Perovskite layered copper oxide

Many of the new tasks would be at the boundary with materials science. There are some that are obviously applications-oriented, like the electronic theory of high temperature superconduction in the layered copper-oxide perovskites, and other aspects of nanotechnology. There are also fundamental valence problems, such as accounting for the structures and properties of quasiciystals. Why is the association of transition metals and aluminium apparently of central importance How do we deal with the valence properties of systems where the free energy of formation or phase transition is dominated by the entropy term ... 

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. 

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. 

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]. 

In the structure Nd2 y zCeySrzCu04, the Cu ions have a fivefold oxygen coordination. However, with a smaller A cation that is stable in eightfold oxygen coordination, and a larger A cation in an AO layer, the family of copper oxide intergrowth stmctures that are related to the perovskite structure becomes extensive. Similar compounds may be represented by their sequence of successive (001) planes as follows ... 

Toda K, Kameo Y, Kurita S, Sato M (1996) Crystal structure determination and ionic conductivity of layered perovskite compounds NaLnTiO (Ln = Rare Earth). J Alloys Compd 234 19-25 Tomchenko AA, Harmer GP, Marquis BT, AUen JW (2003) Semiconducting metal oxide sensor array for the selective detection of combustion gases. Sens Actuators B 93 126-134 Tongpool R, Leach C, Freer R (2000) Temperature and microstructural dependence of the sensitivity of heterocontacts between zinc oxide and copper oxide in reducing environments. 1 Mater Sci Lett 19 119-121 Traqueia LSM, Marques FMB, Kharton VV (2006) Oxygen ion conduction in oxide materials selected examples and basic mechanisms. Bol Soc Esp Ceram 45(3) 115-121... 

In situations where the performance is dependent on the uniqueness of the crystal structure of the material, such as the metal-Insulator transition in vanadium oxide, acousto-optic and acoustic-electric responses of AIN, ZnO, PZT, BaTiOg and SrTi03, superconducting properties of copper oxide based perovskites, A-15 silicides, and NbN, wide band gap and dopability of SiC, transistor action in Si/NiSi2(CoSi2)/Sl epitaxial layers etc., it is important to optimize the deposition conditions for the growth of a %[Pg.395]

The general formulation of these oxides, (ACuOg.x)m(AO)n, reflects for each of them the number m of copper layers which form each perovskite slab, and the number n of AO layers which form each rock salt-type slab (the AO layers which lie at the boundary of the perovskite slabs and rock salt type slabs can only be counted as for 1/2). Thus all these oxides (2-34) can be represented by the symbol [m,n] in which m,n will be integral numbers. In most of these oxides one observes for one compound only one m and n value, corresponding to single intergrowths. 

Bai-xKxBi03 A non-copper containing compound with a Tc of 30K, Ba1 xKxBiOs is of much interest in the study of oxide superconductors (10)(9l). Unlike the cuprate phases which are layered, Ba K BiOg is an ideal cubic perovskite. Most of the work on Ba1 xKxBiOs has been carried out on ceramics. Recently, small... 

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

Many layer-perovskites with Cu as the B cation are superconductors at relatively high temperatures ( 100 K). Although the mechanism of superconductivity is not well understood, a necessary condition is that the oxidation state of the copper ion be around +2.2. In many compounds, such as the well-known YBa2Cu307 (63324) this is achieved naturally through the relaxation of... 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

---