Molecular beam epitaxial growth of layered Bi-Sr-Ca-Cu-O compounds

On a number of compositions in the solid solutions Bi2−xPbxSr2CuO6+δ and Bi2−xPbxSr2Ca1−yY yCu2O8+δ high resolution electron microscopy, X-ray diffraction, magnetic measurements and oxygen content determination were performed. In general substitution of Ca by Y leads to a decrease in Tc, which can be partly suppressed by the substitution of Bi by Pb. With increasing amount of Pb the modulated structure changes from a Bi-type, via a mixed Bi/Pb-type, and a Pb-type of modulation, to a structure in which the modulation is absent. In the Pb-type the modulation waves in neighbouring (Bi, Pb)O layers are in phase with approximately k = 8 b . Over a considerable composition region the Bi-type and Pb-type of modulation coexist in the same region. Models for the Pb-type and the mixed Bi/Pb-type of modulation are postulated, based on the incorporation of extra oxygen in the (Bi, Pb)O layers. It is shown that the balance between the Pb-type and Bi-type of modulation is very delicate, and probably depends on the distribution of the excess oxygen in the two (Bi, Pb)O layers.

Structure and properties of (Bi, Pb)2Sr2(Ca, Y)Cu2O8+δ

From the discovery of oxide superconductors in 1964, there were frequent indications that these materials are metastable. This lack of thermodynamic stability has now become one of the predominant issues in the synthesis of the high‐temperature superconductors. Once prepared, metastable superconductors possess sufficient kinetic stability that we generally need have no concern that they will decompose into more stable phases that are not superconducting. Nonetheless, our understanding of the issues relating to the stability of the known superconductors greatly influences our approaches to synthesizing them. Furthermore, understanding the relationship between instabilities and the mechanism of high‐Tc superconductivity could be the key to the synthesis of superconductors with even higher critical temperatures.

Synthesis of Oxide Superconductors

Electron diffraction and high-accuracy EDX microanalysis in the TEM clearlyrevealed the dependence of the displacive modulation on the Pb content in (Bi, Pb) 2 Sr 2 Ca n −1 Cu n O 2 n +4 high- T c superconductors. A general rule was derived which correlates the amount of Pb in the crystal structure to the wave length of the modulation. With increasing Pb concentration the wave length of the modulation increases linearly and the relation between the Pb content and the wave length of the modulation is the same for both the n =2 and the n =3 material. The maximum Pb fraction was as high as Bi:Pb=0.72:0.28 and resulted in an unmodulated crystal structure. The differences between the Pb modulation and the Bi modulation arise from a decrease of the displacement amplitudes parallel to the b direction and a zero component of the wave vector parallel to the c direction for the Pb modulation. Significant deviations from the ideal compositions 2212 and 2223 were detected by EDX microanalysis. A systematic (Sr+Ca) deficit was found for the n =2 and n =3 phases. EDX microanalysis also showed that as much as 20% of Ca can be substituted by Sr.

Displacive modulation and chemical composition of (Bi, Pb)2Sr2Can−1CunO2n+4 (n=2, 3) high-Tc superconductors

The uses of X-ray anomalous scattering in crystal structure analysis have undergone a major expansion due to the refinement and ease of availability of the necessary X-ray instrumentation and methods. The structural chemistry and biology fields span a similar suite of technical needs but with widely differing molecular systems. The innate synergies between the two research fields brought two of the authors (JRH and VK) together at an Erice Summer School on Synchrotron Radiation in Crystallography in 1985 and took them into a collaboration spanning already 20 years. The authors’ wide perspectives are therefore, if not unique, perhaps rather rare. Thus the breadth of coverage of this review is unusual. However, there are two excellent books on anomalous scattering and its uses that have been published covering the periods up to 1975 and 1994 [S. Ramaseshan, S.C. Abrahams (Eds). Anomalous Scattering, Munksgaard, Copenhagen (1975); G. Materlik, C.J. Sparks, K. Fischer (Eds). Resonant Anomalous X-ray Scatterin...

Anomalous scattering in structural chemistry and biology

The distribution of the bismuth atoms over the cation sites in the 2212 Bi-Sr-Ca-Cu-O superconductor has been determined by anomalous scattering synchrotron crystallography. The analysis of reflection pairs measured at wavelengths of 0.9243 and 0.9600 angstrom shows a delocalization of the bismuth atoms over the calcium and strontium sites. The "mixed" plane between the CuO 2 layers contains 6.0(1.4) percent bismuth (where the number in brackets represents the statistical standard deviation derived from the least-squares refinement of the data), and a much smaller amount of strontium than often assumed. The strontium deficiency is charge-compensated by the creation of electron holes in the CuO 2 layer. The result supports the view that neither extra oxygen nor overlap of the bismuth 6 p and copper 3 d bands is needed to account for the holes, which are an essential feature of the superconductivity mechanism.

https://science.sciencemag.org/content/sci/244/4900/62.full.pdf

Anomalous scattering study of the bi distribution in the 2212 superconductor: implications for cu valency.

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Anomalous scattering study of the bi distribution in the 2212 superconductor: implications for cu valency.