The most promising material for fabricating long-length conductors is (Bi,Pb)-2223 produced by the powder-in-tube process in Ag clad tapes. Intensive studies have shown that the critical current density of fully processed tapes is affected by a large variety of processing parameters, but the nature of the (Bi,Pb)-2223 phase formation is still controversial. In this study certain steps of the 2223 phase formation were investigated at different stages of sintering under the influence of various temperature/time schedules of thermomechanical processing and the important role of Pb is shown. The main effects of Pb in the phase forming process are: (i) Acceleration of the 2223 formation via a rapid generation of (Bi,Pb)-2212; (ii) introduction of grain growth and texture; (iii) increase of the temperature window for the 2223 formation; (iv) decrease of reaction temperature; (v) adjustment of the hole concentration of the 2223 phase; (vi) operation as buffer for charge carriers via the temperature dependent equilibrium between Pb4+ and Pb2+; and (vii) creation of Pb containing pinning centres.

The conditions of melt texturing of bulk (Bi,Pb)-1212 were improved by adaption of (i) the chemical composition of the precursor, (ii) the temperature/time schedule and (iii) the method of post-treatment. It is shown that especially an increase of the Ca and Pb content is suitable and well textured materials are obtained. In melt textured materials the best values are T c, χ =94 K and j c, χ =7.8×10 5 A cm −2 (5 K, 0 T). However, thermomechanical processing has failed to induce a pronounced grain alignment in (Bi,Pb)-1212 PIT tapes and the superconducting properties are still dominated by weak intergrain superconducting couplings.

The room temperature crystal structure of Y-0212 and La-0212 contains double layers of nearly regular (Fe, Cu)O5 quadratic pyramides. A pecularity is the compensation of the pronounced Fe4+ content via the introduction of additional oxygen (O(3)) in 2b. Thus a charge reservoir is created. At 10 K the materials are basically antiferromagnetic. The gradual freezing of the Fe spins is reminescent of concentrated spin glasses.

A combined study of X-ray and neutron powder diffraction on (Pb0.75Cu0.25)Sr2Y0.8Ca0.2Cu2Oz 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 Ca2+ → Y3+ in the system (Pb0.75Cu0.25)Sr2Y1 − xCaxCu2Oz superconductivity is introduced with Tc around 40 K. Doping of the AO layers via a substitution Bi3+ → Pb4+ in the system (Pb0.75 − yBiyCu0.25)Sr2Y0.8Ca0.2Cu2Oz has a more pronounced effect. Transition from a non-superconducting material with y = 0 to a bulk superconductor with y = 0.3 and Tc = 96 K is observed. Post-treatment in flowing Ar shifts Tc for y = 0.3 to 100 K. A simultaneous variation of the carrier concentration in the perovskite blocks in the system (Pb0.75 − yBiyCu0.25)Sr2 − Y1 − xCaxOz is of subordinate effect.

The crystal structure of (Bi, Pb)-0212 was refined from neutron powder diffraction data at room temperature and 10 K as tetragonal (space group I4/mmm). The transition to superconductivity is not accompanied by a structural transition. Between the quadratic Cu/O double pyramides a complex charge reservoir is created via the introduction of extra oxygen (O(3) in 2b). Pb is simultaneously present in two atomic positions with CN8 and 9 and considered as Pb4+ and Pb2+. In as-prepared powdered materials the superconducting volume fraction remains low due to the formation of nonsuperconducting grain boundaries. A significant improvement is obtained by use of Ca2PbO4 or CaBi2O4 as flux. The highest T-c is situated at 86 K.

The crystal structure of both Bi0.5Sr1.5YCu1.5Fe0.5O6 and Sr2Pb0.5Y0.5CuFeO6 is basically composed of double layers of (Cu,Fe)O-5 pyramids. In order to better understand the magnetic properties of this type of compound, a Fe-57 Mossbauer spectroscopy study in the temperature range 4.2-380 K was undertaken. The Mossbauer spectra of both compounds studied present a strong relaxational character. The lines, magnetically split at 4.2 K, become broader and more asymmetrical with increasing temperature. The amount of the paramagnetic phase Increases drastically and the magnetic splitting vanishes at 340 K for Sr2Pb0.5Y0.5CuFeO6 and at 295 K for Bi0.5Sr1.5YCu1.5Fe0.5O6. The results are discussed in terms of the superexchange interactions between the Fe atoms placed in the basal planes of the quadratic pyramidal layers and of the Fe-Cu superexchange. The relaxational character of the Mossbauer spectra offers evidence for a gradual freezing of the Fe spins, reminiscent of the behaviour of concentrated spin glasses.

The crystal structure of Pb-0212 was refined from neutron powder diffraction data at room temperature and 8K as tetragonal (space group I4/mmm). The transition to superconductivity is not accompanied by a structural rearrangement. The disadvantage of the lacking charge reservoir in the 0212 type is compensated by the introduction of Pb2+/Pb4+ as second redox couple in addition to the system Cu2+/Cu3+. An alternative charge reservoir is created between the CuO5 double pyramids by incorporation of Pb in 2a and O in 2b. Preparation of powdered materials at ambient pressure results in the formation of superconductivity. However, the superconducting volume fraction remains low due to the formation of grain boundaries acting as weak links. A significant improvement is obtained by use of Ca2PbO4 or CaBi2O4 as flux. The highest T-c is about 75 K.

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 jcm = 7.4 × 105 Acm−2 (5 K, 0 T) and 2.8 × 103 Acm−2 (77 K, 0 T).

Several cuprates of type Pb-0212 are prepared. The crystal structure is refined from neutron powder diffraction and X-ray diffraction data as tetragonal (space group 14/mmm). interestingly, Pb is simultaneously present in two atomic positions with CN 8 and 9 and considered as Pb4+ and Pb2+ Superconductivity with T-c between 30 and 75 K is observed in the system Pb0.5La1.5-xSr1+xCu2Oy. However, in as-prepared powdered materials the superconducting volume fraction remains low due to the formation of grain boundaries acting as weak links, whereas a significant improvement is obtained in bulk materials by use of flux. Mossbauer and susceptibility measurements of Fe substituted materials indicate the development of a spin glass like behaviour at low temperatures.

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 improvedT c 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.

A series of cuprates of type Bi-0212 has been prepared. The crystal structure was refined from neutron powder diffraction data at room temperature and 10 K as tetragonal (space group I4/mmm). Mössbauer and susceptibility measurements of the Fe substituted material indicate the development of a spin glass-like behavior at low temperatures. Superconductivity with Tc around 30 K is observed in the system Bi0.5La1.5−xYxSrCu2Oy. An increase of the hole-carrier concentration is obtained by suitable incorporation of Bi, Pb, La and Sr on the nine coordinated 4e positions with an improvement of Tc to 75 K.

The superconducting performance of (Bi,Pb)-1212 and (Bi.Pb>0212 is strongly influenced by the method of material processing. It is shown that the application of melt processing results in bulk materials with improved intragrain and intergrain properties. The best pellets of (Bi,Pb)-1212 dispose on a transition temperature of 94K with critical current densities of 7.8 x 10(5) Acm(-2) (5K,0T) and 2.4 x 10(3) Acm(-2) (77K, 0T). For pellets df (Bi,Pb)-0212 a maximal transition temperature of 86K is obtained with superconducting volume fractions around 15%.

In the system Bi-0212 the carrier concentration can be changed by both cation substitution and oxygen content. The crystal structure of Ca substituted material was refined from neutron powder diffraction data for Bi0.5Sr1.5Ca0.5Y0.5Cu1.5Fe0.5Oy. It is shown that extra oxygen is introduced in the cation layers between the double sheets of Cu/O pyramids. In superconducting material the familar dependence ofTc on the hole carrier concentration is observed. The higherTc is situated at 75 K.

Improved material processing of single BiO layered 1212 raises Tc to 102K with a superconducting volume fraction around 30–50% and a critical current density of 2×106 Acm−2 (5K, OT). Moreover, strong indications for the existence of Bi-1223 are present. Bi-0212 is realized for the first time in the cuprate family (Bi, Sr)2YCu2O6+z, and easily obtained for a partial substitution of Fe → Cu. According to neutron diffraction profile refinement at RT and 10K the crystal structure is tetragonal (space group 14/mmm) with double layers of (Cu,Fe)O pyramides. Additionally, the existence of isotypic Pb-0212 is reported.

Studies of the influence of chemical composition and post-treatment on the superconducting properties of the new superconducting family Bi-1212 indicate that the superconducting transition temperatures as well as the superconducting volume fraction are dependent on both parameters. The highest Tc = 84K is obtained for Bi:Pb:Sr:Y:Cu = 0.4:0.3:2:0.6:2.1 after post-annealing in flowing Ar (600°C/50h). From the uniform observation of an increase of the superconducting volume fraction after post-annealing at moderate temperatures (500–700°C) follows the important correlation between a certain crystalline order and the development of superconductivity; actually values up to a Meissner fraction of about 10% are obtained.

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.

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 Å and c = 11.53 ± 0.2 Å, 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.

Bi cuprates of type 1212 have been synthesized above 970°C from the proper starting materials. The oxides of idealized composition (Bi0.5Cu0.5)Sr2YCu2O7−z crystalline in the tetragonal space group P4/mmm (a=3.815(5), c=11.73(1) Å) 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 (Bi0.5Cu0.5)Sr2Y0.8Cu2.2O6.95.

The (Bi, Pb)2Sr2Bin-1FenO3n+3+z compounds are strongly related to the members of the high-Tc superconducto r series (Bi, Pb)2Sr2Can-1CunO2n+4+z. In this study we show that the Fe compounds with n = 2, 3, 4 are antiferromagnets with Néel temperatures of 512, 597 and 620 K. The saturation hyperfine field and isomer shift values correspond to Fe ions in the 3+ oxidation state. For the n = 3 and 4 compounds, the different subspectra belonging to the inequivalent Fe lattice sites were measured, and the temperature dependence of their hyperfine fields was interpreted in the molecular field approximation.