A. Ehmann

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 .

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%.

Material processing of Ca/Y substituted single (Bi, Pb) O-layered 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

Superconductivity and substitution of Bi-1212

Abstract The superconducting properties of bulk (Bi, Pb)-1212 material are strongly influenced by the chemical starting composition, the employed temperature/time schedule and gas atmosphere. It is shown that via the application of melt texturing (Bi, Pb)-1212 material with improved intragrain and intergrain properties is obtained. The best materials are formed for a Y/Ca ratio of 0.7/0.4 with Bi:Pb:Sr:Y:Ca:Cu = 0.4:0.45:1.9:0.7:0.4:2.25 after application of 1030°C as melting temperature and a cooling rate of 35°C/h to 985°C/12 h. The transition temperature is situated at 94 K with a critical current density j cm = 7.4 × 10 5 Acm −2 (5 K, 0 T) and 2.8 × 10 3 Acm −2 (77 K, 0 T).

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

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.

Superconductivity and processing of single Bi−O layered cuprates in the (Bi, Pb)−Sr−(Ca, Y)−Cu−O system

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.

Antiferromagnetism of the new non-superconducting 2201 stacking polytype BiPbSr2Fe1-xMxO6+z with M = Co, Ni and Morin-like spin-flop-transition in BiPbSr2FeO6+z

The effects of improved materials processing on single Bi−O layered cuprates in the (Bi, Pb)−Sr−(Ca, Y)−Cu−O system have been investigatged. For Bi-1212 we have improvedTc to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions (χ ZFC) around 30–40%. The critical current density is 2×106 Acm−2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.

The system BiPbSr2Fe1−xTxO6+z(T≡Co,Ni)

Abstract The magnetic properties and the spin structure of the BiPbSr2Fe1-xMxO6+z compounds with M = Co, Ni, isotypical with the members of the Bi2(T12)Sr2(Ba2)Can-1CunO2n+4 compositional series with n=1, were studied using magnetic susceptibility measurements and 57Fe Mossbauer spectroscopy. All the compounds investigated (with x=0, xCo=0.5 and xNi=0.25) are basically antiferromagnetic. The highest Neel temperature of 212 K is reached for BiPbSr2FeO6+z, where the hyperfine field at the Fe site has with 48.9 T a value close to the expectation for Fe3+ in a high spin state. A comparison between the values of the quadrupole splitting in the paramagnetic and in the magnetically ordered state shows that a spin-flop transition from the basal plane to the c-axis occurs in the temperature range 120-77 K in BiPbSr2FeO6.1. Such a behaviour is reminiscent of the Morin transition in α-Fe2O3, where the spin direction of the antiferromagnetic sublattices rotates from the plane to the trigonal axis. No spin-flop transition takes place in BiPbSr2Fe0.5Co0.5O6.01.

Investigation of superconductivity and physical properties of some spinel-, perovskite- and pyrochlore-type oxides

Abstract The Substitution of cobalt or nickel for iron is reported for the system BiPbSr2Fe1−xTxO6 + z, (T ≡ Co, Ni). The average structure can be described by an orthorhombic subcell. The subcell of BiPbSr2FeO6 + z follows the general building principles of the series (AO)2M2Bn−1FenO3n + 1 + z and consists of perovskite-like slabs with one iron oxygen octahedron intercalated between AO double layers. The electrical resistivity is strongly influenced by the parameter x and decreases with increasing x.