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1.
Complete high-order perturbation formulas are established based on charge-transfer (CT) and crystal-field (CF) mechanisms. The electron paramagnetic resonance (EPR) g-factors of MgTiO3:Cr3+, SrTiO3:Cr3+, and SrTiO3:Mn4+ crystals are calculated from the formulas. The calculations of the EPR g-factors agree well with the experimental values. The contribution rate of the CT mechanism to EPR parameters increases with increasing valence state of the 3d n ions in the crystals. For the higher-valence state 3d 3 Mn4+ ion in the crystals, the elucidation of the EPR parameters rationally involves both CF and CT mechanisms.  相似文献   

2.
The high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters (g factors g∥,g and zero-field splitting D) for 3d8 ions in trigonal octahedral clusters are established. These formulas contain the contributions not only from the crystal-field (CF) mechanism, but also from the charge-transfer (CT) mechanism (which is not considered in the widely used CF theory). From these formulas, the EPR parameters and the impurity-induced defect structures for Ni2+ ions in CdX2 (X=Cl, Br) crystals are studied. The calculated EPR parameters are coincident with the experimental values, and the defect structure of Ni2+ impurity center obtained from the calculation is different from the corresponding structure in the host crystal. The sign of QCT (Qg, Δg, or D) due to CT mechanism agrees with that of the corresponding QCF due to CF mechanism and the relative importance of CT mechanism (characterized by QCT/QCF) increases with increasing covalence of 3d8 clusters and hence with raising atomic number of ligand X. So, in the explanations of the EPR parameters of 3d8 (or other 3dn) ions in crystals with the heavy-element ligand ion (e.g., Br), the calculated formulas based on the two-mechanism (CF and CT mechanisms) model are preferable to those based on only the CF mechanism in the CF theory.  相似文献   

3.
The spin Hamiltonian parameters (g factors, hyperfine structure constants and zero-field splittings D and E) and local structures for Mn2+ and Ni2+ in [Zn(en)3](NO3)2 single crystal are theoretically investigated from the perturbation calculations for trigonally distorted 3d5 and trigonally (or orthorhombically) distorted 3d8 cluster. The trigonal Mn2+ and Ni2+ centres are found to undergo the moderate angular variations Δβ of 4.5° and 5.2°, respectively, related to host Zn2+ site due to size mismatch. The orthorhombic Ni2+ centre shows the relative axial elongation ratio ρ (≈ 2.5%) and the relative perpendicular bond length variation ratio τ (≈0.2%). For Mn2+ centre, the contributions to g-shifts ΔgCT (or hyperfine structure constants ACT and zero-field splitting DCT) from charge-transfer (CT) mechanism are opposite in sign and five times (or 5% and 8%) in magnitude compared with those from crystal-field (CF) mechanism. For the trigonal Ni2+ centre, ΔgCT (or DCT) are the same (or opposite) in sign and 17% (or 2%) in magnitude related to those from CF mechanism. For the orthorhombic Ni2+ centre, ΔgCT and ECT (or DCT) are same (or opposite) in sign and 16% and 48% (or 442%) in magnitude with respect to those from the CF mechanism. The signs and magnitudes of the trigonal distortion angles δβ (≈ ?0.3 and 0.4°) related to an ideal octahedron and the local angular variations Δβ related to the host bond angle are suitably illustrated by those of the axial distortion degree (ADD) and the angular variation degree (AVD) of the systems, respectively.  相似文献   

4.
The spin-Hamiltonian parameters (zero-field splitting D, g-factors g //, g and hyperfine structure constants A //, A ) of Cr3+ and Mn4+ ions at the trigonal Ga3+ site of Ca3Ga2Ge3O12 (CGGG) garnet crystals are calculated from the high-order perturbation formulas based on the two-mechanism model. In the model, besides the contributions to spin-Hamiltonian parameters from the crystal-field (CF) mechanism in the frequently applied CF theory, those from the charge-transfer (CT) mechanism (which is neglected in CF theory) are taken into account. The calculated results are in reasonable agreement with the experimental values. The defect structures of Cr3+ and Mn4+ impurity centers in CGGG crystals are also obtained from the calculations. The calculations show that the relative importance of CF mechanism (characterized by $ \left| {{{Q^{\text{CT}} } \mathord{\left/ {\vphantom {{Q^{\text{CT}} } {Q^{\text{CF}} }}} \right. \kern-0pt} {Q^{\text{CF}} }}} \right| $ , where $ Q = D,\;\Delta g_{\rm{//}} ,\;\Delta g_{ \bot } ,\;A_{\rm{//}}^{(2)} or\;A_{ \bot }^{(2)} $ ) for Mn4+ center in CGGG is larger than that for Cr3+ center. So, for the high valence state dn ions in crystals, the reasonable calculations of spin-Hamiltonian parameters should consider the contributions due to both the CF and CT mechanisms.  相似文献   

5.
The g factor of Cr4+ in Y2SiO5 crystal is calculated from a completed high-order perturbation formula, in which not only the conventional contribution to the g-shift Δg(=gge) from the crystal-field mechanism, but also the contribution from the charge-transfer mechanism (which is neglected in the crystal-field theory) are considered. The calculated result shows good agreement with the observed value. It is found that the calculated Δg due to the charge-transfer mechanism is opposite in sign and about 38% in magnitude, compared with that due to the crystal-field mechanism. So, in the studies of the g factor for a 3dn ion having high valence state in crystals, the contribution due to the charge-transfer mechanism should be taken into account.  相似文献   

6.
The electron paramagnetic resonance (EPR) parameters (g-factors g , g and zero-field splitting D) of two tetragonal 3d3 impurity centers M3d-VMg and M3d-Li+ (where M3d = Cr3+ or Mn4+, VMg is the Mg2+ vacancy) in M3d-doped MgO crystals are calculated from the high-order perturbation formulas including both the crystal-field (CF) and the charge-transfer (CT) mechanisms for 3d3 ions in the tetragonal symmetry. The calculated results are in reasonable agreement with the experimental values. From the calculations, it can be found that the relative importance of the CT mechanism for EPR parameters increases with increasing valence state of the 3d3 ion. So, for the high-valence 3d n ions in crystals, a reasonable explanation of EPR parameters should take into account both CF and CT mechanisms. The defect structures (characterized by the displacement ΔR of O2− in the intervening M3d and VMg or Li+ at the Mg2+ site) for these tetragonal impurity centers are obtained from the calculations. The results are consistent with the expectations based on the electrostatic interactions.  相似文献   

7.
The electronic structures of SrTiO3 crystals doped with Fe3+, Fe4+ and Fe5+ ions have been investigated using the Xα cluster approach. The ground-state eigenvalues show the lower Fe acceptor level, of t2g↓ symmetry, localized inside the SrTiO3 band gap, respectively at 2.8 eV (Fe3+, S = 52), 1.6 eV (Fe4+, S = 1) or 1.1 eV (Fe4+, S = 0) and 1.1 eV (Fe5+, S = 32) above the valence band edge. Other acceptor levels, with eg↓ and eg↑ symmetries, appear inside the gap when the Fe nominal ionicity increases.The theoretical Xα excitation energies of O 2p-Fe 3d transitions confirms the experimental interpretations of acceptor charge transfer bands for the optical absorption spectra of SrTiO3:Fe4+ and SrTiO3:Fe5+ crystals.The large optical excitation energies compared with the thermal transitions are partly due to the O 2p band width.  相似文献   

8.
杨子元 《物理学报》2008,57(7):4512-4520
基于完全对角化方法(complete diagonalization method, CDM), 研究了6S(3d5)态离子在立方对称晶场中的磁相互作用,分析了自旋哈密顿参量(a, gg)的微观起源.研究中除了考虑研究者通常考虑的SO(spin-orbit)磁相互作用外,同时考虑了SS(spin-spin),SOO(spin-other-orbit),OO(orbit-orbit)磁相互作用.研究表明:6S(3d5)态离子在立方对称晶场中的自旋哈密顿参量起源于五种机理,即SO机理,SS机理,SOO机理,OO机理以及SO-SS-SOO-OO联合作用机理.文中研究了五种机理的相对重要性,结果表明:SO机理与SO-SS-SOO-OO联合作用机理在五种机理中最为重要.尽管SS,SOO,OO磁相互作用单独作用时对自旋哈密顿参量的贡献很小,但它们的联合作用SO-SS-SOO-OO机理对自旋哈密顿参量的贡献非常可观.此外研究表明:零场分裂参量a主要来自纯自旋四重态及自旋二重态与自旋四重态联合作用的贡献,而Zeemang(或者Δg)因子主要来自纯自旋四重态的贡献.纯自旋二重态对自旋哈密顿参量ag(或者Δg)的贡献为零.在我们所选择的晶场区域,发现下列关系始终成立:a>0,a(-|Dq|)<a(|Dq|),g(-Dq)=g(Dq),a(-Dq,-ξd,B,C)=a(Dq,ξd, B,C),Δg(-Dq,-ξd, B, C)=Δg(Dq,ξd, B, C).作为本文理论的应用,研究了四种典型的Mn2+掺杂晶体材料,即Mn2+:KZnF3,Mn2+: RbCdF3,Mn2+: MgO,Mn2+: CaO,理论与实验测量符合很好. 关键词: 自旋哈密顿参量 6S(3d5)态离子')" href="#">6S(3d5)态离子 磁相互作用 完全对角化方法(CDM)  相似文献   

9.
The spin-Hamiltonian parameters (g factors g i and hyperfine structure constants A i , where i = x, y, z) of the rhombic Mo5+ center in Ca1?x Y x MoO4 crystal are calculated from the high-order perturbation formulas based on the two-mechanism model for the rhombic d1 tetrahedral clusters with the ground state |d z 2〉. In these formulas, besides the contributions due to the widely applied crystal-field (CF) mechanism concerning CF excited states, those due to the charge-transfer (CT) mechanism (which is omitted in CF theory) concerning CT excited states are considered. The calculated results are in reasonable agreement with the experimental values. The calculations show that because of the great relative importance of CT mechanism for the components of spin-Hamiltonian parameter along x and y axes, the accurate and complete calculations of spin-Hamiltonian parameters for Mo5+ and other high valence state dn ions in crystals should take account of both the CF and CT mechanisms. The defect model of the rhombic Mo5+ center is also confirmed from the calculations.  相似文献   

10.
Fully relativistic calculations of the energy levels, absorption spectra, molecular orbitals (MO) compositions, covalence effects and energies of the charge transfer (CT) transitions for three isoelectronic ions Cr3+, Mn4+, Fe5+ in the SrTiO3 (STO) crystal have been performed. The recently developed first-principles approach to the analysis of the absorption spectra based on the first principles discrete variational multi-electron method (DV-ME) (K. Ogasawara et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. As a result, energy levels of the above ions, their absorption spectra and energies of the lowest CT transitions were all calculated. By performing analysis of the MO population, it was shown that the degree of covalence of the chemical bonds between 3d ions and oxygen ions in SrTiO3 increases in the following order: Cr3+→Mn4+→Fe5+, whereas the CT energies monotonically decrease in the same order.  相似文献   

11.
The high-order perturbation formulas founded on the two-mechanism model are applied in this paper to compute the spin-Hamiltonian parameters (g factors g //, g and zero-field splitting D) of the trigonal Mn4+ centers in Y2Ti2O7:Mn4+ crystal. In this model, besides the contributions from the traditional crystal-field (CF) mechanism (in the CF theory) related to CF excited states, those from the charge-transfer (CT) mechanism connected with CT excited states are contained. The calculated results are reasonably coincident with the observed values. The calculations show that the contributions of CT mechanism to spin-Hamiltonian parameters (in particular, the g factors) for (MnO6)8? clusters are large and cannot be neglected. The defect structure of trigonal (MnO6)8? clusters in Y2Ti2O7:Mn4+ crystals is also evaluated. The results are discussed.  相似文献   

12.
The g factors of a tetragonally-compressed Cu2+ center in NaCl: Cu+ crystal X-irradiated at room temperature are calculated from the high-order perturbation formulas based on the two-mechanism model. In the model, the contribution to g factors from both crystal-field (CF) and charge-transfer (CT) mechanisms are included. The calculations are based on the defect model that the tetragonally-compressed Cu2+center is assigned to the Cu2+ ion (which is caused by Cu+ ion (at the Na+ site) irradiated by X-ray) associated with a nearest Na+ ion vacancy VNa along C4 axis due to charge compensation. From the calculations, the g factors g|| and g are explained and the defect structure (charactering by the displacement ΔZ of the Cl ion intervening in Cu2+ and VNa) of the Cu2+ (or Cu2+-VNa) center is obtained. The results are discussed.  相似文献   

13.
The X-band EPR spectra of Cr3+, Mn2+, and Fe3+ impurity ions in glasses of (CaO?Ga2O3?GeO2) system are investigated in the 77÷300 K temperature range. The experimental data analysis yields the following results: (i) Impurity chromium ions are incorporated into the (CaO?Ga2O3?GeO2) glasses network in Cr3+ (3d3,4F3/2) paramagnetic valence state only and occupy the strong distorted oxygen coordinated octahedral sites. (ii) For all activated and non-activated (CaO?Ga2O3?GeO2) glasses the iron impurity is present at concentration roughly 0.01 wt.%. Isotropic EPR signals atg eff=4.29 andg eff=2.00 are assigned to Fe3+ (3d5,6S5/2) ions in the sites with strong rhombic distortion and in the sites with nearly cubic symmetry respectively. (iii) The manganese EPR spectrum in (CaO?Ga2O3?GeO2) glasses is weakly dependent on temperature, doping procedure as well as manganese concentration. EPR spectra of impurity manganese ions in glasses with Ca3Ga2Ge3O12 and Ca3Ga2Ge4O14 compositions are virtually identical and belong to Mn2+ (3d5,6S5/2) ions. Impurity manganese ions are incorporated into the (CaO?Ga2O3?GeO2) glass network as isolated Mn2+ centres and clusters of Mn2+ ions.  相似文献   

14.
The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of isoelectronic Cr3+, Mn4+, and Fe5+ ions in an octahedral coordination in the SrTiO3 crystal. The parameters of the crystal field acting on the valence electrons of impurity ions are calculated from the available crystal structure data. A special attention is paid to the analysis of dependencies of the crystal field invariants and covalence effects on the impurity ion. It is shown numerically that the covalence effects between the above impurity ions and ligands increase with an increase of the 3d-ion oxidation state.  相似文献   

15.
EPR and optical absorption studies on Fe3+ and Mn2+ doped strontium tetraborate (SrB4O7) glasses are carried out at room temperature. The EPR spectrum of the Fe3+ doped glass consists of signals with g-values 9.04, 4.22 and 2.04, whereas the EPR spectrum of Mn2+ doped glass exhibits a characteristic hyperfine sextet around g=2.0. The spectroscopic analyses of the obtained results confirmed distorted octahedral site symmetry for the Fe3+ and Mn2+ impurity ions. Crystal field and Racah parameters evaluated from optical absorption spectra are: Dq=790, B=700 and C=3000 cm−1 for Fe3+doped glass and Dq=880, B=700 and C=2975 cm−1 for Mn2+ doped glass.  相似文献   

16.
Detailed study of dependence of the crystal field strength 10Dq and lowest charge transfer (CT) energies for different interionic distances in Cs2GeF6:Mn4+ and Cs2GeF6:Os4+crystals is presented. The calculations were performed using the first-principles discrete-variational Dirac-Slater (DV-DS) method. As a result, the functional dependencies of 10Dq and lowest CT energy on the metal-ligand distance R were obtained without any fitting or semiempirical parameters. It was shown that 10Dq depends on R as 1/Rn, with n=4.0612 and 4.3874 for Cs2GeF6:Mn4+ and Cs2GeF6:Os4+, respectively. Two approximations (linear and quadratic) are obtained for the dependence of the lowest CT energy on R; CT energy decreases when R increases with dE(CT)/dR=−638 and −1080 cm−1/pm for Cs2GeF6:Mn4+ and Cs2GeF6:Os4+, respectively, if the linear approximation is used. These values can be used for estimations of the lowest CT energies for Mn4+ and Os4+ ions in other hosts with fluorine ligands. Estimations of the electron-vibrational interaction (EVI) constants, Huang-Rhys parameters, and Stokes shifts for all the above-mentioned crystals were performed using the obtained 10Dq and E(CT) functions.  相似文献   

17.
Optical absorption and the enhancement and bleaching of Fe3+ and Fe5+ electron paramagnetic resonance in SrTiO3: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O2- valence states to Fe4+ and Fe5+. They occur at 2.09 and 2.82 eV for Fe4+ and at 1.99 and 2.53 eV for Fe5+.  相似文献   

18.
杨子元  郝跃 《物理学报》2005,54(6):2883-2892
基于完全对角化方法,研究了4B1(3d3)态 离子在四角对称晶场中的磁相互作用,分析了自旋哈密顿参量(b02, g, g, Δg)的微观起源.结果表明 :在被考虑的大部分晶场区域,人们通常考虑的SO(spin-orbit)磁相互作用的贡献最为重要 ;然而,对于零场分裂参量b02而言,来自其他机理(包 括SS(spin-orbit),SOO(sp in-other-orbit),SO-SS-SOO)的贡献在大部分晶场区域超过了20%;在部分晶场区域,其 他机理的贡献甚至超过SO机理的贡献.详细地分析了Macfarlane 零场分裂参量b02 近似三阶微扰理论的收敛性,结果表明:该理论在大部分晶场区域收敛性较差.讨论了3d3态离子第一激发态2Eg分裂的微观起源.并利用 群论方法解 释了在C4v和C3v对称晶场中2Eg< /sub>态分裂的不同机理. 关键词: 4B1(3d3)态离子')" href="#">4B1(3d3)态离子 磁相互作用 自旋哈密 顿参量 完全对角化方法(CDM) 微扰理论方法(PTM)  相似文献   

19.
The spin-Hamiltonian parameters (g factors g, g and hyperfine structure constants 143A, 143A, 145A and 145A) of the tetragonal Nd3+ center in the low-temperature (T≈4.2 K) tetragonal phase of SrTiO3 are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are attached to the conventional Hamiltonian and a 52×52 energy matrix concerning the ground term 4HJ (J=9/2, 11/2, 13/2, 15/2) is constructed. The Nd3+ center is attributed to Nd3+ occupying the 12-fold coordinated Sr2+ site in SrTiO3. Differing from the defect model assumed in the previous paper that the tetragonal distortion of this Nd3+ center is due to the association of one interstitial oxygen ion at a nearest neighborhood of Nd3+ and the Nd3+ displacement Δz along C4 axis, we suggest that it is due to the distortion of SrTiO3 lattice in the tetragonal phase. The calculated g factors g and g show good agreement with the experimental values, suggesting that our defect model of Nd3+ center in SrTiO3 is reasonable. The experimental hyperfine structure constants were not reported and so our calculated results remain to be checked by EPR experiment.  相似文献   

20.
The local lattice structure and EPR parameters (D, g, g) have been studied systematically on the basis of the complete energy matrix for a d3 configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V2+ ions in CdCl2 and CsMgCl3 crystals, respectively. The results show that the local lattice structure of CdCl2:V2+ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl3:V2+ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V2+ ion is smaller than that of Cd2+ ion or larger than that of Mg2+ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and gfactors (g, g) are discussed.  相似文献   

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