High-temperature superconductors structures

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

High-temperature superconductors show superconductivity at temperatures higher than the boiling point of liquid nitrogen (77 K). Their structures are superstructures of... 

CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series... 

Figure 8.4 Idealized structures of some high-temperature superconductors (a) Bi2Sr2C>6 (=Tl2Ba206) (b) Bi2CaSr2Cu208 (=Tl2CaBa2Cu208) (c) as (b) but with the nominal Cu06 octahedra completed in faint outline (d) Bi2Ca2Sr2Cu30io (=Tl2Ca2Ba2Cu30io) (e) as (d) but with the nominal Cu06 octahedra completed in faint outline.

High temperature superconductors (HTS), 23 814, 826, 829. See also Anisotropic HTS HTS entries applications of, 23 852-872 layered, 23 827, 840 magnetic phase diagram of, 23 838-842 p- and n-type, 23 838 structural anisotropy and fluxon line fragmentation in, 23 841 thallium- and mercury-based, 23 848-850... 

It is found that the compound (Yo.6Cao.4)(SrBa)(Cu2.5Bo.5)07.8 with the ratio of Cu and B atoms equal to 5 1 is a high-temperature superconductor [28]. In principal, the crystal structure of this compound is isomorphic to that of YBa2Cu307 6. It would be interesting to make clear how B atoms substitute for Cu atoms in the crystal. 

The most heavily studied high temperature superconductor is YBa2Cus07 x (x = 0 to 1), whose Cu oxidation state is determined by the oxygen content. The parent structure, YBa2Cus07, contains layers... 

Dissipation or resistive behavior in a superconductor develops when the quantized vortices depin and cut across the current flow. Vortices are established in these superconductors either with an applied magnetic field or from the self field of a current. Visualization of the vortex structure in the high temperature superconductors has been studied with a Bitter decoration technique by Dolan et al. 

Non-stoichiometric compounds are of potential use to industry because their electronic, optical, magnetic, and mechanical properties can be modified by changing the proportions of the atomic constituents. This is widely exploited and researched by the electronics and other industries. Currently, the best known example of non-stoichiometry is probably that of oxygen vacancies in the high temperature superconductors such as YBCO (1-2-3) (YBa2Cu307 J. The structure of these is discussed in detail in Chapter 10. 

Chapter 10 covers the exciting field of superconductors, including high-temperature superconductors, many of which have structures related to the perovskite structure. 

With the availability of a method to produce fullerenes in the laboratory, the topic has become the rage of the day. It has created great excitement in the scientific world comparable only to that of high-temperature superconductors in early 1987. Cgg and C70 have been characterized in terms of the crystal structure, UV-visible, NMR,... 

Calculations using the methods of non-relativistic quantum mechanics have now advanced to the point at which they can provide quantitative predictions of the structure and properties of atoms, their ions, molecules, and solids containing atoms from the first two rows of the Periodical Table. However, there is much evidence that relativistic effects grow in importance with the increase of atomic number, and the competition between relativistic and correlation effects dominates over the properties of materials from the first transition row onwards. This makes it obligatory to use methods based on relativistic quantum mechanics if one wishes to obtain even qualitatively realistic descriptions of the properties of systems containing heavy elements. Many of these dominate in materials being considered as new high-temperature superconductors. 

Another feature of the superconductivity effort has been that most has been devoted to detailed studies of existing materials rather than wide-ranging searches for new compositions. Substantially enhanced properties are necessary if high temperature superconductors are to have a major impact in the commercial sector. The assumption has been that detailed understanding will lead to improved superconductors. Usually, this has not occurred. The complexities of these underdetermined solid state systems restrict the ability to design improved structures. 

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