Copper Valence Research Articles

Correlation of copper valency with product formation in single turnovers of dopamine .beta.-monooxygenase

Chemical- and freeze-quench EPR techniques have allowed single-turnover studies of the copper-containing enzyme dopamine beta-monooxygenase. Reduction of enzyme by a stoichiometric amount of ascorbate followed by rapid mixing with tyramine leads to oxidation of bound copper and formation of hydroxylated product in the expected 2:1 ratio. The tyramine dependence of single turnovers yields a limiting rate of 82 +/- 9 s-1 and Km of 3 +/- 1 mM, in agreement with kinetic modeling based on steady-state parameters. Together these results show that the reduced enzyme is a catalytically competent species, with bound copper acting as the sole reservoir of reducing equivalents. The correlation of copper oxidation and substrate hydroxylation rules out significant antiferromagnetic spin coupling in the enzyme-product complex. Since the enzyme-product complex's Cu2+ EPR signal is absent in the transient approach to the steady state [Brenner, M. C., Murray, C. J., & Klinman, J. P. (1989) Biochemistry (preceding paper in this issue)], this result implies that ascorbate reduces copper in the enzyme-product complex. These findings have important consequences for catalysis and active site structure in dopamine beta-monooxygenase.

Thermogravimetric study of the transition copper valency in the superconductive system Y-Ba-Cu-O

The transition copper valency in the Y-Ba-Cu-O superconductive system was studied with respect to its dependence on the sample processing, doping, the heating and cooling conditions, and on the atmosphere. Significant differences existed in the oxidation reaction between the reduced phases Y(Ce)Ba 2Cu 3O 5.9 and YBa 2Cu 3O 5.9 in the temperature interval 430–550°C. The presence of Ce 3+ accelerated the oxidation reaction.

ChemInform Abstract: Determination of the Copper Valency and the Oxygen Deficiency in the High Tc Superconductor, YBa2Cu3O7‐δ.

AbstractIn order to obtain the relation among the critical temp., the oxygen deficiency, and the effect of preparation conditions, iodometric titrations are carried out for two superconducting phases of YBa2Cu(III)2xCu(II)3‐2xO6.5+x with onset temp. of 60 K (x = 0.23) and 90 K (x = 0.35), and for the insulating one (x = 0) which has been kept in an ambient atmosphere for 72 h. The results show that Tc increases with increasing Cu(III)/Cu(II) and that both Tc and the Cu(III)/Cu(II) ratio depend strongly on the annealing temp. and time.

Superconductivity produced by electron doping in CuO2-layered compounds.

We have discovered that the Ce4+doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the Nd2CuO4-type structure with sheets of Cu-O squares. In contrast to the previously reported high-Tccuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence ofTcon the concentration of doped electrons is shown for these electron-doped superconducting cuprates.

Bismuth cuprate high-Tc superconductors using cationic substitution.

The ${\mathrm{Bi}}_{4}$${\mathrm{Sr}}_{4}$${\mathrm{Ca}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{R}}_{\mathit{x}}$${\mathrm{Cu}}_{4}$${\mathrm{BO}}_{\mathit{y}}$ materials (R is a rare-earth element) were studied to determine their structural and physical properties. For most of the rare-earth elements, a complete solid solution exists up to x=2. Below x=0.5,${T}_{c}$ is not affected and for each added rare-earth element we find that about 0.5 oxygen atom is added to the structure. However, the structural modulation observed along the b axis for the undoped material persists and remains of the same amplitude for the rare-earth-doped samples. When more than one R(x\ensuremath{\ge}1) is substituted, ${T}_{c}$ is depressed and the compound becomes semiconducting beyond x=1.5. The depression in the ${T}_{c}$ from 85 K (x=0) to less than 4.2 K (x=1.5) correlates to a decrease in the formal valence of copper and is independent whether the rare-earth element is magnetic or nonmagnetic. No evidence for magnetic ordering over the range of temperature 1.7--400 K has been observed in all the substituted compounds. The substitution for Cu by 3d metals or for Sr by rare-earth elements fails for the 85-K Bi phase but succeeds for the 10-K Bi phase. Consequently, the following series ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{O}}_{\mathrm{y}}$ (M=Fe, Co) and ${\mathrm{Bi}}_{2}$${\mathrm{RCaCuO}}_{\mathrm{y}}$ (R=La, Pr, Nd, Sm) were made for study. These substitutions result in an uptake of oxygen (0.5 for each substituted element). But the materials become semiconducting even though the formal valence of Cu remains greater than 2. An antiferromagnetic transition at 140 K has been found for the Co sample for which Co is found to be in the +3 state.

Copper valency and anion p band holes in the perovskite superconductor YBa 2Cu 3O 7

Results of photoelectron spectroscopy studies on a molecular Cu (III) complex are compared with those of the perovskite type high T c superconductor YBa 2Cu 3O 7. The evidence from the Cu 2p core level spectra is compatible in terms of formal valency assignments with a model that involves the presence of trivalent copper with strongly covalent Cu/O interaction as the minority Cu component. Attempts to obtain a spectroscopic prove for the simultaneous existence of anion p band holes (critical valence state model) turned out to be inconclusive as a consequence of surface effects.

Oxygen nonstoichiometry and superconductivity in mixed valence copper oxides

The structures of the new superconductors involving bismuth and thallium are described. For all these oxides and the La 2CuO 4-type compounds, the structures result in intergrowth of oxygen deficient perovskite layers and rock salt-type layers leading to the general formulation (AO) n (A'Ca m−1 Cu m O 2 m+1 ) where n and m are the multiplicity of the rock salt and perovskite layers respectively. The oxygen nonstoichiometry in the “123” superconductors is explained by the existence of a partial disproportiona of Cu(II) into Cu(I) and Cu(III). Some extended defects, observed by high resolution microscopy in the bismuth and thallium superconductors, are presented. The role of the mixed valence of copper in superconductivity is discussed. A model is presented, involving a dynamic redox reaction, in the case of the thallium compounds.

Average copper charge and valence states of superconducting copper-oxides

The average charge and possible valence states of copper ions are calculated and tabulated for the superconducting oxide compounds (R 1 − x M x ) 2− y CuO 4− d (the 21 type) and R 1− x M 2− y Cu 3O 8− d (the 123 type).

Oxygen content, lattice constants and Tc of Bi 2Sr 2CaCu 2O 8+δ

The T c of the superconductor Bi 2 Sr 2 CaCu 2 O 8+δ changes over about 40 K upon annealing at 600°C at partial oxygen pressures between 10 −7 and 10 1.7 bar. The oxygen content slightly increases with increasing partial oxygen pressure. This change is too small to affect the formal density of holes and consequently the T c is independent of the formal copper valence. However, the small increase in oxygen content causes a significant decrease in the c -axis while the a -axis remains constant. This indicates that the T c is correlated with a redistribution of holes between the Bi 2 O 2+δ layers and the CuO 2 planes. This interpretation is supported by the unambiguous relation between T c and c -axis lattice constant.

Preparation, structure, and properties of the superconducting compound series Bi2Sr2Can-1CunOy with n=1, 2, and 3.

Crystals of the three Bi-based cuprates of general formula ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{y}$ with $n=1,2, \mathrm{and} 3$ have been isolated and their structural and physical properties investigated. The structures are similar, differing only in the number of Cu${\mathrm{O}}_{2}$-Ca-Cu${\mathrm{O}}_{2}$ slabs packed along the $c$ axis. The insertion of one and two slabs increases $c$ from 24.6 to 30.6 and 37.1 \AA{}. Transmission electron microscopy shows there are stacking faults within the crystals in agreement with our x-ray data and its analysis. Resistivity, ac susceptibility, and dc magnetization measurements demonstrate superconductivity in the $n=1,2, \mathrm{and} 3$ phases at 10, 85, and 110 K, respectively. The observed transition temperatures and the stacking fault densities are dependent upon sample processing, in particular, the annealing temperatures and cooling rates. The transition temperature is, within the accuracy of our chemical titration, independent of the average copper valency that was determined to be 2.15 \ifmmode\pm\else\textpm\fi{} 0.03 for each of the three compounds.

Thermal analysis of some methylhydrazine and methylhydrazinium complexes of copper(II), copper(I) and mixed valence species

Novel methylhydrazine and methylhydrazinium chloride complexes of copper in various oxidation states (1, 4 3 , 2) have been prepared and characterised by elemental analyses and magnetic data. The thermal decomposition of these complexes was studied by simultaneous DTA, TG and DTG. The complex Cu(II)Cl 2(NH 2NH 2CH 3Cl) decomposes on heating with reduction to Cu(I)Cl. Significantly the complex Cu 3(2I, II)Cl 6(NH 2NH 2CH 3) 2 on heating disproportionates as follows 3Cu( 4 3 ) → 2 Cu(I) + Cu(II).

Defect model and transport at high temperature in YBa 2Cu 3O 6+ Y

From 650–850°C in situ conductivity and thermopower measurements it was determined that YBa 2Cu 3 O 6+ Y is a p-type small polaron conductor over the range 0.15 ≤ y ≤ 0.75 with a hopping energy of approximately 0.1 eV. Copper valence distribution on the Cu(1) site was calculated from the thermopower and literature values for the oxygen content, Y. All three valence state of Cu appear to be present with the equilibrium constant of disproportionation, 2 Cu 2+ ⇔ Cu + + Cu 3+ , independent of temperature and oxygen content. The defect model proposed to explain the nonstoichiometry and transport behavior is: Cu + + 1 2 O 2(g) ⇔ O 0 = + Cu 3+ . As Y → 0 at low p O 2 a p-to- n transition is detected, possibly indicating a Cu + Cu 2+ e −-type small polaron mechanism as opposed to the Cu 3+ Cu 2+ h-type mechanism over most values of Y. The phase boundary at low p O 2 prevents further investigation of this n-type mechanism.

Crystallization of high temperature superconductors from nonstoichiometric melts

Crystallization of high-temperature superconductors YBa 2Cu 3O 6.5+ x and (La,Sr) 2CuO 4 - y from homogeneous nonstoisiometric melts has been studied. The phase diagram of the CuO-BaO system was constructed. The real phase diagram of the CuO-BaO system in air having the complete liquidus corresponds to the superposition of two phase diagrams — those of the CuO-BaO and the CuO 0.5-BaO systems. An important part in the process of the formation of superconducting crystals is played by Cu + cations. The formation of peroxide oxygen pairs O 2 2- bound to Ba is briefly considered. These pairs provide the formation of O 2 2- → O-O pairs whose transport could be implemented by charge-density waves associated with a varying copper valence. Platelet-like YBa 2Cu 3O 6.5 + x crystals 8 × 8 × 0.1 mm 3 in size and prismatic La 2 - x Sr x CuO 4 - y crystals 6 × 4 × 10 mm 3 in size were synthesi zed by the method of spontaneous crystallization from homogenized nonstoichiometric melts enriched with copper oxides. La 2 - x Sr x CuO 4 - y single cry stals 8×8×12 mm 3 in size were grown on a seed from homogenized nonstoichiometric melts by the Czochralski technique. The measurements on annealed YBa 2Cu 3O 6.5+ x single crystals yielded: T c = 93 K, Δ T ≈ 0.2 K.

Structure and superstructure of the superconductor Tl2Ca1Ba2Cu2O8 by neutron and electron diffraction

Neutron powder diffraction has been used to refine the structure of the high- T c superconductor Tl 2 Ca 1 Ba 2 Cu 2 O 8 to determine the precise oxygen co-ordination and the effective valence of copper. The results are in excellent agreement with the Dupont X-ray structure, confirming the possible substitution of about 9% of the Ca-sites with Tl in our sample. The Tl-sites may be either 3% Tl-deficient or 7% Ca-substituted. The apparent valence of copper is 2.22. Oxygen in the center of the Tl square implies an average valence of less than 3 for Tl, and it is therefore not suprising that this oxygen is desordered toward the mid-point of the shortest Tl-Tl distance. High resolution electron images and diffraction reveal a quite different superstructure to the 5 x 5.4 Å superstructure seen in Bi 2 Sr 1 Ca 2 Cu 2 O 8 . Very weak and diffuse superlattice spots may be due to a charge density wave in the TlO plane with period 6 x 3.8 Å.

Some aspects of XES pattern of GdBa 2Cu 3O x and EuBa 2Cu 3O x, ceramics

X-ray emission spectroscopy has revealed the mean valence of copper in the Gd Ba Cu O and Eu Ba Cu O superconducting ceramics to be +2.07. It was confirmed that the binding energy of 3d electrons is equal to 3.3 eV. It was found that the chromium impurity reduces the chemical bond efficiency of barium. It was noticed that the ovelap of the 4f orbitals of europium ions with 2p orbitals of oxygen is weaker for the ceramic than for the superconducting single crystals.

ChemInform Abstract: Determination of Average Copper Valence in Superconducting YBa2Cu3On

Abstractis carried out by coulometric titration of Cu(III) using Cu(I) as the reductant.

MIXED VALENCE COPPER OXIDES: RELATIONSHIPS BETWEEN CRYSTAL CHEMISTRY AND SUPERCONDUCTIVITY

The influence of different factors — mixed valence of copper, coordination of this element, low dimensionality of the structure, phase transition and non-stoichiometry — upon superconductivity is analyzed for the two families of higher Tc superconductors La 2 CuO 4 and YBa 2 Cu 3 O 7-type.

Ba2-xLaxYCu3O7+/- delta perovskite compounds: Crystal chemistry.

We report the crystal chemical properties of solid solutions obtained by substitution of lanthanum for barium in the high-${T}_{c}$ superconductor ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. An orthorhombic-to-tetragonal symmetry change occurs at x=0.4 in ${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ with the tripled perovskite supercell maintained. Charge compensation is achieved through reduction of the formal copper valence (\ensuremath{\delta}=0) in the orthorhombic phase and by a mixture of excess oxygen accommodation (\ensuremath{\delta}>0) and formal valence reduction in the tetragonal phase. The resistivity increases by several orders of magnitude, even though the average formal copper valence changes very little.

The valence of copper in YBa 2Cu 3O 6.9 — an X-ray absorption study

The Cu K absorption spectra of YBa 2Cu 3O 6.9 and of a number of mono-, di- and trivalent copper model compounds have been measured at 77 K and at room temperature. YBa 2Cu 3O 6.9 is found to contain 2 divalent Cu ions with a K-edge similar to that in Y 2BaCuO 5 (but dissimilar to that in CuO), with 0.84 chemically trivalent Cu ions as in KCuO 2 and with 0.16 monovalent Cu ions as in Cu 2O. The Cu K-edge of the superconductor cannot be fitted with only divalent copper. The nature of chemically trivalent copper in KCuO 2 and in the superconductor is discussed.

EPR and josephson absorption in YBaCuO high temperature superconductor

Abstract EPR techniques were used to study electron paramagnetic resonance spectra of Cu2+ and to determine microwave Josephson absorption in the Y-Ba-Cu-D superconductor. On the basis of the powder like EPR spectrum we have concluded that superconductivity is independent of the amount of Cu2+, which is present in the material and the valency of copper in HTS phase is Cu1+ or Cu3+. The broad Josephson oscillations spectrum with pronounced maximum at low magnetic fields was applied to determine Tc and Hc1.