Cubic high-2 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... 

As the cubic system was often found to be an important structural class for good superconductors, another myth was generated that suggested one should focus on compounds having a cubic-type crystalline structure, or a structure possessing high symmetry. This myth was also abandoned when lower symmetry systems were found... 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

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... 

Some attention has been drawn to the low dimensional aspects of these materials. While the high Tc materials have two dimensional sheets of Cu-0, and the 90K superconductor has in addition one dimensional Cu-0 chains, Ba(Pb/Bi)03 has an almost cubic structure. Consequently, low dimensionality seems not to be a necessary condition for materials in this class to be superconducting. In any case low dimensional structures will likely result in other materials from low coordination number of the transition metal and by including large electropositive cations. 

L.a Cr,j Nj, crystallizes with a superstructure of rocksalt and is a superconductor below 2.7 K [262], The partial occupancy of the nitrogen positions was fixed at the experimental stoichiometry. LaA NT formed at high pressure reportedly has a structure similar to CsCl [263]. La(NH,)3 is formed under ammonia at 400 MPa and crystallizes with amide groups in approximate cubic eutaxy [264]. 

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 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]. 

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