Valence Of Cu Research Articles

Valence Fluctuation in α-CuVO3 Studied by Photoemission Spectroscopy

We have performed a photoemission study of the mixed-valence semiconductor α-CuVO3 (Cu7-xV6O19-x, x ∼ 0.38). From the measured spectra we have found that both Cu and V are mixed-valent between Cu+ and Cu2+ and between V4+ and V5+. The mean valences of Cu and V deduced from the spectra suggest a considerably larger number of local magnetic moments than that deduced from the Curie-Weiss magnetic susceptibility, indicating that the magnetic moments of the V4+ and Cu2+ ions are partially suppressed. Combining the present results with the NMR and ESR data, we proposed that the V4+ local moments are totally suppressed by the formation of singlet pairs and the Cu2+ local moments are reduced by the Cu+-Cu2+ valence fluctuation. The O 2p to Cu 3d charge-transfer energy deduced from the analysis of the Cu 2p core-level spectrum is found to be small, which may favor the occurrence of the valence fluctuation at the Cu sites.

Oxidation state and electronic configuration determination of copper in tetrahedrite group minerals by L-edge X-ray absorption spectroscopy

L-edge X-ray absorption spectroscopy employing a synchrotron radiation source has been used to study the electronic structure and valency of Cu in the chemically and structurally complex tetrahedrite group of minerals. Mechanical mixtures of Cu2+O and Cu+FeS2 were used to estimate the relative cross sections of Cu2+ and Cu+; the absorption of Cu2+ at 931 eV is 25 times greater than that of Cu+ at 945 eV. Using this calibration, Cu2+/Cu ratios were found to vary from 0.00 to 0.054 in the tetrahedrite samples studied; the highest proportion of Cu2+ occurs in synthetic tetrahedrites with a composition close to Cu12Sb4S13. This study reveals the utility of the technique for determining the valence state of copper in complex minerals, allowing the crystal chemistry to be more fully characterised.

Distribution of the hole-carrier density in Y2Ba4Cu7O14+ delta ( delta =0.0, 0.5, 1.0) superconductors.

The ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14+\mathrm{\ensuremath{\delta}}}$ family of superconductors is composed of alternating layers of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ materials along the c axis. The distribution of the hole-carrier density in the two-dimensional ${\mathrm{CuO}}_{2}$ planes in this compound can be quite complex due to the differing nature of these layers. Results of electronic-structure calculations are presented for three oxygen stoichiometries (\ensuremath{\delta}=0.0, 0.5, 1.0) which shed some light on the distribution of the hole-carrier density in different ${\mathrm{CuO}}_{2}$ planes. We find that the hole density in the ${\mathrm{CuO}}_{2}$ planes associated with the single chains is, generally speaking, lower than for double-chain layers. The difference is, however, relatively small.For the compound ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14}$, we obtain an average value of \ensuremath{\sim}0.110 hole/${\mathrm{CuO}}_{2}$ for the hole density which is slightly above the threshold value of \ensuremath{\sim}0.06 hole/${\mathrm{CuO}}_{2}$ for the appearance of superconductivity to give it a low value of ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}15 K, despite the fact that the single chains in this compound have no oxygen atoms. In ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{15}$ and in ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14.5}$ in the full, empty, full,. . . sequence for single chains, the hole densities are much larger, in agreement with much higher ${\mathit{T}}_{\mathit{c}}$ values observed in these compounds. The effect of oxygen disorder in the single chains on the hole densities has also been investigated for the latter two compounds, and is found to be much less drastic than in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$. This is due to the presence of double chains which act as a cushion. The effective valence of Cu at different sites, which is an alternative way of presenting the hole densities, is also calculated and is shown to depend intimately not only on the oxygen stoichiometry but also on the degree of oxygen ordering in the single chains.

The non-metal-metal transition of Bi 2Sr 2Ca 1−xY xCu 2O 8+δ studied by soft x-ray emission spectroscopy

Cu L α,ß and O k α x-ray emission spectra and the measurements of the occupancy of Cu 3d orbitals in Bi 2Sr 2Ca 1−xY xCu 2O 8+δ are presented. No appreciable change in the valency of Cu is found with increasing x. In contrast to La 2− ySr yCuO 4 and YBa 2Cu 3O 6+y, when going from non-metal to metal, the high-energy shift of O K α band, caused by redistribution of the O 2p states of the apical oxygen atoms, is not observed. The valence states of Cu and O do not essentially redistribute due to Ca doping and the p-d energy separation does not change.

Reaction of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> with Gold, Silver, Bismuth and Lead: Substitution Chemistry and Composite Fabrication

The reaction of YBa2Cu3O(7-beta) with Au, Ag, Bi, and Ph ions or metal is described. Three types of materials were produced: a well-defined series of homogeneous superconductors was obtained for Au ion substitution with little effect on T(sub c); attempted Ag and Bi ion substitution resulted in multi-phase samples with slightly enhanced T(sub c); finally, attempts to produce superconducting metal/superconducting ceramic composites with Pb and Bi powders resulted in multi-phase samples with drastically diminished superconducting properties. For Au- substituted superconductors, YBa2(Cu(l-x)Au(x))3O(7-beta), a substitution series (x = 0 - 0.1) has been synthesized. For x = 0.1 there was no change in the a and b lattice parameters (a = 3.826 A and b = 3.889 A) but a 0.06 A c axis expansion to 11.75 A was observed. The valence of Cu and Au in YBa2Au(0.3)Cu(2.7)O(7-beta) was investigated using X-ray Absorption Near-Edge Structure (XANES). X-ray studies indicate that Au goes into the Cu(l) site and Cu K edge XANES shows that this has little effect on the oxidation state of the remaining copper. A small effect on T(sub c) is observed (T(sub c) = 89 K for x = 0.10). Ag and Bi addition results in a rise in T(sub c) and a decrease in (delta)T(sub c) at low levels (x = 0.10 Ag, T(sub c) = 94 K and (delta)T(sub c) = 0.5 K; x = 0.02 Bi, T(sub c) = 94 K and (delta)T(sub c) = 1K) relative to typical values for YBa2Cu3O(7-beta) (T(sub c) = 91 K, (delta)T(sub c) = 2 K). Attempts at fabrication of Pb- and Pb(1-x)Bi(x)-superconductor composites are described. Cold pressing followed by low temperature (200 C) sintering resulted in a composite which excluded flux below 90 K but did not show zero electrical resistance until the metal (alloy) superconducting transition. X-ray diffraction showed the presence of pervoskite and metal. Processing at moderate (450 C) or high (950 C) temperatures resulted in oxygen-depleted pervoskite and/or metal oxides. These materials displayed greatly degraded superconducting properties. Processing at 800 C resulted in high T(sub c) only for composites containing greater than 90% weight fraction ceramic. Reaction of metal with YBa2Cu3O(7-beta) formed superconducting lead/bismuth-based oxides and other binary oxides.

Strong reduction of the electron-doped superconductor Nd 2 − xCe xCuO 4 − δ: Correlation between crystal and electronic structure

The microstructure, microchemistry and electronic structure of the electron-doped superconductor Nd 2 − x Ce x CuO 4 − δ were studied as functions of cerium and oxygen concentration by means of X-ray diffraction and transmission electron microscopy including selected area electron diffraction, energy dispersive X-ray microanalysis and electron energy loss spectroscopy. In strongly reduced Nd 2 − x Ce x CuO 4 − δ samples at low Ce concentrations ( x < 0.1) we observed some small crystals, about 1 μm to 5 μm in size, with twin lamellae on {110} which had an orthorhombic distorted structure different from the bulk material with the tetragonal 14/mmm structure. The twinning is due to the phase transition 14/mmm to Bmmm during the reduction process which is connected with loss of point group symmetry. The twinned crystals share the orientation of the twin plane and the crystallographic c-axis with {110} and [001] of the tetragonal matrix. During in situ heating experiments on single grains with the electron beam in the high vacuum of the microscope several structural transformations were observed, whereby this orthorhombic modification could be induced by heating the undoped parent compound Nd 2CuO 4 − δ . The concomitant change of the electronic structure, especially the transitions O 1s → 2p and Cu 2p → 3d were determined with EELS. The EELS data indicate that there exist mixed. valences of Cu 1+ and Cu 2+ in the orthorhombic distorted grains.

Structural analysis of the charge transfer mechanism in the superconducting compounds Pb 2Sr 2Y 1−xCa xCu 3O 8+δ

Pb 2Sr 2Ln 1−xCa xCu 3O 8+δ is one of the copper-based superconductors with high critical temperature. Undoped Pb 2Sr 2YCu 3O 8 is not a superconductor and becomes so when some of the trivalent Y cations are replaced by bivalent Ca cations. Microprobe analysis and magnetic measurements have been carried out on several single crystals. The x Ca values are found to be between 0 and 0.42, while T c varies between 0 and 80 K. Structural determinations by X-ray diffraction (6 crystals) show that the introduction of calcium reduces the stress between the CuO 2 and SrO layers. The Y substitution with Ca increases the valence of Cu(2) cations with no appreciable variation of the apical distance Cu(2)-O(1). The oxidation of substituted compounds is accompanied by the suppression of the superconducting state because of the movement of the O(1) atoms towards the Pb 4+ cations and away from the Cu(2) ones. This mechanism of charge transfer is very different from that of YBa 2Cu 3O 7−x.

Transport properties of (Ba 1minus;xSr x) 2Cu 1.1O 2.2+δ(CO 30.9 (0.35 ⩽ × ⩽ 0.6)

Hall-effect measurements on superconducting copper oxide carbonates (Ba 0.55Sr 0.45 2Cu 1.1O 2.2+gd(CO 3) 0.9 have shown that carriers are holes and the carrier number per CuO 2 unit is between 0.1 and 0.2. This coincides with the results of iodometric titration, which show that the average valence of Cu is between 2.13 and 2.20.

Mixed valency of Cu, electron-mass enhancement, and three-dimensional arrangement of magnetic sites in the organic conductors (R1,R2-N,N'-dicyanoquinonediimine)2Cu (where R1,R2=CH3,CH3O,Cl,Br).

The unique molecular conductors with p\ensuremath{\pi}-d mixing band structures (${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$N,N'-dicyanoquinonediimine${)}_{2}$Cu [(${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$DCNQI${)}_{2}$Cu] (${\mathit{R}}_{1}$,${\mathit{R}}_{2}$=${\mathrm{CH}}_{3}$,${\mathrm{CH}}_{3}$O,Cl,Br) were examined. General features of the phase diagram of the DCNQI-Cu system were depicted. A region that is related to the existence of anomalously heavy-metal electrons has been found. The ${\mathit{T}}^{2}$ dependence of the low-temperature resistivity of the alloyed system (${\mathrm{DMe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{MeBr}}_{\mathit{x}\mathrm{\ensuremath{-}}}$DCNQI${)}_{2}$Cu (where Me=${\mathrm{CH}}_{3}$) suggests a large enhancement of the electron mass at the critical situation where the system begins to exhibit a characteristic metal instability. The mixed valency of Cu (${\mathrm{Cu}}^{+1.3}$) in (DMe-DCNQI${)}_{2}$Cu was confirmed by ir experiments performed on neutral DMe-DCNQI crystals and (DMe-DCNQI${)}_{2}$M (M=Li, Ba, Cu).

PHOTOELECTRON SPECTROSCOPIC STUDY OF κ-(BEDT-TTF)2Cu(NCS)2 ORGANIC SUPERCONDUCTOR

Experimental evidence for understanding the transport and electrical properties of the 10.4 K organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 (where BEDT-TTF represents bis(ethylenedithio)tetrathiafulvalene) has been achieved by X-ray and ultraviolet photoelectron spectroscopic (XPS and UPS) measurements. Chemical environment of the atoms in this organic superconductor is discussed based on the XPS core-level results. Binding energies of the inequivalent C and S sites in the cation BEDT-TTF and the anion Cu(NCS)2 are found to be different and an assignment is made. The valences of Cu and N were observed to be typically +1 and −3, respectively. The results indicate that bonding is somewhat covalent in the BEDT-TTF while it is ionic in the Cu(NCS)2, i.e., conduction in this compound occurs essentially on the two-dimensional network of the BEDT-TTF molecules. Measurements of the valence band region revealed a broad valence band structure with several peak features.

Mean-Field Theory of Mixed-Valence Conductors (R1,R2-DCNQI)2Cu

The insulating state of (R 1 , R 2 -DCNQI) 2 Cu salts showing both the mixed valence of Cu + and Cu ++ and the three-fold lattice distortion has been examined by the mean-field theory to the periodic Anderson model where two one-dimensional conduction bands with the electron-phonon interaction are coupled to Cu ions. For the choice of the interaction constant which leads to approximately Cu +4/3 as an average, it is shown that modulations of both charge density and spin density coexist in the ground state and that the phase transition at finite temperature is of the first order.

Oxygen Behavior in the Superconductor (Bi, Pb)2Sr2Ca2Cu3Oy Phase Studied by X-Ray Photoelectron Spectroscopy

Samples annealed in lower and higher oxygen partial pressure than PO2=10-2 atm show Tc shift to the lower temperatures compared with the sample annealed in PO2=10-2 atm, which shows the highest Tc, 107 K. X-ray Photoelectron Spectroscopy (XPS) measurement was carried out to study the behavior of oxygen and the change of chemical environments of each ion in the high Tc phase, dependent on the annealing atmosphere changes becoming the origin of these phenomena. The tendency toward increasing binding energies of Bi 4f7/2 and Cu 2p3/2 with increasing oxygen partial pressures during the heat treatments indicates that the mobile oxygens are located in the (Bi, Pb)-O and the Cu-O layers. We know that the valence of Cu moves to +3 with increasing oxygen content in the structure, and this increase of Cu valence indicates the enhancement of hole creation.

Electronic structure of the organic conductor (DMe-DCNQI)2Cu studied by photoemission spectroscopy

Abstract An x-ray photoemisson study of (DMe-DCNQI) 2 Cu is reported. The line-shape analysis reveals the mixed valency of Cu + and Cu 2+ , and the Cu 2+ : Cu 2+ ratio is found to be ∼ 2 : 1. This suggest that Cu is in a valance fluctuating state in the metallic state and, in other (DCNQI) 2 Cu compounds, a Cu + Cu 2+ charge-ordering takes place below the metal-insulator transition temperature.

Photoelectron spectroscopic study of ϰ-(BEDT-TTF)2Cu(NCS)2 organic superconductor

Abstract An x-ray photoelectron spectroscopic (XPS) study of the 10.4K superconductor ϰ-(BEDT-TTF) 2 Cu(NCS) 2 is reported. Chemical environment of the atoms in this organic salt was discussed based on the XPS core-level results. Binding energies of the nonequivalent C and S sites in the donor BEDT-TTF and the anion Cu(NCS) 2 are found to be different and an assignment is made. The valences of Cu and N were observed to be typically 1 + and 3−, respectively. XPS results indicate that bonding is somewhat covalent in BEDT-TTF while it is ionic in Cu(NCS) 2 . Measurements of the valence band region revealed a broad valence band structure with several peak features. A very low spectral intensity at the Fermi level is observed.

Near-edge study of gold-substituted YBa2Cu3O7−δ

The valence of Cu and Au in YBa2Au0.3Cu2.7O7−δ was investigated using x-ray absorption near-edge structure (XANES). X-ray and neutron diffraction studies indicate that Au goes on the Cu(1) site and Cu K-edge XANES shows that this has little effect on the oxidation state of the remaining copper. The Au L3 edge develops a white line feature whose position lies between that of trivalent gold oxide (Au2O3) and monovalent potassium gold cyanide (KAu(CN)2) and whose height relative to the edge step is smaller than in the two reference compounds. The appearance of the Au L3 edge suggests that fewer Au 3d states are involved in forming the Au—O bond in YBa2Au0.3Cu2.7O7−δ than in trivalent gold oxide.

Formation and superconductivity of 1212-type phase TlSr 2(Sr 0.5R 0.5)Cu 2O 7- δ with R= Sc, Y and lanthanides

Nominal samples TlSr 2(Sr 0.5R 0.5)Cu 2O x with R= Sc, Y and lanthanides, have been prepared, and investigated by X-ray diffraction analysis and resistance measurement. The sample with R= Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu consists mainly of a tetragonal 1212-type phase TlSr 2(Sr 0.5R 0.5)Cu 2O 7- γ , and exhibits a superconducting transition at about 90 K. The sample with R= Ce forms a very pure 1212 phase, but is a semiconductor. The sample with R= Sc forms essentially a 1201 phase, and is a semiconductor. The sample of R= La consists of 1201 phase as its major phase and a 1212 phase as its minor phase, and shows two-step superconducting transitions at 40 K and 90 K. The results are discussed from the point of view of the average valence of Cu and the ionic size of the rare earth.

On the effects of Ti, Zr and Ce substitution in YBa2Cu3O7−δ : transport, XRD, XPS and XANES studies

Results of the XRD, XPS, XANES, iodometric titration and transport studies on freshly prepared Ce, Zr and Ti substituted 1-2-3 samples are reported here. It is argued that at least 2% of Ce, Zr and Ti ions go into the 1-2-3 lattice. It is found that Zr, Ti ions exist in 4+ valence state, Ce is in predominantly 4+ (mixed valence) state and valence of Cu remains unaffected on substitution.

Suppression of the Metal Instability and Continuous Change of the Carrier Density in (MeBr-DCNQI)2Cu1−xLix

Abstract Electrical resistivities and IR spectra of new mixed cation (Cu/Li)-DCNQI systems were examined. (MeBr-DCNQI)2Cu1−xLix (0.25 &amp;lt; x &amp;lt; 0.5) remains in the metallic state down to 4 K. The stretching frequency of C=N(imine) of DCNQI shows a linear dependence on the charge of DCNQIf which is consistent with the mixed valency of Cu (Cu+1.3).

The valence of Cu and Cr ions in Zn 1− xCu xCr 2Se 4 spinel system

The results of extensive experimental and theoretical studies of magnetic properties for CuCr 2X 4 (X = S, Se) spinel compounds are summarized. They concern the magnetometric, neutron and X-ray diffraction, X-ray photoelectron spectra and nuclear magnetic resonance studies, made in the different ranges of temperature. All the experimental data were interpreted within the framework of an ionic model (however, the chemical bonds between anions and cations have there the mixed ionic-covalent character, the ionic contribution is predominant). This work demonstrates the temperature dependence of the valence of the Cr and Cu ions in CuCr 2Se 4 compound. It has been proved that the Lotgering and the Goodenough models, assuming a valence stability of these ions, fail. Evidence for the temperature dependence of the valence for these cations has been presented also in the case of the Zn 1− x Cu x Cr 2Se 4 solid solutions. These systems may be classified as “inter-atomic” intermediate valent to be distinguished from valence fluctuating “intra-atomic” mixed valent systems.

Determination Method of Cu Valence in High-&lt;I&gt;T&lt;/I&gt;&lt;SUB&gt;c&lt;/SUB&gt; Oxide Superconductors by means of EPMA

The intensity ratio of the X-ray characteristic lines L α to K β ,L α /K β decreases exponentially with increasing Cu valence.In ordre to obtain the correct results,we need to make a correction owing to the effects of atomic number,absorption and fluorescence(ZAF correction).The valences of Cu in high-T c oxide superconductors Ba 2 NdCu 3 O 7-δ and Bi 2 Sr 2 CaCu 2 O 8+δ are estimated