Crystal structure superconductors

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

Three common uses of RBS analysis exist quantitative depth profiling, areal concentration measurements (atoms/cm ), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profilii of polymers, high-T superconductors, optical coatings, and catalyst particles. ... 

Among various superconductors, compounds with the A15 (Cr3Si) crystal structure have the highest critical temperatures. This crystal structure has a simple relationship with the Ll2 structure (Ito and Fujiwara, 1994) as illustrated in Figure 8.9. When the unit cells are aggregated, the face-centered pairs of atoms form uniform chains of transition metal atoms along three orthogonal directions. This feature may be related to the relatively stable superconductivity in compounds with this structure. 

In fact, this attraction between negative charges (that violates the principles of electrostatics) is mediated by the crystal structure of the superconductor. In every metal lattice there is a reciprocal stripping of valence electrons between metal sites which results in these metal sites, fixed at lattice positions, assuming a positive charge. As shown in Figure 7, when a moving electron crosses these positive metal sites the metal ions are attracted towards the electron trajectory and disturbed from its equilibrium position. 

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. 

For other examples involving materials see, for example, the determination of the crystal structure of the high Tc superconductor YBa2Cus07by Huang et al. [26], and K20.Nb205 [27]. 

Shibaeva RP, Kaminskii VF, Yagubskii EB (1985) Crystal structures of organic metals and superconductors of (BEDT-TTF)-l system. Mol Cryst Liq Cryst 119 361-373... 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

Few oxide superconductors were known prior to 1985 and we shall now return to these so that we can discuss these materials in reference to their crystal structure classes. There are only three broad structural categories in which most of the oxide superconductors occur. The important structural types include sodium chloride (rocksalt, or Bl-type), perovskite (E2X), and spinel (Hlx). 

The crystal structure of the 1-2-3 superconductor, YBazCusOy- is depicted in Figure 10.8. Figure 10.8(a) depicts only the positions of the metal atoms. If we discuss it in terms of the perovskite structure ABO3, where B=Cu, the central section is now an A-type perovskite unit cell and above and below it are also A-type perovskite unit cells with their bottom and top layers missing. This gives copper atoms at the unit cell corners and on the unit cell edges at fractional coordinates A and Ys. The atom at the body-centre of the cell (i.e., in the centre of the middle section) is yttrium. The atoms in the centres of the top and bottom cubes are barium... 

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,... 

A breakthrough in superconductor technology came with the discovery24 of yttrium barium copper oxide, YBa2Cu307, whose crystal structure is shown here. When heated, the material readily loses oxygen atoms from the Cu-O chains, and any composition between YBa2Cu307 and YBa2Cu3Ob is observable. 

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