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15. Single-phase Bi2 x(Ca,Sr)n 1 Cun O2n 4 δ, n = 0, 1, 2, 3 AND ∞, and some substitutional DERIVATIVES

Author

Tallon, J. L., Buckley, R. G., Presland, M. R., Gilberd, P. W., Brown, I. W. M., Bowden, M., and Goguel, R.

Abstract

The preparation, structure and properties of five members of the homologous series Bi2+x(Ca,Sr)n+1 Cun O2n+4+δ are discussed, namely for n = 0, 1, 2, 3 and ∞. The two end members are insulating phases with a fixed oxygen stoichiometry but the members n = 1, 2 and 3 are superconducting phases with Tc depending on the calcium to strontium ratio and on oxygen stoichiometry as determined by annealing temperature and oxygen partial pressure. Maximum zero resistance Tc's obtained are n = 1: 76 K, n = 2: 91 K and n = 3: 106 K. The effects of Pb-substitution in n = 2 and n = 3 are discussed with particular reference to the stabilisation of the latter phase. Rare-earth substitution is shown to raise Tc for n = 2 to at least 101 K without the introduction of the n = 3 phase.

Published
1989
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16. High-Tcsuperconducting phases in the series Bi2.1(Ca, Sr)n+lCunO2n+4+δ

Author

Tallon, J. L., Buckley, R. G., Gilberd, P. W., Presland, M. R., Brown, I. W. M., Bowden, M. E., Christian, L. A., and Goguel, R.

Abstract

Following the report of Maeda et al.1on high-Tcsuperconductivity in samples of nominal composition BiSrCaCu2Oxwe have identified three distinct phases in the homologous series Bi2.1(Ca, Sr)n+lCunO2n+4+δwith n = 1,2 and 3. These have zero resistance Tcs of 80 K, 91 K and 105 K respectively. The structures in the homologous series appear to be based on alternating double bismuth Bi2O2layers and perovskite (Sr, Ca)O–CuO2layers, with higher members obtained by intercalating additional 3.1 Å Ca-CuO2bilayers. The structures can be indexed on a pseudo-tetragonal subcell with a = b = 5.4 Å and c = 24.4 Å (n = 1), 30.76 Å (n = 2) or 36 Å (n = 3). We were unable to prepare the n =4 member. In each structure a 2× superlattice structure in the c-direction arises as a natural consequence of a 19/4 incommensurate structure in the b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature, Tc, for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.

Published
1988
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17. Enhancing Tc in Bi2.1CaSr2Cu2O8+δ from 90 to 101 K by yttrium substitution.

Author

Tallon, J. L., Buckley, R. G., Staines, M. P., Presland, M. R., and Gilberd, P. W.

Subjects
COPPER oxide superconductors, HIGH temperature superconductors, SUBSTITUTION reactions
Abstract

The Y and rare-earth substitution for Ca in Bi2.1CaSr2Cu2O8+δ is described with particular emphasis on Y substitution. We have prepared single phase, fully substituted material which exhibits an 8.0×b-axis incommensurate structure, rather larger than the 4.75× structure in the parent superconductor. The infrared absorption spectra of the insulating fully substituted material are shown to have features very similar to that for YBa2Cu3O7-δ. A solubility range is observed only at substitutions up to 5% at which point zero-resistance Tc is raised from 90 to 101 K. This is shown to arise from 2 copper-layer material and not from 3 copper-layer material. Combined Y and Pb substitution yields n=2 material for which Tc remains fixed at 90 K, insensitive to annealing temperature and oxygen loading. [ABSTRACT FROM AUTHOR]

Published
1989
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