Perovskites, superconductivity

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... 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

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... 

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. 

Studies of Superconducting Oxides with the Perovskite-type... 

The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

During this same period of time, Michel and Raveau reported (141) the synthesis of La3BasCu6014+y, a new compound having some close relationship to the perovskite structure. This compound was the precursor to a variety of different copper-oxide derivatives a more complete paper on its structure was published (142) in 1987, and the recent physical property measurements indicated (143) no superconductivity in either the quenched or oxygen-annealed phases. 

The ability of copper to take various coordinations as well as the great flexibility of the perovskite structure combine to allow significant deviations in oxygen stoichiometry that do not really change the structure but dramatically affect the superconducting properties. 

There have been many solution techniques used to synthesize the superconducting perovskite-related phases, but no experiment has really achieved a substantial improvment of the synthesis, compared to conventional solid state reaction. 

Suzuki, M., Enomoto, Y. Murakami, T. and Inamura, T., Thin Film Preparation of Superconducting Perovskite-Type Oxides by rf Sputtering. Jpn. J. Appl. Phys. 20(s 20-4) 13 (1981). 

Crystal Chemistry of Superconducting Bismuth and Lead Oxide Based Perovskites... 

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