Perovskite superconducting materials

The compound MgCNi3 has the perovskite structure, as determined by neutron diffraction, and is a superconducting material with 7c = 8 K. 

RE2CUO4 perovskites exhibit important and varying magnetic and electrical characteristics, and they are broadly studied as potential high-Tc superconductive materials. At room temperature, they show p-type semiconducting behaviors, and are used as electrode materials in fuel batteries. The catalytic properties of the perovskite oxides also make them effective in various oxidation and reduction reaction, hence they are considered as promising substitutes to the classical Pt/Rh-based catalysts applied to automotive pollution control. 

Bismuth oxide forms a number of complex mixed-metal phases with the divalent metal oxides of calcium, strontium, barium, lead, and cadmium, and these show a wide variety in composition. With transition metal oxides, mixed-metal oxide phases have been observed which are based upon a Perovskite-type lattice (10) containing layers of Bi202. It is notable that the high Tc superconducting materials which include bismuth also have this Perovskite-type of lattice with layers of copper oxide interleaved with bismuth oxide layers. 

In addition to catalytic applications, the perovskite backbone is a key component in modern high-temperature superconductive materials. By definition, a superconductor exhibits no resistance to electrical conductivity, and will oppose an external magnetic field, a phenomenon referred to as the Meissner effect (Figure 2.19). Many pure transition metals e.g., Ti, Zr, Hf, Mo, W, Ru, Os, Ir, Zn, Cd, Hg) and main group metals e.g., Al, Ga, In, Sn, Pb) exhibit superconductivity, many only when exposed to high-pressure conditions. These materials are referred to as Type I or soft superconductors. 

Oxides form the most common and interesting compounds with perovskite structure. Almost all the metallic natural elements in the periodic table are found in stable perovskites. Also, materials with this structure can be obtained by partial substitution of one or more metallic elements in the A site and/or in the B site. The wide range of properties shown by perovskite-type oxides find applications in catalysis, magnetism, solid oxide fuel cells, and superconductivity. Proper combination or partial substitution of the A site and/or B site atoms introduces abnormal valences or lattice defects, which in turn gives rise to interesting changes in their properties. 

Permonosulfuric acid (PMS), 26 392 Permselective diaphragms, 9 656-657 Permutations, in Latin hypercube sampling, 26 1009-1010 Pernicious anemia, vitamin B12 and, 25 804 Perovskite carbides, 4 692 Perovskite ferrites, 22 55, 56t, 57 Perovskite material, mercury-base superconducting, 23 801 Perovskites, 5 590-591 22 94-96, 97 ... 

The special electric, magnetic, optical, superconductive and catalytic properties of perovskite-typed oxides make this group of materials attracting and widely used. Perovskites were named according to the similarity of their structure with the CaTiOs compoimd. The... 

These copper-oxide compounds crystallize in the perovskite structure and superconductivity is based on the (hole or electron) doping in the copper-oxide planes. This is the reason why these materials can be regarded as being 2D. The first compound of the family was La2 i Sr i Cu04 with Tc 38 K, which soon led to YBa2Cu307 5 with Tc — 92 K for 5 < 1 (Bums, 1993). The non-copper oxide electron-doped perovskite Bai-jcK cBiOa exhibits superconductivity near 30 K for 0.3 < X < 0.5 (Cavaeta/., 1988). 

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. 

At this point, it may be informative to present a chronological listing of the different discoveries in oxide superconductors reported prior to 1975. In this listing, Table 5, we present the year that the oxide compound was first reported, then the year in which superconductivity was first observed in the system and the group credited for the discovery. Of particular interest is the compound Ba(Pb1 xBix)Os discovered by Sleight at du Pont in 1975. This oxide material adopts the perovskite-type structure and contains no transition metals. 

There has been only one report to date on superconductivity in the BaPbj S Oj perovskite series (11). From BaPbOs, where no superconductivity is observed, Tc rises to approximately 3.5K for x=0.25-0.3 and drops rapidly to below 1.5K for larger values of x. Because the BaSbOs perovskite does not exist, a complete solid solution is not expected. The limiting Sb concentration appears to be x=0.5. The electronic properties have not been studied in any detail, but the visual appearance of the materials suggests considerable changes within the solid solution... 

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