Perovskite-based compounds superconductors

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]. 

Another significant development was the superconducting system of a mixed oxide of yttrium reported by Wu, Chu and co-workers. The compound has a formula YBa CujO, where x is between 6 and 7 and a T, 93 K which means that the expensive liquid helium can be replaced by cheap liquid nitrogen (77 K). For simplicity, the above system is called the 1-2-3 system based on the ratio of the metals present. This also has a perovskite structure. Similar compounds substituting yttrium by other lanthanides like europium, samarium etc., are known. These are called warm superconductors. A lot of research to obtain high T are going on and superconductors with Tj, as high as 164 K have been reported. 

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