Stoichiometry of superconducting YBa2Cu3Oy. Determination of Cu(III)/Cu(II) ratio and oxygen content

The electrical properties of the high T_c superconductor YBa₂Cu₃O_y depend on its oxygen content. The oxygen content is indirectly determined by iodimetric measurement of the oxidation state of copper. A combination of two titrations, with and without addition of KI, prior to dissolving the sample permits the determination of the two species Cu(II) and Cu(III). A simplified automatic titration with potentiometric detection of the end-point is described. The method is suitable for rapid and reliable determinations of the Cu(III)/Cu(II) ratio and total copper content and for controlling the stoichiometry of the compound.Samples of the superconductor were analysed and the stoichiometric coefficient y for oxygen was determined with excellent results. Typically, y=6.811 ± 0.0063 (s.d.) (n=5). A comparison of the total copper content (found by direct analytical determination) with the copper concentration calculated from the stoichiometric formula gives an evaluation of possible deterioration of the sample.     

Application of model of ideal solution of interaction products (ISIP) and thermodynamic modeling (TM) for description of superconducting phases in the system YBaCuO

The thermochemical properties of Cu₂O₃ were calculated. The stability of Cu₂O₃ was studied with the help of TM methods. It was shown that this oxide can exist in oxygen atmosphere (P=10⁵ Pa) at temperatures below 380 K. A variant of the ISIP model was used with TM to determine the Cu⁺, Cu²⁺ and Cu³⁺ contents and oxygen indexes in YBa₂Cu₃O_x (123-O_x), YBa₂Cu_3.5O_y (123.5-O_y), YBa₂Cu₄O_z (124-O_z), YBa₂Cu₅O_q (125-O_q) and YBa₂Cu₆O_m (126-O_m) solutions at 100–1200 K in oxygen medium (P=10⁵ Pa). Methods for determination of some thermodynamic properties and oxygen indices are suggested for a superconductor family in the system YBaCuO.     

Cu-superconductors in organic catalysis I. Isomerization and hydrogenation of allyl alcohol on YBa2Cu3O7?x

On the superconductor YBa₂Cu₃O_7–x between 150 and 300°C, allyl alcohol is isomerized to propionaldehyde (70%) and hydrogenated to 1-propanol (30%) in hydrogen.     

Synthesis and ac susceptibility of YCa2Cu3O7−δ

The bulk superconducting YCa₂Cu₃O_7−δ compounds are prepared at an ordinary pressure of oxygen by conventional solid-state reaction method. The formation of sample is tested by means of XRD and is studied for their ac susceptibility below room temperature up to 77.5 K. The samples are found single-phase orthorhombic structure and found superconducting at 83.5 K. It is shown that the analysis is consistent with published data on YBa₂Cu₃O_7−δ oxide superconductor.     

The decomposition of the oxalate precursor and the stability and reduction of the YBa2Cu4O8 superconductor studied by TG coupled with FTIR and by XRD

The 124 superconductor YBa₂Cu₄O₈ was prepared from the oxalate precursor Y₂(C₂O₄)₃. ·4BaC₂O₄·8CuC₂O₄·xH₂O at one atmosphere oxygen pressure. In O₂ the precursor decomposes in one step at 300°C and more gradually (300°–600°C) in Ar. The stability of the superconductor is strongly dependent on the gas atmosphere: in O₂ and in air there is no significant weight change as long as the temperature does not exceed 800°C, whereas in a 1% O₂-99%N₂ mixture decomposition starts at about 670°C with the formation of CuO and YBa₂Cu₃O_x withx<7. The reduction of YBa₂Cu₄O₈ in a 5% H₂-95% Ar mixture takes place in at least four major steps with formation of products such as Y₂O₃, BaO, Cu₂O, Cu, BaY₂O₄ and Ba₄Y₂O₇.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

The oxygen potentials corresponding to the YBa2Cu3O6+x phase in the YBaCuO system

Phase relations in the vicinity of the YBa₂Cu₃O_6+x in the CuO-rich part of the YBaCuO system were studied by the equilibration and quenching technique. At 950°C, the system is characterized by the large number of solid four-phase combinations. The oxygen potential as a function of temperature for two four-phase combinations, i.e., YBa₂Cu₃O_6+x + BaCuO₂ + BaCu₂O₂ + Y₂BaCuO₅ and YBa₂Cu₃O_6+x + BaCu₂O₂ + Y₂BaCuO₅ + Cu₂O, were studied by means of solid state EMF measurements. At higher oxygen potentials the pair of YBa₂Cu₃O_6+x and Y₂BaCuO₅ is stable in contact with BaCuO₂. At lower oxygen pressures this pair coexists in equilibrium with BaCu₂O₂ or Cu₂O.     

Subsolidus phase diagram of Cu2OCuOMoO3 system

Five chemical compounds, CuMoO₄, Cu₃Mo₂O₉, Cu₂Mo₃O₁₀, Cu₆Mo₄O₁₅, and Cu_4?x Mo₃O₁₂ (0.10 ? x ? 0.40), were identified in the system Cu₂OCuOMoO₃ and characterized by DTA, X-ray powder patterns, ir spectra, and magnetic properties. Cupric molybdates CuMoO₄ and Cu₃Mo₂O₉ are stable in air up to 820 and 855°C, respectively, melting at these temperatures with simultaneous decomposition (oxygen loss). Congruent mp of cuprous molybdates Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅, in argon, are 532 and 466°C, respectively. Nonstoichiometric phase Cu_4?x Mo₃O₁₂ = Cu²⁺₃Cu⁰_1?xMo⁶⁺₃O₁₂, melts in argon between 630 and 650°C depending on the value of x and at 525–530°C undergoes polymorphic transformation. Areas of coexistence of the above-mentioned phases are determined. The μ_eff of Cu²⁺ ions and θ values are: 1.80 B.M. and 28°K for CuMoO₄, 1.71 B.M. and ? 12°K for Cu₃Mo₂O₉, and 1.74 B.M. and ? 93°K for Cu_4?xMo₃O₁₂. Below 200°K CuMoO₄ becomes antiferromagnetic. Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅ show weak temperature-independent paramagnetism.     

Interface analysis of the system Si/YBa2Cu3O7?x

Summary The influence of different preparation conditions and substrate surface orientations on the superconducting properties of thin YBa₂Cu₃O_7–x (YBCO) films on silicon was studied. Comparative electrical and surface spectroscopic measurements were performed. SAM and SIMS depth profile analysis show an enrichment of barium at the interface between the superconductor and silicon for samples with T_c<76 K. Comparison with XPS data obtained for thin silicon films on YBCO indicates the formation of barium and yttrium silicates at the interface under these conditions.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Application of ferrous-chromate and iodometric titration for the determination of copper oxidation states in the superconductor YBa2Cu3Oy

Summary Oxidation-reduction titration methods, Fe²⁺-Cr₂C ₇ ^2– and I^–-S₂O ₃ ^2– , were applied to the determination of the oxidation state of copper in the superconductor YBa₂Cu₃O _y , and related compounds. The former method presented problems in the sample dissolution and titration steps. The dissolution of the sample in low concentrations of Fe⁺-phosphoric acid and Fe²⁺-perchloric acid takes place in two steps, the oxidation of Fe²⁺ to Fe³⁺ and the liberation of oxygen gas, when the liberation results in low analytical values for Cu⁺. In addition the coexistence of cuprous ion and acids induces the oxidation of ferrous ion by dissolved oxygen and air. The problems were resolved by dissolution in 0.1 mol/l Fe²⁺-phosphoric acid and titration in an argon atmosphere. The latter method gave good results by controlling the amounts of potassium chloride, the concentration of acetic acid, and by elimination of the dissolved oxygen in acetic acid solution. The results of the two titration methods coincided with each other.

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Anwendung der Eisen(II)-Chromat- und der iodometrischen Titration zur Bestimmung der Oxidationsstufe des Kupfers im Supraleiter YBa₂Cu₃O _y

     

Metal-organic deposition of YBa2Cu3Ox and Bi2Sr2Ca1Cu2Ox films on various substrates starting from different fluorine-free metallorganic compounds

YBa₂Cu₃O_x (Y-123) and Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) films on various substrates have been prepared by Metal-Organic Deposition starting from different metallorganic fluorine-free compounds and using a very simple instrumentation. The processing conditions include a rapid pyrolysis step in air and an annealing step in oxygen for Y-123 and in air for Bi-2212. The films obtained have been characterized by X-ray diffraction (XRD) and the formation of a superconducting phase of Y-123 or Bi-2212 was confirmed measuring the critical temperature (T _c) with Ac-susceptibility and resistive measurements. Microstructure and final cationic ratios have been studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).     

Ein alternativer Weg zur Darstellung von Bi2Sr2CaCu2Ox,YBa2Cu3O7−δ, und YBa2Cu3−xMxO7−δ (M  Ni,Ag und x ≤ 33 Mol%)

An Alternative Method for Preparation of Bi₂Sr₂CaCu₃O_x, YBa₂Cu₃O_7?δ, and YBa₂Cu_3?xM_xO_7?δ (M ? Ni, Ag and x ≤ 33 Mol%) Oxidation of quenched melts of metals in a well defined argon/oxygen atmosphere has been found as an alternative method to prepare hight purity samples of Bi₂Sr₂CaCu₂O_x without Pb-stabilization. A special equipment for the oxidation of powder samples will be described. By our new method it is also possible to prepare YBa₂Cu₃O_7?δ and derivatives of this compound, where Cu is substituted by high amounts of Ni or Ag.     

Synthesis, crystal structure and magnetic properties of the spin ladder compounds La2(Cu1−xZnx)2O5 and La8(Cu1−xZnx)7O19

The influence of Zn-doping on the crystal structure and magnetic properties of the spin ladder compounds La₂Cu₂O₅ (4-leg) and La₈Cu₇O₁₉ (5-leg) have been investigated. The La₂(Cu_1−xZn_x)₂O₅ and La₈(Cu_1−xZn_x)₇O₁₉ solid solutions were obtained as single phases with x=0-0.1 via the solid-state reaction method in the temperature range between 1005-1010 °C and 1015-1030 °C in oxygen and air atmospheres, respectively. The lattice parameters a and c of the monoclinic crystal structures as well as the unit cell volume V increase with increasing x, while b and β decrease for both series. The magnetic susceptibilities χ of both series show a very similar behavior on temperature as well as on Zn-doping, which is supposed to be due to the similar Cu-O coordination in both La₂Cu₂O₅ and La₈Cu₇O₁₉. For low Zn-doping (x?0.04), a spin-chain like behavior is found. This quasi-one-dimensional behavior is strongly suppressed in both series for x?0.04. Here, the maximum (characteristic for spin chains) in χ(T) disappears and χ(T) decreases monotonically with increasing temperature.     

Influence of partial substitution of zinc for copper on electronic structures of YBa2Cu3O y

Summary By use of an approximate band-structure treatment based on the EHMO approach, the energy band structures for the Zn-doped superconductor YBa₂Cu_3–x Zn _x O _y were calculated in the present paper and the influence of partial substitution of zinc for copper on the electronic structures for orthorhombic YBa₂Cu₃O_y was studied. From analysis of the band structures and the densities of states for YBa₂Cu_3–x Zn _x O _y , it was demonstrated that the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system have a direct and dominant influence on superconductivity, whereas the role of the 1D Cu-O ribbons and the O(4) atoms is also of some importance.     

Thermogravimetric study of the nonstoichiometric regions in the Y-Ba-Cu-O system

The paper presents the results of studies on thermal reduction and oxidation of the nonstoichiometric phases from the Y-Ba-Cu-O in air.The thermogravimetric (TG, DTA) experiments were performed in air in order to establish the ranges of stoichiometry and temperature of oxidation and reduction of YBa₂Cu₃O_x, YBa₄Cu₃O_x, YBa₅Cu₂O_x and Y₃Ba₈CusO_x.It has been found, that at 950°C in air there are four oxygen deficient ternary cuprates: YBa₂Cu₃O_6.02, YBa₄Cu₃O_8.01, YBa₅Cu₂O_8.35, Y₃Ba₈Cu₅O_16.45 and stoichiometric Y₂BaCuO₅. When these nonstoichiometric cuprates are cooled slowly to room temperature in air they oxidize to the following compositions: YBa₂Cu₃O_6.98, YBa₄Cu₃O_8.97, Y₃Ba₈Cu₅O₁₈ and YBa₅Cu₂O_8.97.This work has been supported by the CPBR 6.6.64 research programme.     

A simple and convenient solid state microanalytical technique for identification and characterization of the high temperature superconductor YBa2Cu3O7?x

Summary The application of a new electroanalytical technique, abrasive stripping voltammetry, is described for the purpose of identification and characterization of orthorhombic YBa₂Cu₃O_7–x. The method is based on the peculiar property of this compound that the copper ions therein cannot be reduced below the +1 oxidation state, whereas all other copper compounds exhibit this property. The total analysis takes about 2 min and is very reliable.

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Eine einfache und bequeme mikroanalytische Festkörper-Technik zur Identifizierug und Charakterisierung des Hochtemperatur-Halbleiters YBa₂Cu₃O_7–x

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On leave from Deakin University, Geelong, Victoria 3217, Australia     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

Positron annihilation studies in highT c superconductor YBa2Cu3Cd x O y (x=0, 0.05)

Positron lifetime and Doppler broadening of the annihilation line measurements were performed in highT _c superconducting samples YBa₂Cu₃Cd _x O _y ,x=0, 0.05 and 6.9<y<7, as a function of temperature in the region of 14–300 K. It was found that the positron lifetime and theS parameter values are lower in the Cd doped sample than those in the undoped one. It was also observed that the positron annihilation parameters show similar temperature dependence for the undoped and Cd doped samples. We conclude that the Cd doping in highT _c superconductor YBa₂Cu₃Cd _x O _y , 6.9<y<7 fills defects associated with oxygen vacancies probably in oxygen deficient regions which can trap positrons.     

Structural and electrical changes in NdSrNiO4−δ by substitute nickel with copper

A novel series of the formula NdSrNi_1−xCu_xO_4−δ were synthesized for various values of x ranging from 0 to 1 in 1 atm of O₂ gas flow using conventional solid-state methods and were characterized by powder X-ray diffraction and electrical resistivity measurements. The compounds have been shown to adopt the K₂NiF₄-type structure. The oxygen stoichiometry of the compounds was determined from thermo-gravimetric analysis (TGA). An analysis of the micro-structure of the neodymium strontium nickel copper oxide is described. All the samples were semi-conducting from room temperature down to 77 K. The effect of Cu²⁺ incorporation on the structural and electrical properties of NdSrNi_1−xCu_xO_4−δ, 0?x?1, are discussed in terms of Jahn-Teller distortion of the (Ni/Cu)O₆ octahedra and mixed valence character of copper.