Oxidic superconductors, crystal structure

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... 

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). 

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. 

The crystal structure of many oxide superconductors can be regarded as based on the perovskite structure, which can be modified by atomic replacements, displacements, vacancies, and changes in the order of stacking layers. 

Figure 4.8-21 Crystal structure of oxidic superconductors YBa2Cu307 (a) and Bi2CaiSr2Cu208 (b).

Maeno et al.m found superconductivity in a sample of SnRutTt (Tc 1 K). This has the same crystal structure as La2CuC>4 and attracts much attention in exploring the nature of the superconductivity in relation to the Cu oxide high-Tc superconductor. 

In 1986, Bednorz and Muller of IBM Zurich discovered an oxide superconductor based on La-Ba-Cu-0 with a critical temperature Tc of 30K [1]. The crystal structure was determined by the Tanaka and Kitazawa group at Tokyo University to be La2- cBa cCu04 [2], In the following years oxide superconductors with higher Tc have been found [3-7] ... 

The high-temperature superconductors based on copper oxides have a principally perovskite structure. The crystal structures of four typical perovskite superconductors, BaPbA Bii- 3 (BPB), La (214), Y (123), and T1 (1223) are shown in Fig. 2. BPB is a simple perovskite, and the others are layered per-ovskites. The perovskite superconductors are classified in Table 1 from the viewpoint of crystal structure and constituent elements. Class 1 is the simple cubic perovskite with ABO-i composition. Typical of class I, BaPbArBii- Os was first synthesized by Sleight et al. in 1975 [9]. The parent compound is BaBiOa, whose Bi3+ site is partially substituted by Pb +. After the discovery of the high-temperature superconductors, it was found that the replacement of Ba by K also gives rise to a superconductor, Bai- cKjcBi03, with Tc = 28 K [10]. 

The structural label perovskite applies to an enormous number of multi-element oxides. The ideal perovskite (CaTiOa) structure is cubic with one formula unit per unit cell, with ions on the body center (Ca), cube comers (Ti) and face centers (O). Identification of crystal structures as distortions (tetragonal, orthorhombic, trigonal) of this has allowed extension of the class even to the wedding-cake layer structures of the high-To superconductors (see, e.g., Phillips 1989). The properties of rare-earth perovskites (prior to the discovery of high-Tc s) are reviewed by Khattak and Wang (1979). 

Before the discovery of high-temperature superconductors in 1986, there already existed oxide ceramic superconductors such as SrTi03 (Tc = 0.4 K) 7, BaPbOs (Tc = 0.4 K) and Ba(Pb,Bi)03 (Tc = 12K). They have a cubic perovskite structure. In those, the Ba(Pb, Bi)03 attracted much interest from scientists due to its high Tc despites of relatively small carrier density. In 1998, Cava et al. discovered that (Ba, K)Bi03 with same cubic perovskite structure has an extremely higher Tc of 40 K [8] in cubic perovskite superconductors. Figure 8.1.2 shows the crystal structure of cubic perovskite. The unit cell is... 

FIGURE 8.1.5 Crystal structure of homologous series of Bi system oxide superconductor. 

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