Perovskite superconductors

The A-site ordered double perovskite (Nag jKg jlBajBi Ojj has a superconducting transition temperature T of approximately 27 K. Tlie structure is cubic, space group Imlm (229), a=0.8550nm, with ordering of the (Na, K) and Ba atoms in layers to give a doubled perovskite unit cell. As the formula suggests, one set of A-sites (Al) is only partially occupied and contains the Na and K cations and a 

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Figure 1 Relationship between the perovskite structure ABOs (left) and the defect-perovskite superconductor YBajCugOy (right). Metal atoms are shaded. Note the missing oxygen atoms in the latter drawing that result in formation of copper-oxygen sheets (above and below the Y atoms), and copper-oxygen chains (between the Ba atoms).

MgO has emerged recently as a usefiil substrate on which to grow perovskite superconductor and ferroelectric materials, because it has a good lattice match with these phases. MgO has also been used as a protective layer in plasma displays. A range of metal-organic precursors has been described, including Mg(C5H5)2 in the presence of O2, [Mg(CH3)(OtBu)]4, Mg(dmto)2, Mg(tmhd)2(tmeda), ... 

The field of perovskite superconductors has witnessed an explosion of interest and activity over this past year. While much remains to be done to better understand these compounds, some general structural and chemical features necessary for the superconducting behavior are beginning to emerge. These can be summarized as ... 

The low dimensional electronic structure of the Cu-O sublattice appears to be a general and important feature in all of the perovskite 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]. 

Figure 2 Crystal structure of four types of layered 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... 

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

A unit cell of the mineral perovskite, which has a structure similar to that of some of the ceramic superconductors, is shown here. What is its formula ... 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

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... 

CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series... 

Perovskite structures, 5 598 Perovskite-type layered superconductors, 23 852... 

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

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... 

Figure 10 The first stage in the building of the molecular structure of the superconductor YBa2Cu307-x starting from CaTi03 perovskite units. The formation of YBa2Cu309. (a) Common representation (b) stereoscopic representation...

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