Bi-Based Superconductors

PMP Powder-Melt Process known method that uses finely milled and then pressed mixture of powders of BaCu02, CuO and Y2BaCu05 phases involved later in a crystallization process this method is similar to the so-called two-powder-process in the case of Bi-based superconductors QMG Quenched-Melt Growth original method of achieving smaller Y2BaCu05... 

Figure 1 Schematic representation of the ideal structures of the homologous series of Bi- and Tl-based superconductors showing the coordination polyhedra about Bi or T1 (octahedra) and Cu (octahedra, square pyramids, or square planes).

Figure 8 Atomic displacements in the M-O (M = La, Bi, Tl, Pb) rock salt-like layers of the copper-based superconductors. The view is perpendicular to the MO sheet. (A) Ladder-like arrangement of M and O atoms observed in orthorhombic La2Cu04, Bi2Sr2Can.1Cun 02n+4, and Tl2Ba2Cu06. (B) and (C) Atomic arrangement of Bi and O atoms as islands and chains that form when the atoms are displaced from the mirror positions shown in (A) (see text).

J. G. Wen, Ph.D. Thesis, TEM study on the structures and defects of Bi-based high Tc superconductor , Institute of Physics, Chinese Academy of Sciences (1991). 

The same methods to introduce grain boundaries, i.e. bi-crystal substrates, step-edges and bi-epitaxial techniques, also apply to the other superconductors [14.52, 14.107-14.109], The details of the interfacial interactions between the substrates and the thin films need to be investigated in order to find the optimum conditions and geometries for each individual system. The chemical interactions must also be considered when integrating the superconductors into multilayer structures. Intermediate non-superconducting layers that are inert to the YBCO may react chemically with the other superconductors. It is also necessary to carefully consider the ex situ processes of the Tl- and Hg-based superconductors producing multilayer structures. 

There appears to be a common basis for superconductivity in Cu and Bi based oxides. To date there is insufficient data to properly characterize many of the copper oxides, especially in those cases with shorter Cu-Cu separations than similar superconductors. More copper free superconductors would be of great value in testing and developing theoretical models. 

Wire production process has been well developed for Bi-based high-temperature superconductors, Bi-2223 and Bi-2212. There are two major techniques for... 

Emge TJ, Leung PCW, Beno MA, Schultz AJ, Wang HH, Sowa LM, Williams JM (1984) Neutron and X-ray diffraction evidence for a structural phase transition in the sulfur-based ambient-pressure organic superconductor bis(ethylenedithio)tetrathiafulvalene triiodide. Phys Rev B30 6780-6782... 

Misaki Y, Higuchi N, Eujiwara H, Yamabe T, Mori T, Mori H, Tanaka S (1995) (DTEDT) [Au(CN)2]o.4 an organic superconductor based on the novel tc-electron framework of vinylogous bis-fused tetrathiafulvalene. Angew Chem Int Ed Engl 34 1222-1225... 

Tanaka H, Ojima E, Eujiwara H, Nakazawa Y, Kobayashi H, Kobayashi A (2000) A new K-type organic superconductor based on BETS molecules, K-(BETS)2GaBr4 [BETS = bis (ethylenedithio)tetraselenafulvalene]. J Mater Chem 10 245-247... 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

The following sections will outline specific methods for the synthesis of Bi- and Tl-based cuprate superconductors. Because the synthetic methods and historical evolution of the compounds are different, the bismuth and thallium families are described separately. 

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

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