J. Schneck

Phase diagram of the system Bi1.6Pb0.4Sr2CuO6-CaCuO2 between 825°C and 1100°C

Abstract The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C–1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+ x . It involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus (∼855°C), the sequence of stable { n } phases is {1}, {1}+{2}, {2}, {2}+{3}, and {3}. The {3} phase (110K) exists as the dominant phase between ∼835°C and ∼875°C for 3 n t >∼900°C) mainly contains the transformation and melting lines of CuO, (Ca, Sr) 2 CuO 3 , and (Ca, Sr) 14 Cu 24 O y . The liquids involves a eutectic point at (∼930°C, n =2) and a critical end point of miscibility of the two liquids L 1 (bismuth rich) and L 2 . The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.

Resistivity anisotropy in lead-substituted Bi2Sr2CaCu2O8+y

We show that lead-substitution in single crystals of Bi2Sr2CaCu2O8+y induces a decrease of both the structural and resistivity anisotropies. In the first place we have found that lead substitution can reduce the resistivity anisotropy ρcρab by two orders of magnitude and that ρc changes its behaviour from a semiconductor-like to metallic-like. In the second place, the existence of a threshold value in the amount of lead-substitution, above which iodine intercalation is no longer possible is consistent with the occurence of a strengthening of the binding force between adjacent Bi-O planes.

Structural characterization of epitaxial BiSrCaCuO films on MgO substrates

Abstract We have studied the epitaxial growth of thin Bi 2 Sr 2 Ca 1 Cu 2 O 8 films on (001) oriented MgO substrates. These films were grown using the in-situ laser ablation process (700°C, 0.1 mbar oxygen), and they showed a perfect c -axis alignment normal to the substrate, characterised by narrow mosaic distributions and by channeling effects (low χ min values) along this direction. Despite the large difference between their lattice parameters (9%), the epitaxial growth of the BiSrCaCuO film on MgO substrate has been found evidence for using X-ray diffraction in transmission geometry, by the observation of three azimuthal adjustments of the a - and b -axes of the films with respect to the a -axis of the substrate (0°, 13° and 45°). The possible origins of such epitaxial relationships are presented and discussed.

Influence of lead substitution on the properties of BiSrCaCuO superconducting ceramics and crystals

Abstract We have examined systematically the influence of substitution of lead to bismuth in Bi2Sr2CanCun+1O2n+6, (2 2 n n+1), superconductors. The stabilisation of the (2223) phase is greatly promoted for substitution between 0.5 and 0.7 Bi in the above formula, and virtually single phase ceramics have been obtained, with zero resistivity at T=108K. In (2212) single crystals, we have observed an intermodulation of the incommensurate wave already present in the lead free compounds and the wave specific of lead doping. The wavelength of the latter modulation increases with the lead content. These results are discussed with respect to the increase of the strengh of the bonding between Bi-O layers, present in the structure of these materials, induced by the lead substitution.

Spin susceptibility behaviour in the Bi2Sr2Can−1CunO2n+4 compounds : from underdoped to overdoped regime

Abstract We have studied the thermal behaviour of the spin susceptibility, χ spin (T), of the CuO 2 planes in the normal phase. χ spin (T) was measured by 17 O and 43 Ca NMR in various compounds of the Bi 2 Sr 2 Ca n −1 Cu n O 2 n +4 family : a n=3 sample almost optimally doped, an overdoped n=2 sample and a more strongly doped n=1 sample. By contrast to the underdoped case, in the overdoped one, χ spin (T) increases when cooling. Furthermore, this behaviour is enhanced by increasing the doping. χ spin (T) is compared to the macroscopic susceptibility behaviour. On the other hand, our results are consistent with a decrease of the number of charge carriers per copper plane for increasing n.

Phase diagram of the system Bi1.6Pb0.4Sr2CuO6-CaCuO2 above 825°C

Abstract The phase diagram of the lead-substituted Bi2O3-SrO-CaO-CuO system has been investigated, in the temperature range 825°C–1100°C, and in static air, along the line Bi1.6Pb0.4Sr2Can−1CunO2n + 4 + x. For n~3, the ration SR Ca has been systematically varied. The diagram involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus(~855°C), the sequence of stable {n} phases is {1}, {1} + {2}, {2}, {2} + {3}, and {3}. The {3} phase (110K) exists, as dominant phase, between ~835°C and ~875°C, for 3 ~900°C), mainly contains the transformation and melting lines of CuO, (Ca, Sr)2CuO3, and (Ca, Sr)14Cu24Oy. The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.

Influence of lead substitution on the magnetic irreversibility line of 2212 superconducting single crystals

Abstract An experimental study of the magnetic irreversibility lines has been carried out on superconducting single crystals of Bi 2− x Pb x Sr 2 CaCuO 2 O 8 , with and without lead. The irreversibility field values B irr ( T ) are almost identical for x = 0 and x = 0.4. However, the power-law fits of B irr ( T )∼1/ T α at low temperatures provide different exponents α in both cases, α = 6 in the lead-free sample and α = 4 in the lead-substituted sample. At high temperatures a regime change is observed in both samples and B irr vanishes as ( T c − T ) β with β = 1 for x = 0 and β = 0.37 for x = 0.4. A discussion of these results is achieved within the TAFF and vortex solid melting models taking into account a possible “dimensional crossover” of the vortex system behavior.

17O NMR investigation of the properties of the various types of CuO2 planes in (Bi,Pb)2Sr2Can−1CunOy

Abstract In the normal state of the (2223) phase of bismuth cuprates the 17 O Knight shift of the CuO 2 planes with pyramidal coordination for copper is nearly temperature independent except in the vicinity of Tc where it presents a steep decrease. The 17 O Knight shift of the CuO 2 planes with square plane coordination for copper is significantly lower and presents a continuous decrease with decreasing temperature which is already detected just below room-temperature. We discuss. these results on the basis of a recent theoretical prediction for the existence of an inhomogeneous charge carriers distribution among the two types of CuO 2 planes, and also on the basis of similar available data, for other high Tc superconductors.

Pseudo binary phase diagram of the system (Bi1.6Pb0.4Sr2CuO6)z−(CaCuO2)1−z

Abstract A pseudo-binary phase diagram, between 800°C and 1020°C, has been investigated by preparing and characterizing ceramics of 20 distinct nominal compositions Bi 1.6 Pb 0.4 Sr 2 Ca n−1 Cu n O y with 1

Discommensuration patterns in the BiSrCaCuO high Tc superconductors

Abstract We present a transmission electron microscopy study of the discommensuration (DC) walls in the superconducting bismuth-cuprates. High resolution evidence for the translation slip across a wall is obtained for the first time in these materials. A consistent relationship is established between the observed characteristics of the walls in several phases of this structural family: number of walls merging at a node, average distance between DCs, and value of the modulation wavenumber. We show that a wide range of ‘latent’ commensurate periods, between 4 b and 9 b , determine the characteristics of the walls.