Copper Valence Research Articles

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.

Nd 1+ xBa 2− xCu 3O y processed in high pressure oxygen. Effects of enhanced oxygen uptake on structure and superconductivity

The T c of Nd 1+ x Ba 2− x Cu 3O y (0.1⩽ x⩽0.6) has been increased by synthesizing and annealing the compounds in 200 bar oxygen pressure. The oxygen content, y, increases to compensate the effect of Nd 3+ substitution and the formal average copper valence (FACV) remains unchanged at ∼2.3 throughout the composition range. Although the FACV remains constant, T c falls systematically with increasing x, suggesting that Nd 3+ at the Ba 2+ site binds a portion of holes. an increasingly negative slope of the weight-temperature plot with larger x, indicating that the enthalpy for binding the oxygen in the CuO chain increase when oxygen per formula unit increases beyond seven.

Determination of the Average Valence of Thallium in High-Tc Superconductors by Isotachophoresis

Abstract Capillary-tube isotachophoresis was conducted to determine the average valence of thallium in high-temperature Tl-Ba-Ca-Cu-O superconductor samples. A superconductor sample (5—7 mg) was dissolved in 2.5 ml of 0.3 M (M = mol dm−3) HCl. The solution was diluted to 25 ml with distilled water. By injection of a 50 μl sample solution into the analyzer, thallium(I) was determined at 0.05—5 μg with a relative standard deviation of 3.3%. Hydrochloric acid (5.0 × 10−3 M) and LiCl (1.0 × 10−2 M) were selected as leading and terminating electrolytes, respectively. The order of the mobility of the metal ions was Tl+ > Ba2+ > Ca2+ > Cu2+. Thallium(III) ion was not detected as a zone. The total of thallium was determined after the reduction of Tl3+ to Tl+ with ascorbic acid. The average valence values of thallium in the superconductors were 2.93—2.97. The interference of the Tl3+ ion in the determination of the average valence of copper was examined. When the valence of copper was determined with KI by extraction-spectrometry, the Tl3+ ion was found to affect the absorbance of the ion-pair complex extracted into chloroform. The effect of this ion was eliminated by a suitable selection of the wavelength.

The preparation, structure, magnetic and electrical properties of CuICuIIMg3(PO4)3

Abstract Single-phase Cu2Mg3(PO4)3 has been prepared by a high temperature synthesis technique. The structure was deduced from powder X-ray diffraction data and shown to be of the alluaudite type, with the monoclinic space group C2/c and lattice constants: a=11.726(5) A , b=12.613(4) A , c=6.457(3) A , β=114.43(4), Z=4 . Magnetic susceptibility measurements give an effective molar magnetic moment μeff=1.75 μB which indicates an equimolar mixed copper valency. Impedance spectroscopy data (567–810 K) show a bulk electrical conductivity with Ea(total)=0.955±0.005 eV. Electromotive force (E.m.f.) measruements (721–863 K) for the activity cell: Cu|Cu2Mg3(PO)4)3||Au0.2Cu0.8, were compared with those using the Cu+ ion conductor CuZr2(PO4)3. From the ratios of the e.m.f.s it was found that the transport number tCu+(Cu2Mg3(PO4)3)=0.29±0.02 over this temperature range if Cu+ conductivity is assumed. This indicates not only the presence of mixed conductivity, but also that the activation energies for ionic and electronic conductivity are similar (i.e. both Ea≈0.955 eV), hence suggesting a coupling of the two mechanisms, Beyond this, a novel electronic vehicle mechanism is discussed as a second but less probable possibility. At 800 K the bulk ionic conductivity amounts to 1.1 × 10−5 S cm−1. The electrical behaviour in Cu2Mg3(PO4)3 is compared with that incertain superionic phases of the Nasicon and hollandite structures.

Role of calcium in suppressing the superconductivity of Y1-xCaxBa2Cu3O7- delta.

Partial substitution of ${\mathrm{Ca}}^{2+}$ at the ${\mathrm{Y}}^{3+}$ site in the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ system shows several interesting features, namely (1) creation of oxygen vacancies without any change in the lattice parameters, (2) no change in the effective copper valence, and (3) decrease in the critical temperature ${\mathit{T}}_{\mathit{c}}$ with substitution, accompanied by a decrease in the normal-state conductivity. A host of measurements such as low-temperature resistivity, magnetic susceptibility, x-ray diffraction, oxygen stoichiometry, and the effective copper valence, reported here, suggest that ${\mathit{T}}_{\mathit{c}}$ depression with Ca substitution is unlikely to be due to an overdoping effect. An alternate explanation, which takes stock of the above observations, and involves the creation of disorder in ${\mathrm{CuO}}_{2}$ planes, is invoked.

Role of oxygen vacancies in the decomposition of N 2O over YBa 2Cu 3O 7−δ and Gd 2CuO 4 oxide systems

The activity of high T c oxides, YBa 2Cu 3O 7−δ (0.7⪢δ⪢0.1) and that of nonsuperconducting Gd 2CuO 4 was evaluated for N 2O decomposition in the temperature range of 450–800 K. The structural and compositional changes were examined at different stages of reaction using X-ray powder diffraction, thermogravimetry and iodometric titration methods. The N 2O decomposition on the oxygen-deficient tetragonal form of high T c cuprates commenced at 450 K and at each temperature these oxides showed much higher catalytic activity than the oxygen-rich orthorhombic homologues. The higher activity was related to the number of vacant oxygen lattice sites. The oxygen produced during N 2O decomposition led to the tetragonal→orthorhombic transformation. On the other hand, the tetragonal gadolinium cuprate showed poor activity and N 2O decomposition was observed only at temperatures above 650 K. The results support a reaction mechanism involving adsorption of N 2O at oxygen vacancy sites adjacent to copper ions, followed by a change in copper valence state and release of nitrogen. An equilibrium between N 2O adsorption at vacancy sites and the generation of such sites on thermal release of lattice oxygen decided the catalytic behaviour of the oxide systems studied at higher temperatures.

Preparation of a single phase Pb 2Sr 0.8La 1.2Cu 2O 6.1+δ superconductor with a citrate precursor method

A single phase Pb 2Sr 0.8La 1.2Cu 2O 6.1+δ superconductor was prepared by a polymeric citrate precursor method. Metal nitrates of the nominal compositions were used as the starting materials. After pyrolysis of the gel-like polymeric citrates, the precursor obtained was calcined at 800 °C for 4 h, sintered at 790 °C for 6 h in the flowing nitrogen atmosphere followed by furnace cooling. The processing period was greatly reduced by this method compared with the conventional solid state reaction method. The chemical titration found that all the Pb ions were in the +2 state and the average copper valence was 1.601 indicating the δ value of 0.101 equivalent to hole concentration of 0.202 in the Cu-O plane of this superconductor.

Catalytic Decomposition of Nitrogen Monoxide over Valency-Controlled La2CuO4-Based Mixed Oxides

Abstract The direct decomposition of nitrogen monoxide (NO) to N2 and O2 was carried out over valency-controlled La2−xA′xCu1−yB′yO4 (A′ = Sr, Ba, Ca, Ce; B′ = Zr, Al) mixed oxide catalysts with a K2NiF4-type structure. The catalysts were characterized by XRD, XPS, redox titration, and oxygen TPD. It was confirmed by XPS that the surface and bulk compositions generally agreed and by redox titration that the average oxidation number (AON) of copper varied widely from 1.60 to 2.30. XPS showed that the valency of surface copper agreed with AON for AON ≤ 2, while it was in the Cu2+ state for AON > 2.0. The amounts of oxygen desorbed in TPD and the oxygen nonstoichiometry of La2−xSrxCuO4 increased with x in parallel. The catalytic activity for NO decomposition showed a maximum at x = 0.4—0.5, and a good correlation was found between the catalytic activity and AON for all of the catalysts tested. The decomposition of dinitrogen oxide (N2O) was also carried out for La2−xSrxCuO4 for a comparison. Based on the much slower rate of decomposition for NO than that for N2O and the significantly high rate of oxygen TPD, it was concluded that the oxygen desorption was not the rate-determining step in the steady state. The relative activities of the catalysts studied are all well explained by a mechanism in which the active sites for NO decomposition over these catalysts are coordinatively unsaturated Cu 2+ ions on the surface that can be easily oxidized to Cu3+ upon NO adsorption.

Kinetics of Nonisothermal Crystallization of Bi2Sr2CaCu2Ox Glasses with Different Copper Valence States

The kinetics of the nonisothermal crystallization process in Bi2Sr2CaCu2Ox glasses with different copper valance states, R(Cu+) = Cu+/(Cu++ Cu2+) = 0.79–0.99, were examined by using differential scanning calorimetry. Four different kinetic equations were used for the data analyses. It was found that the values of activation energy for crystal growth, Ea, decreased with increasing Cu+ content. The value of Ea, which was estimated from the modified Ozawa equation, for the glass with R(Cu+) = 0.79 was 483 kJ·mol‐1 and for the glass with R(Cu+) = 0.99 was 353 kJ·mol‐1. In the former glass, the number of crystal nuclei was almost independent of the heating rate. But, in the latter glass, the number of crystal nuclei varied significantly with the heating rate. The crystallization mechanism in the glasses was closely related to the thermal stability and viscosity of glasses: the glass with R(Cu+) = 0.99 showing a high thermal stability and a low activation energy for viscous flow had a small value of Ea. The plot of In (Tp2/α) against 1/Tp, where Tp and α are crystallization peak temperature and heating rate, respectively, gave a reasonable value of Ea for the glass with R(Cu+) = 0.79 but was unsuitable for the evaluation of Ea for crystal growth of the glass with R(Cu+) = 0.99.

Synthesis and Structure of Cu 1.35Fe 3(PO 4) 3 and Cu 2Mg 3(PO 4) 3: New Mixed Valence Compounds of the Alluaudite Structure Type.

The first example of a phase in the copper-iron-phosphorus-oxygen quaternary system (Cu I,Cu II) 1.35 (Fe II,Fe III) 3(PO 4) 3, has been prepared by a high temperature synthesis technique. The crystal structure was determined from single-crystal data and shown to be of the alluaudite type, but with differences in copper coordination and occupancy compared to the alkali and alkaline earth metal analogues. It crystallizes in the monoclinic space group C2/ c with lattice constants a = 11.894(4) Å, b = 12.629(4) Å, c = 6.440(1) Å, β = 115.24(2)°, Z = 4. The structure contains chains of edge sharing FeO 6 octahedra linked together via the common corners of PO 4 tetrahedra, yielding two distinct one-dimensional channels for copper ion occupation. The compound exhibits both copper and iron mixed valency. The copper-magnesium phosphate analogue Cu ICu IIMg 3(PO 4) 3 has been prepared and is shown to be of the same structure type, a = 11.85(2) Å, b = 12.55(2) Å, c = 6.38(1) Å, β = 115.9(2)°. Magnetic susceptibility measurements confirm the presence of one Cu 2+ ( d 9) ion per unit formula with μ eff = 1.78 μ B (gt; 50 K).

The effect of Yttrium on the Ca and Sr planes of Y-doped Bi 2Sr 2Ca 1Cu 2O 8

Photoemission spectromicroscopy experiments on a series of Y-doped Bi 2Sr 2Ca 1Cu 2O 8 single crystals show that the doping changes the valence of copper as required for the observed modification of the critical temperature. The doping, however, affects with substitutional reactions not only the CaO planes, but also the SrO planes. Des expériences de photoémission sur une série de monocristaux de Bi 2Sr 2Ca 1Cu 2O 8 dopés avec de l'yttrium montrent que le dopage change la valence du cuivre, comme on dopage, pourtant, ne modifie pas seulement les plans CaO par des réactions de substitution, mais également les plans SrO. Esperimenti di fotoemissione condotti su una serie di monocristalli di Bi 2Sr 2Ca 2O 8 con impurezze di Y mostrano che tali impurezze modificano la valenza del rame, com'é indipendentemente reso necessario dal fatto che si osservano dei cambiamenti della temperature di transizione. Si nota peraltro che le impurezze non modificano solamente i piani CaO mediante reazioni di sostituzione, ma anche i piani SrO.

Structure and properties of the LaCuO3- delta perovskites.

The high-pressure synthesis and electric and magnetic properties of the ${\mathrm{LaCuO}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ system have been studied. The copper valence can be varied in this oxygen-defect perovskite almost continuously from +3 to +2, and three distinct ordered phases are observed over its 0.0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.5 stability field. A tetragonal phase exists over the composition range 0.0\ensuremath{\le}0.2, and the structure of a sample with \ensuremath{\delta}=0.05 was refined by high-resolution synchrotron powder x-ray diffraction in space group P4/m, with a=3.818 97(4), c=3.972 58(6), and R=0.0812. The tetragonal phase and also a monoclinic form stable between 0.2\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.4 are metallic with room-temperature resistivities of (1.0--3.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ \ensuremath{\Omega} cm. The phase is orthorhombic for larger \ensuremath{\delta}, becoming an insulator as \ensuremath{\delta}\ensuremath{\rightarrow}0.5. Superconductivity was not observed in any of the metallic samples. Ferromagnetic behavior is observed in the tetragonal materials, but the moment disappears at the \ensuremath{\delta}\ensuremath{\simeq}0.2 phase boundary. The small moment is attributed to the existence of a canted antiferromagnetic array in the tetragonal phase. The results are discussed within the contexts of schematic band models and rationalized in terms of one in which the Fermi level moves through partially overlapping copper 3${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ and oxygen \ensuremath{\pi}(2${\mathit{p}}_{\mathrm{\ensuremath{\perp}}}$${)}^{\mathrm{*}}$ bands as \ensuremath{\delta} increases.

An Improved Coulometric Titration Method for Oxygen Content and Valence Determinations in Bismuth and Lead Cuprates

A two-step coulometric titration procedure originally published for the determination of the average valences of both copper and bismuth in the bismuth based cuprate superconductors was modified in order to avoid the effects of an observed harmful side reaction. Furthermore, the improved method was found to be applicable to the analyses of lead containing materials as well. The first titration experiment, which consists of the reactions of Cu(III), Bi(V), and Pb(IV) with Cu(I) followed by an anodic oxidation of the excess Cu(I), reveals the oxygen content of the sample. In the second experiment the valences of bismuth and lead are distinguished from the Cu-valence by taking advantage of the reaction with Fe(II). The selectivities of the proposed wet chemical processes were studied using YBa 2Cu 3O v BiNaO 3, PbO 2, and Pb 3O 4 as reference materials for Cu(III), Bi(V), and Pb(IV), respectively. Additionally, the oxygen content and the average metal valences were determined in a single phase superconducting sample of (Bi,Pb) 2Sr 2CaCu 2O 2.

On the crystallochemistry and superconductivity of the (Bi 1±xCu ±x) 8Sr 6Ca 6Cu 9O 24+z

A series of solid solutions of the (Bi 1−xCu x) 8Sr 6Ca 6Cu 9O 24+z type with 0.08 ≥ x ≥ −0.225 has been prepared in air at 850°C and identified through X-ray, density, average valency of copper and low temperature resistivity measurements. The Cu-rich side of the series starts from x ≅ 0.06, whereas that of Bi-rich one, comparatively wider, reaches composition very close to x = −0.202. Within the single phase region, lattice parameter a of the 2212-structure type unit cell, remains practically constant (around 0.54 nm). At the same time, the parameter c exhibits a regular decrease vs. growing concentration of bismuth and finally it reaches the value 3.0625 nm at x = −0.202. The average valency of copper within the series shows a minimum value 2.17 at x = 0.00. Starting from this value it grows up linearly in both directions, slightly along the Cu-rich side and more efficiently along the Bi-rich one. The atomic content of the elementary unit cell (Z), expressed as number of molecules of MeO 1+y-type (Me = sum of the metallic structure components in their atomic fractions) and calculated from sample densities, indicates that the structure deficiency is characteristic of all the series. However, within its Bi-rich part the deficiency occurs to be much deeper than in the Cu-rich equivalent. Thus, the intermediate composition with x = 0 i.e. Bi 8Sr 6Ca 6Cu 24+9V(Cu)/2, characteristic of minimum deficiency, minimum valency of copper and minimum T c, divides the series into two sub-regions, each of extremely different type of structure deficiency and different critical temperature behaviour.

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.

Relative corrosion reactivity and surface microstructure of yttrium barium copper oxide (YBa2Cu3O7-x) samples with different oxygen contents

The relative chemical reactivity toward water within the YBa[sub 2]Cu[sub 3]O[sub 7-x] series (O < x < 1) is found to be YBa[sub 2]Cu[sub 3]O[sub 6.59] < YBa[sub 2]Cu[sub 3]O[sub 7.00] << YBa[sub 2]Cu[sub 3]O[sub 6.05]. Thus, factors other than copper valence, such as internal strain and lattice vacancies, are likely to be responsible for the high reactivity of the oxygen-deficient phase. For the two orthorhombic samples, YBa[sub 2]Cu[sub 3]O[sub 7.00] and YBa[sub 2]Cu[sub 3]O[sub 6.59], the reactivity follows the expected trend based on the copper valence. Additional useful information related to the mechanism of corrosion is acquired from an examination of the surface microstructure of water-degraded YBa[sub 2]Cu[sub 3]O[sub 7-x] samples. Accordingly, inter- and intragrain cracking phenomena occur during water degradation of YBa[sub 2]Cu[sub 3]O[sub 7-x] specimens and serve to enhance the rate of decomposition of the high-[Tc] lattice. Interestingly, the surface microstructure of corroded samples reveals features with appear to be related to the twinning structure of the host lattice. 30 refs., 3 figs., 2 tabs.

Charge and spin states of DCNQI-Cu salts

Magnetic susceptibility, lattice parameters, heat capacity and magnetoresistance under pressure were measured systematically as functions of temperature in (DMeDCNQI)2Cu and (DMeODCNQI)2Cu, which are metallic down to the lowest temperature measured. These compounds are found to be marginal metals where the copper valence is fluctuating giving marginal properties between a localized state having a magnetic spin and delocalized one in the d-π mixed band. A relatively low pressure or a small amount of lattice distortion brings the system into the well-defined localized state where a band gap opens at the Fermi level of the conduction band. The gap is ascribed to the periodic potential formed by the charge ordering of Cu2+. Origins of the so-called reentrance phenomena are examined in relation to the lattice distortion.

Effects of the oxygen uptake on the crystal structure of 110KBiPbSrCaCuO superconductor

Abstract The sensitivities of the crystal structure and physical properties to the oxygen uptake have been studied for 110 K Pb doped Bi superconductor. The formal copper valency in the single superconducting phase samples increasing between 2.188 and 2.312 was determined by chemical technique. There are no lower T c phases and no line splitting in the x-ray diffraction pattern of the reannealed samples. However, two extra peaks were observed and these peaks become stronger with increasing oxygen content, the symmetry of the crystal structure stays unchanged. Furthermore, TGA experiments under different atmospheres have shown that the oxygen stoichiometry can be controlled in a reversible and systematic way by changing the oxygen partial pressure.

Crystallochemical characterization of Bi 8Sr 8− xCa 4+ xCu 9O 33− y type solid solutions

A series of solid solutions of nominal composition Bi 8Sr 8− x Ca 4+ x Cu 9O 33− y (0 ⩽ x ⩽ 4) have been prepared and characterized by X-ray diffraction, copper valency and density measurements. The series is characteristic of the 2212 structure type with a domain of homogeneity which at 860 °C only slightly exceeds x Ca = 3. Within the calcium concentration interval studied, the lattice parameters (mainly the c parameter), average valency of copper and sample density all exhibit a smooth decrease vs. x Ca. The decrease in sample density is not as large as expected for the (Sr 1− x Ca x )-type substitution performed. The series can be described analytically via a structural model in which the oxygen sublattice (32 + δ atoms) remains practically intact throughout the range of homogeneity but the metallic sublattice exhibits an x Ca-dependent deficiency. The total number of vacancies present in the 28-atom metallic sublattice decreases vs. x Ca and at the terminal composition of the series ( x Ca = 3) it becomes zero. The observed effect of a steady densification of the Me sublattice explains satisfactorily why even in the case of isoelectronic cation substitution a substantial decrease in the average valency of copper can be observed. The paper also contains some important conclusions concerning local charge imbalances existing in Bi-based cuprates and their role as the main source of the overall power factor for oxidation higher than 2+ observed in these phases irrespective of whether they exhibit superconducting or non-superconducting character.

Defect chemistry of high Tc superconductors

High Tc superconuctors have the perovskite-related structure with copper ions on B sites, and possess large oxygen nonstoichiometry. The critical temperature is shown to have a close relationship with the oxygen nonstoichiometry. From the relationship between high temperature conductivity and oxygen nonstoichiometry, the carrier concentration and mobility are calculated and compared with those determined by other methods. The valences of copper and bismuth ions are separately determined by the chemical analysis. Contribution of bismuth ions to oxygen nonstoichiometry is shown to be more than copper ions. Detailed study on Ba (Pb1-xBix) O3 from the viewpoint of defect chemistry indicates that the oxide has large vacancy concentrations of Schottky type.