Electron hopping in Fe1−xCuxCr2S4 (0<x<0.5)

A theoretical expression for the line shape of the Mössbauer spectra in the presence of electron hopping between Fe²⁺ and Fe³⁺ is obtained by using a simple stochastic model. Analyses based upon this expression show that the origin of the complicated Mössbauer spectra observed in the magnetic semiconductors Fe_1?xCu_xCr₂S₄ (0<x<0.5) at 77 K is electron hopping between Fe⁺² and Fe³⁺ This hopping occurs at a rate of a few MHz. Quantitative estimates are given for some parameters; the isomer shifts, the internal magnetic fields, the quadrupole splittings and the proportions of Fe²⁺ and Fe³⁺. The valence distribution in this system is determined from the results. For example, the distribution Fe²⁺_0.69Fe³+_0.29Cu¹⁺_0.02Cr³⁺_1.72Cr²⁺_0.28S^2?₄ is obtained for x = 0.02. The existence of Cr²⁺ is concluded.     

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe_{1? x} Ni _x O₃ (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Mott's variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe³⁺–O–Fe³⁺ and Fe³⁺–O–Ni³⁺ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.     

The Surface Metal Site in Blc. viridis Photosynthetic Bacterial Reaction Centers: Cu2+ as a Probe of Structure, Location, and Flexibility

Metal ion binding to a surface site on photosynthetic reaction centers (RCs) modulates light-induced electron and proton transfer events in the RC. Whereas many studies have elucidated aspects of metal ion modulation events in Rhodobacter sphaeroides RCs, much less is understood about the surface site in Blastochloris viridis (Blc. viridis) RCs. Interestingly, electron paramagnetic resonance studies revealed two spectroscopically distinct Cu²⁺ surface site environments in Blc. viridis RCs. Herein, Cu²⁺ has been used to spectroscopically probe the structure of these Cu²⁺ site(s) in response to freezing conditions, temperature, and charge separation. One Cu²⁺ environment in Blc. viridis RCs, termed Cu_A, exhibits temperature-dependent conformational flexibility. Different conformation states of the Cu_A²⁺ site are trapped when the RC is frozen in the dark either by fast-freeze or slow-freeze procedure. The second Cu²⁺ environment, termed Cu_B, is structurally invariant to different freezing conditions and shows resolved hyperfine coupling to three nitrogen atoms. Cu²⁺ is most likely binding at the same location on the RC, but in different coordination environments which may reflect two distinct conformational states of the isolated Blc. viridis RC protein.     

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V₂O₅-MnO-TeO₂ system, containing two transition-metal ions, have been measured. The dc conductivity _dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V^4+M and V⁵⁺ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V₂O₅-TeO₂ glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( _ac = Aω^s , with s < 1) is well exhibited by the ac conductivity σ_ac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σ_ac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.     

Spin-Hamiltonian Parameters and Defect Structure of the Copper (2+) Center in Lithium Borate

The spin-Hamiltonian parameters (the anisotropic g factors g _|| and g _⊥ and the hyperfine structure constants A _|| and A _⊥) for the impurity Cu²⁺ in Li₂B₄O₇ are theoretically investigated using the high-order perturbation formulas of these parameters for a 3d ⁹ ion in tetragonally elongated octahedra. In the calculation formulas, the tetragonal field parameters D_s and D_t are determined from the superposition model, by considering the relative axial elongation of the oxygen octahedron around Cu²⁺ due to the Jahn-Teller effect. Based on the calculations, the relative axial elongation of about 0.21 Å for the tetragonal Cu²⁺ center was found.     

ESR and optical study of Cu2+-doped bis-(5,5′-diethylbarbiturato)bis picoline Zn(II)

ESR studies were conducted on Cu²⁺-doped bis-(5,5′-diethylbarbiturato)bis picoline Zn(II). Two Cu²⁺ lattice sites, Cu²⁺(I) and Cu²⁺(II), were identified. These sites exhibit two sets of four hyperfine lines in all directions. The g factor and hyperfine splitting were calculated from ESR absorption spectra: g_x ?=?2.0201?±?0.002, g_y ?=?2.0900?±?0.002, g_z ?=?2.1634?±?0.002, A_x ?=?(30?±?2)?×?10^?4?cm^?1, A_y ?=?(40?±?2)?×?10^?4?cm^?1 and A_z ?=?(154?±?2)?×?10^?4?cm^?1. It was found that Cu²⁺ enters the lattice substitutionally. The ground-state wavefunction of the Cu²⁺ ion in this lattice was determined from the spin Hamiltonian constants obtained from the ESR studies. With the help of an optical absorption study, the nature of the bonding in the complex is also discussed.     

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr₂RCu₂O₇ (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr²⁺ for R³⁺ converts this semiconductor to a 90 K superconductor TlSr₂(R_1–y Sr _y )Cu₂O₇. A combination substitution, Sr²⁺ for R³⁺ and Pb⁴⁺ for Tl³⁺, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr₂(R, Sr)Cu₂O₇. The results are explained in the framework of the mixed Cu²⁺/Cu³⁺ valence.     

EPR and Magnetic Susceptibility of Na2O–CuO–P2O5 Glasses

Abstract

(50?x/2)Na₂O–xCuO–(50?x/2)P₂O₅ glasses (x=1, 5, 15, or 30 mol%) have been prepared and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. The shape of the Cu²⁺ EPR spectrum depends on the Cu content, and the corresponding computer simulations suggest that the Cu²⁺ ions occupy two different sites in these glasses: one of them is preponderant at low Cu content and the other is preponderant at high content, in which the Cu²⁺–Cu²⁺ interactions are more important. From EPR parameters, it was found that for the site at low content, the covalency of copper ion bonding with the surrounding ligands is appreciable. The magnetic susceptibility data appear to follow the Curie–Weiss law (χ=C/(T?θ_p)) with negative paramagnetic Curie temperature θ_p indicating antiferromagnetic interactions between Cu²⁺ ions that are more significant in the samples with high Cu content, in agreement with EPR results.     

EPR Studies of Copper-Doped Potassium Dihydrogen Citrate (C<Subscript>6</Subscript>H<Subscript>7</Subscript>KO<Subscript>7</Subscript>) Single Crystal

The magnetic environments of Cu²⁺-doped potassium hydrogen citrate (C₆H₇KO₇) complex have been identified by electron paramagnetic resonance (EPR) technique. The angular variation of the EPR spectra has shown that three different Cu²⁺ complexes are located in different chemical environments, and each environment contains one magnetic Cu²⁺ site occupying substantial positions in the lattice and showing a very high angular dependence. The principal g and the hyperfine structure parameter (A) values of three sets of Cu²⁺ complex groups are determined. The covalency parameter, mixing coefficients and Fermi-contact term of the complex are obtained, and the ground-state wave function of the Cu²⁺ ion in the lattice has been constructed.     

Investigation of the local structure of Cu2+ ions doped in alkali lead tetraborate glasses by their electron paramagnetic resonance and optical spectra

The local structure of the Cu²⁺ centers in alkali lead tetraborate glasses was theoretically studied based on the optical spectra data and high-order perturbation formulas of the spin Hamiltonian parameters (electron paramagnetic resonance g factors g_∥, g_⊥ and hyperfine structure constants A_∥, A_⊥) for a 3d⁹ ion in a tetragonally elongated octahedron. In these formulas, the relative axial elongation of the ligand O^2? octahedron around the Cu²⁺ due to the Jahn–Teller effect is taken into account by considering the contributions to the g factors from the tetragonal distortion which is characterized by the tetragonal crystal-field parameters Ds and Dt. From the calculations, the ligand O^2? octahedral around Cu²⁺ is determined to suffer about 19.2% relative elongation along the C₄ axis of the alkali lead tetraborate glass system, and a negative sign for A_∥ and a positive sign for A_⊥ for these Cu²⁺ centers are suggested in the discussion.     

Theoretical calculations of spin-Hamiltonian parameters and defect structures for Cu2+ in trigonally-distorted tetrahedral sites of ZnO and GaN crystals

The spin-Hamiltonian (SH) parameters (g factors g _//, g ⊥ and hyperfine structure constants ⁶³ A _//, ⁶³ A ⊥, ⁶⁵ A _//, ⁶⁵ A ⊥) for Cu²⁺ ions in the trigonally-distorted tetrahedral sites of ZnO and GaN crystals are calculated from a complete diagonalization (of energy matrix) method (CDM) based on a two spin-orbit parameter model for d ⁹ ions in trigonal symmetry. In the method, the Zeeman and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM. The calculated results are in good agreement with the experimental values. The calculated SH parameters are also compared with those using the traditional diagonalization method or perturbation method only within the ² T ₂ term. It appears that, for exact calculations of SH parameters of d ⁹ ions in trigonal tetrahedral clusters in crystals, the present CDM is preferable to the traditional diagonalization method or perturbation method within the ² T ₂ term. The local structures of Cu²⁺ centers (which differ from the corresponding structure in the host crystal) in ZnO : Cu²⁺ and GaN : Cu²⁺ are obtained from the calculations. The results are discussed.     

Electronic structures of Fe in La1−x Ba x FeO3−y (0≤x≤0.70)

Mössbauer spectra of La_1–x Ba _x FeO_3–y recorded at room temperature for various values of x show a six-line and/or a single-line subspectrum. The six-line subspectrum with IS=0.41 mm/s and H=52 T results from an orthorhombic perovskite containing only Fe³⁺ ions. The single-line subspectrum at 0.17 mm/s from a cubic perovskite can be assigned to neither Fe³⁺ nor Fe⁴⁺ but to an intermediate valence state, which may be due to electron hopping between the Fe³⁺ and Fe⁴⁺ ions on the identical octahedral sites. The temperature dependence of electron hopping in the compound La_0.40Ba_0.60FeO_3–y is presented.     

Insights into copper coordination in the EcoRI–DNA complex by ESR spectroscopy

The EcoRI restriction endonuclease requires one divalent metal ion in each of two symmetrical and identical catalytic sites to catalyse double-strand DNA cleavage. Recently, we showed that Cu²⁺ binds outside the catalytic sites to a pair of new sites at H114 in each sub-unit, and inhibits Mg²⁺-catalysed DNA cleavage. In order to provide more detailed structural information on this new metal ion binding site, we performed W-band (～94 GHz) and X-band (～9.5 GHz) electron spin resonance spectroscopic measurements on the EcoRI–DNA–(Cu²⁺)₂ complex. Cu²⁺ binding results in two distinct components with different g_zz and A_zz values. X-band electron spin echo envelope modulation results indicate that both components arise from a Cu²⁺ coordinated to histidine. This observation is further confirmed by the hyperfine sub-level correlation results. W-band electron nuclear double resonance spectra provide evidence for equatorial coordination of water molecules to the Cu²⁺ ions.     

X-ray photoelectron spectroscopy studies on core-shell structured nanocomposites

Core-shell nanostructures were grown in silica-based glasses. Copper-copper oxide and iron-iron oxide structures had diameters in the range 3-6 nm, with shell thicknesses ∼1-2 nm. Silver-lithium niobate core-shell nanostructures had diameters in the range 4.2-46 nm and thicknesses varying from 2.2 to 22 nm. X-ray photoelectron spectroscopy studies were carried out on all these specimens. The analyses of these results show the presence of Cu⁺/Cu²⁺, Fe²⁺/Fe³⁺ and Nb⁴⁺/Nb⁵⁺ valence states in the above three systems. Electrical resistivity data were fitted satisfactorily to the small polaron hopping model in the case of copper and iron-containing specimens. The presence of ions in the lithium niobate shell provides direct evidence of the formation of localized states between which variable range hopping conduction can be effected.     

Single crystal polarized Raman spectra of the solid electrolyte Cu2HgI4; attempt frequencies for ion motion in Ag2HgI4

The polarized Raman scattering from small single crystals of Cu₂HgI₄ provided assignments for the more prominent Raman features to specific irreducible representations. The E symmetry assignment, mass dependence, and pressure dependence of the 36 cm^?1 band in Cu₂HgI₄ and 24 cm^?1 band in Ag₂HgI₄ indicate that these features approximate the attempt frequency for ion hopping. The unusually high pre-exponential factor in the Arrhenius expression for ion hopping is discussed in light of the observed attempt frequency; we conclude that despite the high activation energy the conduction mechanism is similar to other heavy-metal solid electrolytes.     

Dielectric-spectroscopic and a.c. conductivity investigations on copper doped layered Na1.8K0.2Ti3O7 ceramics

Dielectric studies on copper doped derivatives of polycrystalline layered mixed alkali trititanate Na_1.8K_0.2Ti₃O₇ ceramics indicate that the losses are of mixed type and decrease on copper doping. However, the temperature dependent permittivity plots are characteristic of the diffuse nature of a possible ferroelectric phase transition and hence give indication of relaxor ferroelectric behaviour. From the EPR spectra, recorded at room temperature, it can be seen that the substitution of copper occurs at Ti⁴⁺ as preferred site with a divalent oxidation state (Cu²⁺) for all compositions. Also, copper doping enhances the transition temperature, which is indicative of the stabilization of the existing ferroelectric phase up higher temperatures. Besides bolstering electron hopping conduction, acceptor doping restrains the interlayer ionic conduction. Moreover, electron hopping (polaron) conduction is dominant over the lower temperature region, while interlayer ionic conduction prevails in the higher temperature region.     

Fe掺杂对自旋梯状化合物Sr14(Cu1－yFey)24O41的结构和电输运性质的影响

采用常规的固相合成法分别制备了Fe³⁺掺杂和2/3Fe³⁺+1/3Fe²⁺混合Fe离子掺杂的两组Sr₁₄(Cu_1-yFe_y)₂₄O₄₁系列样品.X射线衍射分析显示，当Fe³⁺离子的掺杂量y≤0.03以及2/3Fe³⁺+1/3Fe²⁺混合Fe离子掺杂量y≤0.02时，样 关键词： 强关联电子系统 自旋梯状结构化合物 晶体结构 电输运性质     

Investigation of the EPR g factors and defect structure for the Cu-VNa center in the X-irradiated NaCl: Cu crystal

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 V_Na 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 V_Na) of the Cu²⁺ (or Cu²⁺-V_Na) center is obtained. The results are discussed.     

Defect Structure and EPR Parameters of the Cu2+Center in MNB Ternary Glasses

The electron paramagnetic resonance (EPR) parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) for 15MgO-15Na₂O-69B₂O₃ (MNB):Cu²⁺ ternary glasses were calculated based on the high-order perturbation formulae of 3d⁹ ion in a tetragonal symmetry. From the calculations, the defect structures of MNB:Cu²⁺ ternary glasses were obtained and a negative sign for A _// and A_⊥ for the Cu²⁺ center is suggested in the discussion.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2−x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O²⁻→Cu²⁺+O⁻, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2−x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.