Formal Copper Valence Research Articles

The bond valence model as a prospective approach: examination of the crystal structures of copper chalcogenides with Cu bond valence excess.

Bond valence analysis has been applied to various copper chalcogenides with copper valence excess, i.e. where the formal valence of copper exceeds 1. This approach always reveals a copper bond valence excess relative to the unit value, correlated to an equivalent ligand bond valence deficit. In stoichiometric chalcogenides, this corresponds to one ligand electron in excess per formula unit relative to the valence equilibrium considering only CuI. This ligand electron in excess is 50/50 shared between all or part of the Cu-atom positions, and all or part of the ligand-atom positions. In Cu3Se2, only one of the two Cu positions is involved in this sharing. It would indicate a special type of multicentre bonding (`one-electron co-operative bonding'). Calculated and ideal structural formulae according to this bond valence distribution are presented. At the crystal structure scale, Cu-ligand bonds implying the single electron in excess form one-, two- or three-dimensional subnetworks. Bond valence distribution according to two two-dimensional subnets is detailed in covellite, CuS. This bond valence description is a formal crystal-chemical representation of the metallic conductivity of holes (mixing between Cu 3d bands and ligand p bands), according to published electronic band structures. Bond valence analysis is a useful and very simple prospective approach in the search for new compounds with targeted specific physical properties.

Synthesis, physical and photo-electrochemical properties of Gd2CuO4

Gadolinium cuprate Gd2CuO4 was synthesized by nitrate route and its physical and electrochemical properties were investigated for the first time. The cuprate was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The Diffuse reflectance infrared Fourier transform (DRIFT) spectrum confirms the characteristic peaks of Gd2CuO4 while the Brunauer-Emmett-Teller (BET) measurement gave a specific surface area of 20.45 m2 g−1. An optical gap of 1.41 eV was obtained from the diffuse reflectance (DR) and the transition is directly allowed. The electrical conductivity indicated semiconducting behavior with activation energy of 0.1 eV that correlates with the formal valence of copper. Indeed, the insertion of oxygen into the layered crystal lattice induces p-conductivity with mixed valences Cu3+/2+ and mechanism conduction by small polaron hopping. The carrier density (1.17 1019 cm−3) and flat potential (Efb = −0.33 VSCE) were determined from the (capacitance−2 – potential) characteristic. The conduction band (−1.64 VSCE), positioned at 3.11 eV below vacuum, is made up of Gd3+: (5d, 6s) hybridized orbital while the valence band (−0.23 VSCE/4.52 eV) derives mostly from Cu2+: 3d orbital.

Anion Rearrangements in Fluorinated Nd2CuO3.5.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anion Rearrangements in Fluorinated Nd2CuO3.5

Nd 2 CuO 3.5 , a derivative of Nd 2 CuO 4 with anion-deficient (CuO 1.5 □ 0.5 ) layers, has been fluorinated with XeF 2 at temperatures between 100 and 400 °C. Samples fluorinated at 200-300 °C exhibit superconductivity with critical temperatures of 6-11 K. Electron microscopy investigations revealed a complicated microstructure in both the initial and fluorinated phases. Nd 2 CuO 3.5 is characterized by the presence of numerous twins which are caused by the lowering of the symmetry after a topotactic oxygen removal from the parent Nd 2 CuO 4 phase. Fluorination at 200°C leads to an increase of the formal copper valence and to a restoration of the T'-type structure. It is suggested that fluorine replaces oxygen in the Nd 2 O 2 slab and the released oxygen atoms migrate into the (CuO 1.5 □ 0.5 ) layers thus forming conducting (CuO 2 ) planes. This rearrangement shows a close resemblance to that previously found in fluorinated A 2 CuO 3 (A = Ca, Sr). Local areas of fluorinated phases with larger values of the c parameter were found in the matrix of T' phase. At 300 °C the monoclinic Nd 2 Cu(O,F) 5 phase is also formed (a ≃ 13.2 A, b ≃ 5.5 A, c ≃ 5.8 A, β ≃ 92°, space group C2/c) with a chainlike ordering of the empty and filled anion positions in the Nd 2 (O,F) 3 blocks and buckled (CuO 2 ) planes due to a tilt of the Cu(O,F) 6 octahedra.

The effect of calcium substitution in deoxygenated R 1− xCa xBa 2Cu 3O 6.1 systems (R=Y, Eu): appearance of superconductivity, insulator to metal transition

The changes in crystallographic, magnetic, and transport properties with calcium substitution in deoxygenated R 1− x Ca x Ba 2Cu 3O 6.1 (R=Y, Eu) system have been investigated. The superconductivity is induced at a calcium content of x=0.26–0.3. However, the induced superconductivity exhibits a low critical field. The system R 1− x Ca x Ba 2Cu 3O 6.1 undergoes an insulator to metal transition. The increase in the effective magnetic moment with Ca substitution is detected. The crystal structure undergoes slight modifications. Although, the observed variation of crystal structure forced by Ca doping does not reproduce the structure modifications characteristic of oxygen doping. The discussed changes in properties of R 1− x Ca x Ba 2Cu 3O 6.1 should be mainly related to hole doping, observed as an increase in formal copper valence, which was confirmed by iodometric titration.

High Pressure Synthesis of (La1− x Sr x )8O20, (0.25 ≤ x ≤ 0.65) with a Wide Solubility Range and High Cu-Valence

With an attempt to increase the Cu valence, we synthesized the (La1−x Sr x )8Cu8O20−δ (8-8-20 phase) compounds with large x using an O2-HIP apparatus. By heat treatment in a high-pressure of oxygen gas (40 MPa), the 8-8-20 compounds were stabilized in the range of 0.25 ≤ x ≤ 0.65. Their crystal structures, characterized by x-ray Rietveld refinement and electron diffraction, are tetragonal, space group P4/mbm. The formal copper valence was found to increase with Sr concentration from 2.24 at x = 0.25 to 2.63 at x = 0.65, keeping the oxygen content constant around 20.0. Electrical resistivity and magnetic susceptibility showed only metallic behavior from room temperature down to 5 K with no sign of superconductivity.

Superconducting Properties of the Family of Mercurocuprates and Role of Layered Structure

The various phases of the homologous series HgBa2 Can&1CunO2 n+2+$ were synthesized earlier using the program-controlled high pressure chamber. We observed that dependence of Tc(a), where a is the in-plane lattice constant (and consequently oxygen content $) have a typical bell-shaped character .T c versus lattice constant has a similar bell-shaped curve for several values of n, with the maximum Tc value measured for n=3. We have analysed this dependence (and of the formal copper valence) for different members of mercurocuprate family in terms of the phenomenological model of layered superconductors taking into account possible charge redistribution. The experimentally observed nonmonotonic bell-shaped dependence Tc(n) was consistent with our model.

Au-1212: A new superconducting gold cuprate with Tc above 80 K

We have synthesised by the high pressure-high temperature technique a new layered gold cuprate of chemical formula AuBa 2(Y 1−x,Ca x)Cu 2O 7 (0≤x≤0.4) (hereafter denoted as Au- 1212). The structural properties of this compound are investigated by means of electron microscopy and powder x-ray diffraction. The average structure (Pmmm, a≈3.83Å,b≈3.84Åc≈12.11Å) is similar to that of YBa 2Cu 3O 7, with square-coordinated Au 3+ cations forming chains along b by corner sharing. Electron microscopy reveals the presence of a superstructure with double b parameter, which is attributed to a long range ordered zig-zag conformation of the chains. For x=0, the formal copper valence is 2+ and the compound is insulating. Doping of the conducting CuO 2 planes is achieved by the partial substitution of yttrium by calcium, up to x≈0.4. Upon substitution, the compound becomes superconducting with a maximum Tc at ≈82K for x≈0.4.

AuBa 2(Y 1− x, Ca x)Cu 2O 7: a new superconducting gold cuprate with Tc above 80 K

We have synthesised by the high pressure-high temperature technique a new layered gold cuprate of chemical formula AuBa 2(Y 1− x ,Ca x )Cu 2O 7 (0 ≤ × ≤ 0.4) (hereafter denoted as Au-1212). The structural properties of this compound are investigated by means of electron microscopy and powder X-ray diffraction. The average structure (orthorhombic Pmmm, a = 3.8298(5) A ̊ , b = 3.8420(5) A ̊ , c = 12.111(1) A ̊ for nominal composition x = 0.4) is very similar to that of YBa 2Cu 3O 7, with Au 3+ cations in square planar coordination sharing corners to form chains running along the b axis. Electron microscopy reveals the presence of a superstructure with double b parameter, which is attributed to a long range ordered zig-zag conformation of the square chains. For x = 0, the formal copper valence is 2 + and the compound is insulating. Doping of the conducting CuO 2 planes is achieved by the partial substitution of yttrium by calcium, up to x ≈ 0.4. Upon substitution, the compound becomes superconducting with a maximum T c at ∼ 82 K for x ≈ 0.4.

Suppression of superconductivity and the overdoped region in HgBa 2CuO 4+δ

For the first time an overdoped, non-superconducting Hg-1201 phase was obtained by a high-pressure technique. The formal copper valence determined by iodometric titration was found to be +2.28(3). Superconducting Hg-1201 with T c = 35 K and v Cu = +2.20(3) was obtained by using the same technique and the same precursor, but which had been previously heat-treated. Further heat treatment of the sample increased T c up to 91 K and reduced v Cu to +2.13(3). We show that in the overdoped region of the Gh-1201 system the Cu valence can be changed by varying the precusor heat treatment. The instability of the overdoped phases makes it difficult to achieve the same results by heat-treating the as-prepared samples. A neutron powder diffraction study carried out on the overdoped non-superconducting sample resulted in a value of δ = 0.23(3). The formal copper valence obtained by iodometric titration and in the structural study can be reconciled if the presence of CO 3 2− defects on the Hg layers is assumed, or if a formal valence of −1 is adopted for the doping oxygen atoms.

Synthesis of the new high pressure Vernier phase Sr0.74CuO2

The strontium oxide–copper oxide binary phase diagram has been reinvestigated under oxygen pressures of up to 600 bar, in the temperature range 600–850 °C. A new, high-pressure phase, Sr0.74CuO2, has been isolated and characterised. The oxygen stoichiometry, confirmed by thermogravimetry, gives an average formal copper valence of ca. 2.5. X-Ray and electron diffraction analyses suggest that Sr0.74CuO2 has a Vernier structure incommensurate in b. The subcells are body-centred orthorhombic: aI= 6.8182(12)A, bI= 3.7113(7)A, cI= 11.0203(17)A, and face-centred orthorhombic: aII= 6.8182(12)A, bII= 2.7290(3)A, cII= 11.0203(17)A. The subcells combine into a coincidence (super)cell with b= 92.79(1)A, which is 25bI and 34bII. The structure of one of the subcells appears to be related to that of Ca0.82CuO2 and NaCuO2. Sr0.74CuO2 is non-superconducting, at least above 4 K.

Stability of Ba, Sr and Ca cuprate phases as a function of oxygen pressure

Alkaline earth-copper oxide binary mixtures were reacted in elevated oxygen pressure up to 200 bar in the temperature range 880–950°C. The phases which were formed have been identified by X-ray powder diffraction analysis. The compounds which become stabilized at higher P(O 2) are Ba 2Cu 3O 5+ δ , Sr 2CuO 3+ δ , (Sr 14Cu 24) 1- x O 41 and Ca 4Cu 5O 10. These compounds have formal copper valence (FCV) in the range of 2.4 to 2.8 and are therefore favored at higher P(O 2) over the ABO 2 and A 2BO 3 compounds obtained at 1 bar which have FCV=2. The transition pressures to the more oxidized cuprates are ∼10 bar for Ca 2CuO 3/Ca 4Cu 5O 10, 17.5 bar for BaCuO 2/Ba 2Cu 3O 5+ δ and 40 bar for SrCuO 2/(Sr 14Cu 24) 1- x O 41 at 880–950°C.

Preparation and Characterization of New 1-2-1-2 Compounds MA2RECu 2O 8-δ and Nb 1- xRu xSr 2RECu 2O 8-δ ( M = Nb or Ta, A = Ba or Sr, and RE = Pr or Sm)

New compounds MA 2 RE Cu 2O 8-δ ( M = Nb or Ta, A = Ba or St, and RE = Pr or Sm) have been prepared and characterized. Structural analysis revealed that they crystallize in a 1-2-1-2 TaBa 2LaCu 2O 8-δ structure type similar to that of YBa 2Cu 3O 7. Cations such as Nb 5+, Ta 5+, and Ru 4+ are accommodated in the basal plane and the double CuO 2 pyramidal planes are maintained. All the compounds show semiconducting behavior. With Ru substitution the formal copper valence increases from +2 to about +2.14.

XAS and XPS study of electronic structure of the trivalent cuprate La2Li0.5Cu0.5O4− δ

Well-characterized stoichiometric La 2 Li 0.5 Cu 0.5 O 4− δ ( δ ≈0.04) has been studied using high resolution CuL 3- and OK-edges absorption (XAS) and Cu 2p 3 2 X-ray photoelectron spectroscopy (XPS) techniques. The system is found not to have any Cu 3 d 8 configuration in its ground state but instead reveals a high density of the holes created by doping of monovalent lithium for the divalent copper in the oxygen 2 p levels resulting in a 3 d 9 L copper configuration by charge transfer. The formal valence of copper in it is found to be as high as ≈2.84 in agreement with chemical analysis. The empirical correlation between the shift of the main XPS line (3 d 10 L) and intensity ratio of the satellite (3 d 9) to the main line is examined and, also, a new correlation between the energy spread of the XPS satellite and the energy separation of the CuL 3 absorption lines due to transitions from (3 d 9) and (3d d 9 L) states is suggested.

Superconductivity of Bi-2201 (n=1) as influenced by the substitution of Pb and/or rare-earths (R=La, Nd and Pr)

Preparation conditions for stabilizing the bulk superconductivity (T c=9.5 K ) in the nominal Bi 2 Sr 2 CuO ∼ 6 composition corresponding to the n=1 phase in the Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 system are described. The samples Bi 2.1 Sr 1 .9 CuO y and Bi 2.2 Sr 1.8 CuO y become superconducting at 10 K and 12 K, respectively. Single-phase compounds (space group Amaa) could be synthesized for the range 0⩽x⩽1.0 in the series Bi 2 Sr 2−x La x CuO 6+y but the orthorhombicity decreased gradual ly with increase in x and the structure became tetragonal at x=1.0. The T c (R=0) value increased to a maximum of 19 K for x=0.4, corresponding to a value of ∼2.2 for the formal valence of copper. For x⩾0.5 a sudden loss of superconductivity is observed. Simultaneous substitution of Pb for Bi, and, La for Sr yields further improvement in superconducting characteristics, hitting a peak of 26 K (R=0) for x=0.4 in the series Bi 2−x Pb x Sr 2−x R x CuO y (R=La). The R=Nd- and and Pr-s ubstituting compounds showed lower values of T c for the same extent of doping (x=0.4). Interestingly, doping of Pb alone (i.e., Bi 2−x Pb x Sr 2 CuO y ) results in suppression of superconductivity altogether. The electrical resistivity, magnetic susceptibility and the cell parameter data are presented for varying levels of different substituents and discussed.

X-ray absorption measurements on Tl 1− xBi xSr 2CuO 5 and Tl 0.5Bi 0.5Sr 2CaCu 2O 7 Valences of Bi, Tl and Cu

Near-edge X-ray absorption spectroscopy measurements of Bi and Tl L3-edges and Cu K-edge in a series of compounds Tl 1− x Bi x Sr 2CuO 5 and Tl 0.5Bi 0.5Sr 2CaCu 2O 7 have been presented. The results provides strong evidence for the existence of mixed-valent Bi and Tl in these compounds. Moreover, the formal valence of copper in Tl 1− x Bi x Sr 2CuO 5 and Tl 0.5Bi 0.5Sr 2CaCu 2O 7 appears to be much higher than 2+. It is suggested that the Tl(Bi) 6s band could cross the Fermi level, resulting in electronic correlations between the Tl(Bi)-O and Cu-O layers, which is significant in determining the physical properties of these compounds.

Structure and copper valence of hydrogen-doped Nd2CuO x H y

There has been an explosion of activity in the field of high-temperature superconductivity since the discovery of superconductivity in Laz_xBaxCuO 4 [1]. Subsequently, superconductivity near 100 K has been found in the layered-perovskite family [1-4]. There are controversies as to whether holes in oxygen play an important role in superconductivity or not. Recently, an n-type superconductor of Ndz_xCexCuO 4 (x = 0.15) showing a critical temperature (To) of 30 K has been reported by Tokura et al. [5]. Superconductivity was also found in fluorine-doped NdzCuO 4 [6]. As well as the doping with fluorine, the introduction of hydrogen into transition-metal oxides has been employed to produce n-type semiconductors [7]. In fact, hydrogendoped YBazCu3O7_~Hy was prepared by the direct reaction of these superconducting oxides with hydrogen gas at a relatively low temperature [8-10]. In this study we attempted to prepare hydrogen-doped NdzCuO4, and the effects of introduced hydrogen on the crystal structure, the formal valence of copper, and superconductivity are examined. Stoichiometric mixtures of Nd203 and CuO were heated to 950 °C in air for 10 h. After the calcination was repeated twice, the mixtures were fired at 1100°C in air for 10h, and slowly cooled in a furnace. The resulting samples were confirmed to be a single phase of NdzCuO 4 by X-ray diffraction (XRD). Hydrogen-doped samples were prepared as follows: tlhe single-phase powders of NdzCuO 4 were heated at 250 °C in 101.3 kPa H 2 for 20 h, and quenched to room temperature. The presence of hydrogen contained in the hydrogen-doped sample was detected by a pulsed hydrogen nuclear magnetic resonance (H-NMR; Bruker PC20C) instrument. In order to obtain the relationship between the hydrogen concentration and the initial intensity of a free induced decay (FID) curve, FID curves were taken for mixtures of ( 1 x) CuO and xCuSO4"5H20, where x ranged from 1 to 0.05. It was confirmed that a linear relationship existed between x and the initial intensity of the FID curves. From this calibration curve, the hydrogen content of the H-doped sample, Nd2CuOxHy, was estimated. The oxygen content of the hydrogen-doped sample was determined by hydrogen reduction using a thermogravimetric analysis system (Shimazu TGA-30); the hydrogen-doped sample was heated in flowing hydrogen (50 cm 3 min -1) to 700 °C at 5 °Cmin -1. The formal oxidation state of copper in the hydrogendoped sample was determined by the iodiometry method. These hydrogen-doped samples were pressed into pellets, and then the superconducting

Oxidation and reduction of BaLa 4Cu 5O 13+δ

The stability of the metallic, nonsuperconducting cuprate, BaLa 4Cu 5O 13+δ, was investigated using thermogravimetric analysis and X-ray and electron diffraction. In air or oxygen this highly oxidized compound only loses small amounts of oxygen; and at 1000°C the formal copper valence is close to 2.3 +. To probe the chemical and structural factors that stabilize this high valence state, methods were developed to reduce the 145 structure. Under helium gas a new structure with δ ≈ −0.5 was obtained; however, the removal of additional oxygen could only be accomplished by reducing the samples at low temperatures under 15% H 2 85% Ar gases. Under these conditions two additional structures were isolated for phases with δ ≈ −1.0 and −2.0. The differences in the X-ray patterns of these new phases result from a change in the symmetry of the perovskite subcell from cubic to orthorhombic to tetragonal. Models for the reaction sequence involved a systematic reduction in the coordination of the octahedral Cu groups in the oxidized structure to square pyramidal, square planar, and finally linear coordination in the fully reduced samples. We propose that the stability of the fully oxidized structure is primarily a result of unfavorable Cu coordinations in the partially reduced samples.

Synthesis of the defect perovskite series LaCu03–δ with copper valence varying from 2+ to 3+

MANY important questions about the origin of superconductivity in the high-temperature copper oxide superconductors could be addressed by the study of a copper oxide compound exhibiting a wide range of oxygen stoichiometry while maintaining the basic perovskite structure. A number of compounds are known in which copper occurs formally as Cu3+ (refs 1, 2), but most are structurally unrelated to the superconductors, so that a direct comparison of their chemical and physical properties is not appropriate. Here we report the synthesis, structure and preliminary characterization of a new oxygen-deficient perovskite series, LaCuO3–δ. Although superconductivity has not been observed in this system, its oxygen stoichiometry range, 0.0 < δ < 0.5, is the widest yet found in a copper oxide compound, allowing the formal copper valence to be varied from 2+ to 3+. This series may thus serve as a model system with which to address such questions as the nature of the Cu/O hole states in the superconductors, and the way in which magnetic and transport properties vary with carrier concentration.

Oxygen content, lattice constants and T c of Bi 2Sr 2-xLa xCuO 6+δ with 0.0 ⩽ x ⩽ 1.0

Compounds in the solid solution Bi 2Sr 2- x La x CuO 6+δ with 0.0 ⩽ x ⩽ 1.0 were investigated by X-ray powder diffraction, magnetic and electrical measurements. The samples are prepared starting from spray-dried powders. The oxygen contents were obtained from determinations of the oxidizing powder. The T c was found to increase with increasing lanthanum content up to 25 K at x = 0.4. Samples with x > 0.6 are not superconducting. The oxygen content increases with lanthanum content while the formal copper valence remains approximately constant.