S. Lösch

New family of superconducting copper oxides: Bi cuprates of type 1212

Abstract Bi cuprates of type 1212 have been synthesized above 970°C from the proper starting materials. The oxides of idealized composition (Bi 0.5 Cu 0.5 )Sr 2 YCu 2 O 7− z crystalline in the tetragonal space group P4/mmm ( a=3.815(5), c=11.73(1) A ) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature (≈500°C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi 0.5 Cu 0.5 )Sr 2 Y 0.8 Cu 2.2 O 6.95 .

Structural analysis and superconducting properties of Pb-free Bi2Sr2Ca3Cu4Oy (n=4, nominal composition)

Abstract Bi-based high- T c superconductors of nominal composition Bi 2 Sr 2 Ca 3 Cu 4 O 12+ z ( n = 4 phase) were synthesized by an extensive annealing of several hundreds of hours in order to come close to thermodynamic equilibrium. The microstructure reveals an intergrowth of lamellae with different n ( n = 1−5, 7) with an average volume fraction of the n = 4 phase of about 15%. The highest T c is about 110 K and superconducting phases with T c > 110 K are not detectable. However, the distribution of phases with different n is not homogenous but varies considerably between the bulk and the grain boundaries. Especially the n = 2 phase is preferentially located at the grain boundaries and inhibits a continuous superconducting path for the majority n = 3 phase in the ceramic material. In addition to the intergrowth of lamellae with different n special grain boundaries of pure twist and pure tilt type are encountered; the possible role of grain boundaries for flux pinning is discussed.

Correlation between oxygen-hole concentration and Tc in YBa2Cu3O7−δ from Cu-LIII X-ray absorption

Abstract A series of YBa2Cu3O7−δ compounds with chemically determined oxygen stoichiometries δ in the range from = 0 to 1 is studied by soft x-ray absorption at the Cu-LIII threshold. From a comparison with Cu-LII,III spectra of CuO, NaCuO2, and Rb2KCuF6, a spectral feature due to electronic holes in the oxygen-ligand band is identified, which is found to be related in intensity with Tc and δ.

High Tcsuperconductors in the system BiPbSrCaCuO

Abstract The physical properties and stability of the high Tc members of the series Bi2Sr2Can − 1CunO2n + 4 are strongly influenced by the substitution of lead for bismuth (system Bi2 − xPbxSr2Can − 1CunO2n + 4 + z). For n = 2 (Bi2 − xPbxSr2CaCu 2O8 + z) a continuous series is formed with 0 ⩽ x

System (AO)mM2B2n − 2FenO4n + z with m = 2 and n = 1,2 for A ≡ Bi, Pb and M, B ≡ Sr

0.75. Reduction of the oxygen content by argon treatment yields, for x = 0.25 and 0.5, an increase in Tc from approximately 70 K (air preparation) to approximately 90 K; the amount of superconducting phase in the 90 K compound is approximately 40%. Nearly pure members of the n = 3 series (Bi2 − xPbxSr2Ca2Cu3O10 + z) are formed for x = 0.25 and 0.5 with Tc between approximately 100 and 110 K. For x = 0.5 the amount of superconducting phase is approximately 35%.

Superconductivity and substitution of Bi-1212

Determination des parametres cristallins par diffraction RX pour BiPbSr 2 FeO 6,04 , Bi 1,5 Pb 0,5 Sr 4 5Fe 2 O 10,04

The onset of oxygen diffusion in YBa2Cu3O7 − δ: An investigation by neutron diffraction

Abstract For superconductors of type Bi-1212 the influence of substitution of (i) Pb for Bi (ii) rare earth elements (Ln) for Y and (iii) Ca for Y was studied. It is shown that a partial Pb→Bi substitution stabilizes the 1212 type and an incommensurate superstructure develops (λ≈4.8). Similar to the Y-123 case a Ln→Y substitution is working without destroying the superconductivity. The highest transition temperature of 90 K is obtained with a combined Ca→Y and Pb→Bi substitution in materials of composition (Bi, Pb, Cu) Sr2 (Y, Ca) Cu2O7-z. Bi-based 1212 powdered materials are characterized by non-superconducting grain boundaries, resulting in a pronounced weak link behavior. This situation can be improved by introducing an intermediate step of partial or total melting in the conditions of preparation.

High-resolution transmission electron microscopy of (1212) Bi-cuprate superconductor materials

Abstract In order to obtain information on the oxygen mobility in orthorhombic YBa 2 Cu 3 O 7 − δ , we carried out in situ neutron diffraction measurements at the D2B high resolution diffractometer at Grenoble in the temperature range 300 K ⩽ T ⩽ 1000 K under a constant pressure of 1 atm O 2 . From the data evaluation using the Rietveld method we conclude a maximum oxygen occupancy at T ≈ 600 K with δ = 0.09. The mean square amplitude B 11 of the O4 site rises almost linearly up to 800 K and then increases sharply. We take this temperature as the onset of enhanced oxygen diffusion occurring at the order-disorder phase transition within the orthorhombic phase. We use the temperature dependence of the mean square delocalization of O4 to obtain an estimate of the energy barrier ΔV PER hindering diffusion. ΔV PER starts to decrease appreciably as soon as the temperature exceeds 800 K and comes down to a value ΔV PER ≈ 1200 K at a temperature of 950 K. Thermal activation is therefore strong enough for diffusion to occur.

Substitution and magnetic properties of Pb-1212

Abstract Powders as well as ion-thinned compressed powder specimens in the form of pellets are investigated by 300 keV high-resolution transmission electron microscopy and selected area electron diffraction techniques. They reveal, as the majority and superconducting phase, an oxide with the bulk composition Bi0.5Cu0.5Sr2Y0.8Cu2.2O6.9, as previously determined. High-resolution electron micrographs are compared with contrast simulation, based on the dynamical theory of electron diffraction. This comparison reveals the position of the metal atoms within the unit cell. The lattice parameters of the tetragonal unit cell, consistent with previous X-ray measurements, are a = 3.75 ± 0.08 A and c = 11.53 ± 0.2 A , as measured in selected area electron diffraction patterns. The crystal structure deduced from the high-resolution micrographs corroborates the 1212-structure type suggested on the basis of X-ray powder data (Ehmann et al., 1992). The material is virtually free of planar defects.

Influence of the preparation technique on the electrical properties of the 2212 high Tc superconductor Bi1.75Pb0.25Sr2CaCu2O8 + Z

Abstract A combined study of X-ray and neutron powder diffraction on (Pb 0.75 Cu 0.25 )Sr 2 Y 0.8 Ca 0.2 Cu 2 O z indicates that the rocksalt-like AO monolayers between the perovskite blocks are strongly distorted. By variation of the carrier concentration in the perovskite blocks via a substitution Ca 2+ → Y 3+ in the system (Pb 0.75 Cu 0.25 )Sr 2 Y 1 − x Ca x Cu 2 O z superconductivity is introduced with T c around 40 K. Doping of the AO layers via a substitution Bi 3+ → Pb 4+ in the system (Pb 0.75 − y Bi y Cu 0.25 )Sr 2 Y 0.8 Ca 0.2 Cu 2 O z has a more pronounced effect. Transition from a non-superconducting material with y = 0 to a bulk superconductor with y = 0.3 and T c = 96 K is observed. Post-treatment in flowing Ar shifts T c for y = 0.3 to 100 K. A simultaneous variation of the carrier concentration in the perovskite blocks in the system (Pb 0.75 − y Bi y Cu 0.25 )Sr 2 − Y 1 − x Ca x O z is of subordinate effect.