Cubic perovskite superconductors

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... 

Several other research groups (89-91) picked up on this discovery and obtained Ba/K/Bi/O materials with an onset transition temperature of 34 K. The structure of this oxide superconductor is cubic, perovskite-type, having unit cell dimensions, 4.288-4.293 A. This system appears to be a three-dimensional superconductor and has... 

Bai-xKxBi03 A non-copper containing compound with a Tc of 30K, Ba1 xKxBiOs is of much interest in the study of oxide superconductors (10)(9l). Unlike the cuprate phases which are layered, Ba K BiOg is an ideal cubic perovskite. Most of the work on Ba1 xKxBiOs has been carried out on ceramics. Recently, small... 

The compound Bai j,Kj Bi03 3, with Tg SOK for X 0.4 has the cubic perovskite structure. It is the first oxide superconductor without copper with a transition temperature above that of all of the A-15 compounds. The... 

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 structure of the 92 K-superconductor YBa2Cu307 (Fig. 6a) is orthorhombic with all parameters closely to the ap parameter of the cubic perovskite (a b ap c 3ap). It is derived from the stoichiometric perovskite by removing rows of oxygens parallel... 

The superconducting oxides include both perovskites and Ruddlesden-Popper compounds which have an orthorhombic arrangement of cubic cells, alternatively of the perovskite and sodium chloride structures. The common feature of all of these is the presence of copper as a major component. The first ceramic superconductor was a lanthanum-strontium substituted cuprate (Lai Sr Cu04 z), which is a perovskite, but subsequently the inter-oxide compound Y203 2BaO 3CuO, commonly referred to as a 123 compound, was shown to have superior performance. The speculation concerning the conduction mechanism is that this involves either Cu3+-Cu2+ positive hole... 

Another way of representing the perovskite structure is to move the origin of the coordination axes such that the Ca2+ ions are now at the centre of the cubic cell, Figure 9. The latter representation allows a better understanding of the structure of a superconductor such as... 

There are only three broad structural categories into which most of the reported oxide superconductors can be classified i.e., sodium chloride, perovskite, and spinel. It is interesting to note that these three structures possess cubic symmetry in their most idealized state. A detailed discussion of the research performed on oxide compounds derived from these three structures will be presented in Section 2.0 below. But before we continue with the general study of superconductivity in other materials, an overview of the oxide work is given in chronological fashion (to 1975) in the following Section. 

Fig. 7.33 (el Unit cell of the 1-2-3 superconductor, orthorhombic, space group Pmtnm. Qne-dirnensional CuOi chains run along the b axis, and two-dimensional CuO, layers lie in the ab plane, (b) The cubic stricture of perovskite. SrTiO,. Three unit cells are shown sleeked vertically, (c) The unit cell of the 1-2-3 superconductor in the context of the surrounding crystal. Copper atoms are surrounded either by five oxygen atoms in a square pyramid or four oxygen atoms in a square plane. (From Holland. C. F. Stacy, A. M. Ace. Chem. Res. 1988,21, 8-15. Reproduced with permission.]... 

For example, the family of perovskite minerals and high-temperature ceramic superconductors exhibits this descent of symmetry, from the cubic "ideal" perovskite structure (space group Pm3m, the real mineral perovskite is orthorhombic, space group Pnma, with a fourfold larger unit cell than the ideal cubic one) to orthorhombic structures for the highest-critical... 

The materials described below include diamond, cubic BN (the hardest material next to diamond), SrCu02 (from which two types of superconductors have been derived by compositional modification), MgSiOs (a geologically interesting perovskite), and Ge02 (amorphized underpressure at room temperature). 

Some high-temperature superconductors adopt a crystal structure similar to that of perovskite (CaTiOs). The unit cell is cubic with a Ti" " ion in each comer, a Ca " ion in the body center, and 0 ions at the midpoint of each edge, (a) Is this unit cell simple, body-centered, or face-centered (b) If the unit cell edge length is 3.84 A, what is the density of perovskite (in g/cm ) ... 

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). 

---