ligand bonds in center I experience the relative elongation of 5% along the C₄ axis due to the Jahn-Teller effect. The calculations of the superhyperfine parameters for the centers I, II and III reveal that the unpaired spin densities are f_s（≈0.28, 0.30 and 0.31%） and f_σ（≈1.55, 2.39 and 2.68%）, respectively.2) The high order perturbation formulas of the spin Hamiltonian parameters for a 3d⁹ ion under rhombically （or orthorhombically） elongated （or compressed） octahedra are applied to the studies of the EPR spectra and the local structure properties for the relevant systems. i) The anisotropic g factors gx, gy and gz are reasonably explained for the orthorhombic Cu²⁺ center in Y₂BaCuO₅, and the oxygen octahedron is found to undergo the elongation of about 0.05（A|°） along c axis and the relative planar bond length variation of about 0.1（A|°） along a and b axes due to the Jahn-Teller effect. ii) The g factors and the hyperfine structure constants are satisfactorily interpreted for the rhombic Cu²⁺ center in TiO₂. The planar impurity-ligand bond angle is found to be about 5.8°larger than the host value due to the Jahn-Teller effect via bending the planar bonds, which considerably reduces the rhombic distortion of the system. In addition, the experimental optical spectral data are also uniformly interpreted. iii) The high order perturbation formulas of the spin Hamiltonian parameters for a 3d⁹ ion under orthorhombically compressed octahedra are established and applied to the Cu²⁺ centers in AWO₄（A= Zn, Cd and Mg）. The planar impurity-ligand bond lengths are found slightly different from those in the hosts, i.e., the orthorhombic distortions can be described by the relative planar bond length variations 0.096、0.021 and 0.028 （A|°） along X and Y axes for CdWO₄, ZnWO₄ and MgWO₄, respectively.3) Based on the cluster approach, the perturbation formulas of the spin Hamiltonian parameters for a 3d⁹ ion under tetragonally distorted tetrahedra are established, and the related parameters （e.g., the tetragonal field parameters and the molecular orbital coefficients） are correlated with the local structures and the optical spectral data of the systems. These formulas are applied to the tetrahedral tetragonal Ni⁺ centers in ABS₂ （A = Cu, Ag; B = Al, Ga）, and the EPR and optical spectral data are satisfactorily interpreted. Around the impurity Ni⁺ replacing the host A⁺, the local impurity-ligand bond angle may be different from the host value due to the size mismatching substitution. It is found that the local bond angles are about 1.73, 1.44 and 4.54°smaller than the host values for CuAlS₂, CuGaS₂ and AgGaS₂, respectively. In addition, the ligand orbital and spin-orbital coupling contributions should be considered in view of the significant covalency of the systems.4) The perturbation formulas of the anisotropic g factors and the superhyperfine parameters for a 4d7 ion under tetragonally elongated octahedra are established, and the related molecular orbital coefficients and the unpaired spin densities are determined from the cluster approach in a uniform way. These formulas are applied to the tetragonal Pd³⁺ centers in AgX （X=Cl, Br）. From the calculations, the [PdX₆]^3- clusters are found to suffer the elongations of about 0.01 and 0.06 （A|°） along the C4 axis for AgX and AgBr, respectively, due to the Jahn-Teller effect. Interestingly, the EPR behaviours for a 4d⁷ ion under tetragonally elongated octahedra are quite similar to those for a 3d⁹ ion under tetragonally compressed octahedra, e.g., both exhibiting the same ²A_1g ground state and anisotropy of g_s（≈2.0023）≤g//≤g⊥. As compared with the previous works, the calculated superhyperfine parameters based on the uniform formulas and the fewer adjustable parameters in this work show good agreement with the experimental data, and the shortcoming of the unpaired spin densities obtained by fitting the two experimental superhyperfine parameters in the previous treatments is thus overcome.更多还原