Cu-O Distances Research Articles

Synthesis, composition, and structure of superconducting (Y, Ca)Sr, Ba)2Cu2GaO7??

The superconducting properties of Y{sub 1-y}Ca{sub y}Sr{sub 2}Cu{sub 2}GaO{sub 7-{delta}} have been examined and related to the Ca content, y, and the use of annealing treatments at 350 bar oxygen. Superconductivity with {Tc} up to 41 K was found only for high-pressure-annealed samples, and the structural effects of Ca substitution and high-pressure treatment were examined using powder neutron diffraction. Small but significant changes in Cu-Cu and Cu-O distances were found and suggest that the Cu ions are more highly charged in superconducting samples. Partial substitution of Ba for Sr was found to be possible (up to 20%) to give samples which, after annealing in high-pressure oxygen, were superconducting at temperatures up to 68 K.

CRYSTAL STRUCTURE ANALYSES OF TWO CROWN ETHER COMPLEXES OF COPPER(II) NITRATE: [Cu(NO3)2(12-crown-4)] AND [Cu(OH2)2(15-crown-5)][NO3]2

Abstract Two complexes of copper(II) nitrate, [Cu(NO3)2(12-crown-4)] and [Cu(OH2)2(15-crown-5)][NO3]2, have been prepared by direct reaction of the hydrated salt with the appropriate crown ether in 3:1 CH3CN:CH3OH. The crystal structures of both complexes have been determined. [Cu(NO3)2(12-crown-4)] crystallizes with a directly coordinated crown molecule and two tight ion pairs. Due to the relatively small size of Cu2+, the nitrate anions are monodentate giving Cu2+ a coordination number of six and a very distorted octahedral geometry. [Cu(OH2)2(15-crown-5)][NO3]2 is seven-coordinate with Cu2+/O interactions to the crown ether and two water molecules. The two anions reside in the hydrogen-bonded secondary coordination sphere. There is a fairly wide range of Cu-O(crown ether) distances in both complexes: Cu-O(12-crown-4) = 2.040(4)-2.387(4)A, Cu-O(15-crown-5) = 2.010(5)-2.343(8)A.

Sr0.735Ba0.265Cu2(BO3)2 and Sr0.661Ca0.339Cu2(BO3)2

The crystal structures of Sr 0.735 Ba 0.265 Cu 2 (BO 3 ) 2 and Sr 0.661 Ca 0.339 Cu 2 (BO 3 ) 2 , which are isostructural with SrCu 2 (BO 3 ) 2 , have been refined from single-crystal X-ray diffraction data. The structures consist of slightly puckered layers with the composition [CuBO 3 ] − stacked along the c axes, with Sr, Ba or Ca atoms eight-coordinated between the layers. The interlayer separation is directly affected by the substitution of smaller Ca or larger Ba atoms in place of Sr. In both structures, each Cu atom is coordinated by four O atoms with Cu-O distances ranging from 1.91 to 1.94 A. The borate groups are distorted from D 3h to C 2v symmetry

Chemical pressure effects on the optimization of T C in T′ electron doped superconducting cuprates

We report a systematic variation of the superconducting properties with the Ln 3+ size and x in (Ln) 2−xCe xCuO 4−δT′ compounds. Progressive higher T C's with increasing interatomic Cu-O distances are found at the rate dT C0/d(d Cu−O)∼;240 KÅ −1 in x-optimized samples, having the same density of carriers in CuO 2 layers. Charge transfer mechanisms have been isolated from all the rest modifying T C under pressure. The first can positively contribute to T C in overdoped samples, while the second are estimated to yield dT C0/dP∼;-0.11 K/Kbar.

Comparative high-pressure structural studies of YBa 2Cu 4O 8 and La 1.85Sr 0.15CuO 4 using single-crystal x-ray diffraction

The structures of YBa 2Cu 4O 8 (YBCO) and La 1.85Sr 0.15CuO 4 (LaSCO) have been determined to pressures of 7.5GPa and 8.6GPa respectively, using high-pressure single-crystal x-ray diffraction techniques. Our results show that, despite the similar initial pressure-dependence of T c, the structural variations with pressure for both materials are quite different. The apical Cu-O distance for YBCO decreases at a rate significantly greater than would be expected from a contraction that scales with the c-axis, but the contraction of Cu-O in LaSCO is hardly more than the overall c-axis compression. In YBCO, the heavy Ba atom also appears to undergo a significant shift along the c-axis with pressure reducing the pucker of the Ba-apical-O layer. For LaSCO, on the other hand, the shift of the La(Sr) atom and hence the change in the pucker of the La(Sr)-apical-O layer is very small.

EXAFS studies of changes in the local environment of Cu ions in Fe- and Zn-doped YBa2Cu3O7- delta superconductor

EXAFS spectra have been measured at the Cu K edge in Fe- and Zn-doped YBa2Cu3O7- delta using a laboratory EXAFS spectrometer. The dopant concentration was varied and the changes in the near-neighbour environment around the Cu ions were examined by determining the best-fit structural parameters after curve fitting to the first-shell EXAFS function. The average Cu-O distance is found to decrease as Fe concentration increases, and is found to increase as Zn concentration increases. The results indicate that Fe and Zn affect the Cu-O environment significantly but in different ways. The mobile hole concentration in the YBa2Cu3O7- delta superconductor is known to change with doping, and our results indicate that the Cu-O distance and the mobile hole concentration are probably related, the Cu-O distance increasing as mobile hole concentration decreases and vice versa.

Surface carboxylate species on Cu(100) studied by angle-scanned photoelectron diffraction and LCAO-LDF calculations

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu 60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20 kcal mol . The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm −1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 Å, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.

Neutron powder diffraction study of the crystal structure of HgBa 2Ca 4Cu 5O 12+δ at room temperature and at 10 K

The crystal structure of the superconductor HgBa 2Ca 4Cu 5O 12+δ has been analyzed by neutron powder diffraction at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.85145(8) and c=22.1066(9) Å. The unit cell contains three (CuO 2) layers with copper in four-fold planar coordination and two (CuO 2) layers with copper in five-fold pyramidal coordination. The Cu-O apical distance for this last copper is 2.85(1) Å, i.e. of the same order as those found in other members of the homologous series. The compound has an oxygen composition corresponding to δ=0.32(3). This extra oxygen is located on the layer of the mercury atoms at the sites ( 1 2 0). The superconducting transition temperature for the “as-prepared” sample with this composition is 101 K. In the structure of Hg-1245 there is no evidence for substitution of mercury by copper and for the presence of oxygen at the sites ( 1 2 , 0, z). The copper and oxygen atoms are coplanar for the (CuO 2) layer at z= 1 2 , and almost coplanar for the other layers. No phase transition has been observed down to 10 K. The AC susceptibility versus T curve of the sample used in the experiment shows the presence of a superconducting minority phase with a T c≈114 K. This phase could be Hg-1256, whose presence in the sample has been established by neutron and electron diffraction.

Crystal structure and spectroscopic study of di-μ(1,1)-azido-di-μ(0,0-nitrato)tetrakis(3,5-lutidine)dicopper(II), [Cu(N3) (NO3) (3,5-lutidine)2

A mixed ligand 1∶2 complex of copper(II) azide with 3,5-lutidine, namely di-μ(1,1)-azido-diμ(0,0)-nitrato)tetrakis(3,5-lutidine)dicopper(II) has been prepared and characterized by X-ray crystallographic and spectroscopic methods. The dimeric molecule, which possesses a crystallographic inversion center, contains twoμ(1,1) bridging azido ligands. Each copper(II) atom in the cyclic Cu2N2 unit is further coordinated by two oxygen atoms from twoμ(0,0-nitrato) bridges at Cu-O distances of 2.465(3) and 2.568(3) A and two nitrogen atoms from the lutidine molecules [Cu-N=2.003(3) and 2.012(3) A] to give a distorted tetragonal bipyramid. Both azido ligands are linear [N-N-N angle=179.0(4) and asymmetric Nα-Nβ=1.207(4) and Nβ-Nτ=1.135(5) A]. The azido bridges produce a rather short Cu ⋯ Cu distance of 3.013(2) A. Infrared and electronic data are presented and discussed.

Darstellung und Struktur dreier verschiedener Bis(ethylendiamin)-kupfer(II)-tetraborate / Preparation and Structure of Three Different Bis(ethylenediamine) Copper(II) Tetraborates

Three different bis(ethylenediamine) Copper(II) tetraborates have been prepared: Cu(en)2[B4O5(OH)4] · 2B(OH)3 (A), 2{Cu(en)2B(OH)3 · H2O[B4O5(OH)4]} · H2O (B) and {Cu(H2O)2[B4O5(OH)4]2}(μ-H2O){Ag(en)2} · (μ-H2O){Na(H2O)2) (C). The crystal structure analyses of A (monoclinic, P21/c, a = 1282.5(1), b = 1020.8(1), c = 1510.5(2) pm; β = 96.66(1)°, Z = 4), B (monoclinic, Pc, a = 1254.4(1), b = 914.1(1), c = 1594.2(2) pm; β = 94.79(1)°, Z = 2) and C (triclinic, P1, a = 846.3(3), b = 917.9(3), c = 928.1(7) pm; α = 93.71(6)°, β = 104.79(3)°, γ = 103.23(2)°; Z = 1) show that all compounds contain the tetraborate ion [B4O5(OH)4]2-. In A and B, there are tetraborate ions and boric acid molecules. The coordination sphere of the Cu2+ ion is Jahn-Teller distorted, coordinated by 2 en groups with 4 Cu-N distances between 196 and 206 pm, as well as by 2 OH groups with Cu-O distances of 251 to 274 pm. In C, the Cu2+ ion is surrounded by 6 OH groups, and the Ag+ ion by 2 en and 2μ-H2O molecules.

Electron- and powder neutron-diffraction studies of YBa 2(Cu 1−yCo y) 3O 6+x with 0.05

Power samples of YBa 2(Cu 1- y )Co y ) 3O 6+ x compounds with y >0.05, in the oxidized and reduced state, have been characterized by thermogravimetry, and X-ray, neutron-, and electron-diffraction methods. Neutron experiments showed that (1) the Co atoms substitute exclusively for the Cu(1) atoms up to y∼0.2; (2) extra oxygen atoms are needed to give the Co atoms the appropriate coordination number which is in average equal to 5; (3) the Cu-O distances were found to be a function of the Co concentration and the oxygen stoichiometry. Co-doping yields a lengthening of the distance between the plane Cu and the apical oxygen and a corresponding shortening of the distance between the chain Cu and the same oxygen. For 0.025⩽ y⩽0.10 the electron- diffraction experiments showed the presence of diffuse scattering indicating the existence of two superposed fields of transverse displacements modulated with long periodicities (4-5)( a+b) along the |110| directions. For higher Co concentration diffuse scattering still exists, but indicates shorter modulation periodicities along the same directions. This diffuse scattering is due to local orthorhombic-like distortions induced by the Co insertion and distribution. The Co atoms tend to form dimers or small clusters which favor micro-twinning. The bond-distance analysis showed that the formal valence of the plane Cu atoms is the important factor in determining the value of T c, provided that only the chain Cu atoms are substituted. Our results are in agreement with those obtained by other authors for the Fe-doped 123 compounds.

Stabilization of superconducting phases in thallium-barium-copper oxide system by fluorine substitution [TlBa 2CuO 5−xF x

Partial substitution of oxygen by fluorine in TlBa 2CuO 5 has been achieved by heating Tl 2O 3, BaO 2, BaO and CuO under one kbar external pressure using TlF as a source for fluorine. A single phase region exists for 0.1 ≤ x ≤ 0.5 and the X-ray powder diffraction data indicated a tetragonal structure (space group: P4/ mmm) for all the compounds. The in-plane Cu-O distance calculated from the a-lattice parameter showed a systematic increase as x was varied from 0.1 to 0.5. This due to an increase in the formal oxidation state of copper as F 1− substitutes for O 2− in the 1201-type structure. The c-lattice parameter also showed a slight increase, as the fluorine content in the reaction mixture is increased. Superconductivity is observed for x values within the range 0.1 to 0.6 with transition temperatures in the range 35–75 K.

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa2Cu3O7 indicate that this compound is strictly isostructural with its superconducting RBa2Cu3O7 (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa2Cu3O7 series as a function of R3+ ionic radius. The sole structural anomaly attributable to the presence of Pr3+ in the YBa2Cu3O7 structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa2Cu3O7 supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa2Cu3O7 structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr3+ (f2) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa2Cu3O7 remains unknown; however, superconductivity is “turned back on” for Nd3+ (f3), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa2Cu3O7 and nonmetallic PrBa2Cu3O7 are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.

銅を含む層状ペロブスカイト酸化物の合成と酸素ドープ

The oxygen doping was tried under high oxygen pressure for various layered perovskite cuprates whose structures are described as ordered perovskite having distinct copperoxygen and nonmagnetic metal-oxygen layers. The oxygen doped samples showed lower resistivity than undoped ones but, in general, semiconductive. Some samples, for example, such as La1-xSr1+xGaCuO5+z, have enough oxidation state of copper of 2.15-2.25 for the onset of superconductivity, but showed no superconductivity. The Cu-O distances obtained by Rietveld analysis were larger than that of hole doped copper superconduc-tors, which may spoil the effective hole dope in CuO2 layer.

Synthesis and spectral and structural characterization of a bridging chloropicolinatocopper(II) complex, Cu(C5H4NCOO)Cl

The title complex of copper(II) chloride with picolinic acid was prepared and characterized by spectroscopic and X-ray crystallographic methods. The complex, Cu(C5H4NCOO)Cl, crystallizes tetragonal, space group P42/n (No. 86),a=976.4(1),c=1499.6(4) pm,N=8;Rw=0.048 for 543 observed MoKα diffractometer data. In the structure of the complex two μ-chloro bridges form only slightly bent Cu2Cl2 rings [Cu-Cl=224.2(4) and 275.6(4) pm] with Cu...Cu separation of 359.4(2) pm and Cl...Cl separation of 348.7(5) pm. These edge-sharing copper coordination polyhedra are further linked via the N and O donor atoms of the picolinato anions at Cu-N distances of 199.6(12) pm and Cu-O bond lengths of 195.7(8) and 200.6(10) pm, to form a two-dimensional layer structure in which these layers are arranged along theab plane. Each picolinate anion functions as a tetra-dentate ligand: N(1) and O(2) are coordinated to the same Cu(II) center whereas O(1) is bonded to a neighbouring Cu(II) center. O(2) is further bonded to the latter Cu(II) center at a long Cu-O distance of 256.5(8) pm. The electronic, infrared and Raman spectra of the solid complex are reported and discussed.

Mechanisms for hole doping in thallium cuprate superconductors having single copper-oxygen sheets

This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-O sheets interleaved with single and double Tl-O layers. These studies strongly suggest that, for Tl 2Ba 2CuO 6, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x 2− y 2 bands, i.e., the Tl-O layer creates holes in the CuO 2 layers. This is in contrast to TlBa 2CuO 5-type phases, where the metal chemistry forces the oxidation of CuO 2 layers. However, studies on TlSr 2− x La x CuO 5 and TlBa 1− x Sr x LaCuO 5 systems indicate that for Tl-O single rock-salt layer phases with a short in-plane Cu-O distance, such as TlSrLaCuO 5, the CuO 2 layer x 2− y 2 band is raised high enough in energy for the Tl-O layer to create holes in the CuO 2 layer

Superconductivity of the single phase La 4-xCa xBa 3Cu 7O y system

A series of single phase materials with tetragonal triple-perovskite structure and nominal compositions of La 4- x Ca x Ba 3Cu 7O y (0.50⩽ x⩽1.375) was prepared by the solid state reaction method. Due to the differences in the size and valence between tha Ca ions and La ions, increasing the Ca substitution for La resulted in a decrease in unit cell axes, volume and the oxygen stoichiometry. A rapid decrease in the T c was observed when the a-axis was larger than 0.38 nm indicating the importance of the r Cu-O distance in the copper-oxygen planes for the superconductivity. Evidence of hole doping was observed by Ca substitution till x=1.00 which had the highest T c and a maximum hole concentration in the La 4- x Ca x Ba 3Cu 7O y superconductors.

In situ X-ray studies of [formula omitted] sorbents reacting with coal flue gas

X-ray absorption fine structure (XAFS) spectroscopy was used to study the local structure around the Cu ion in CuO Al 2O 3 sorbents in situ. The measurements were performed between room temperature and 400 °C in a reaction cell with and without coal flue gas flow. Similar measurements were carried out using standard copper compounds. No significant changes were observed in the XAFS spectra of the sorbents when the measurements were performed in the presence of nitrogen. In contrast, new near-edge features appeared in the spectra of the sorbents in the presence of flue gas. A strong peak appeared at an energy characteristic of monovalent copper. No significant changes were detected in the Cu-O nearest-neighbour distance for any of the samples. The near-edge spectra were analysed to obtain the partial ratios of monovalent to divalent copper as a function of temperature and metal loading of the sorbents. The standard compounds showed no evidence of a reaction with the flue gas that might suggest the presence of monovalent copper. The present results indicate the complex nature of the reactions taking place on the Cu ion site during removal of SO 2. The dominant phase after reaction gives the characteristic parameters of copper sulfates.

CuII and PdII complexes of 3-hydroxy-1,3-bis(2-thienyl)prop-2-en-1-one

The 1:2 complexes formed between Cu II or Pd II and the anion of 3-hydroxy- 1,3-bis(2-thienyl)prop-2-en-1-one [LH: Baxter, Blake, Heath & Stephenson (1990). Acta Cryst. C46, 508-510], bis[(3-hydroxy-kO')-1,3-bis(2-thienyl)prop-2-en-1-onato(1-)-kO]copper(II) and bis-(3-hydroxy-kO')-1,3-bis(2-thienyl)prop-2-en1-onato(1-)-kO]palladium(II), crystallize with the metal ions occupying crystallographic inversion centres and coordinated by approximately square planes of four O atoms. The similarities within the pairs of M-O, C-C and C-O bonds in the chelate rings indicate the latter are behaving as delocalized systems. The mean Cu-O and Pd-O distances are 1.910 (3) and 1.980 (3) A, respectively

The effect of the Ca/Ba ratio on the superconductivity of the La 3Ca χBa 4−χCu 7O y system

A series of samples with nominal composition of La 3Ca χBa 4−χCu 7O y (0≤χ≤4.00) was prepared by the conventional powder reaction method. Single-phase materials with triple-perovskite structure were obtained for χ≤1.25 samples of which the lattice parameters monotonically decreased with the increase of Ca substitution. Superconductivity was observed for χ≤3.50 samples with 37 K≤ T conset≤86 K. A T c plateau above 80 K was found in the 1.00≤χ≤1.25 samples with a-axis length of 3.879≥ a≥3.872 Å. In other words, an optimal r Cu-O distance of ca. 1.94 Å in the Cu-O planes was the most appropriate value for superconductivity in the La 3Ca χBa 4−χCu 7O y system.