Chemical design of copper-oxide superconductors:-Homologous series and oxygen engineering-Homologous s

Abstract

Two-dimensional CuO2 planes in which copper atoms nominally possess an intermediate valence value between II and III have appeared to be essential for superconductivity of copper oxides with holes as charge carriers. Contrary to this, in n-type superconductive copper oxides, copper atoms have an intermediate valence state of I/II. In both cases, control over the charge carrier concentration has been achieved by tuning the precise mixed-valence value of copper atoms in the superconductive CuO2 plane(s). In terms of valence tuning, structures allowing for oxygen nonstoichiometry are advantageous. At the same time, within the conductive copper-oxide framework the charge has to be uniformly distributed between each copperoxygen polyhedron to ensure itinerancy of the charge carriers. Therefore high electrical conductivity is expected only in CuO2 planes that consist of equivalent Cu-O polyhedra. In line with this, high-Tc superconductivity occurs in layered structures built up with (i) conductive copper-oxide layers in which the oxygen content is stoichiometric and (ii) spacing layer(s) with oxygen and/or cation nonstoichiometry, i.e. tunable charge. In the structure of the first high-Tc superconductor, (La,Ba)2CuO4±δ?[1], a single CuO2 plane is combined with a ?(La,Ba)O1 plusmn;δ2?2 double-layer block only. Since this discovery, a variety of superconductive copper-oxide phases with a number of different layers being involved have been synthesized. Depending on the piling sequence of the layers various structure blocks with different characteristics in terms of charge balance and doping are formed. Based on thetypes of layers involved, the structures of superconductive and related layered copper oxides have successfully been classified into "two main categories" ?2,3?, and further understood as members of different "homologous series" [4-6], viz. Chapter 2. With such a large number of superconductive copper-oxide phases synthesized and thoroughly characterized, it has become easier to recognize the "yet-missing" phases and design the strategies for searching for such. It has also become possible to find various trends and draw general conclusions related to the functions of the different structure blocks in terms of redox characteristics and charge balance, as being reviewed in Chapters 3 and 4, respectively. Here one of the key concepts is "oxygen engineering" [7]. It is also important to recognize that, even though ideal rather than real structures are considered for the purpose of classification, it is the small and delicate deviations from the ideal structures that control the material properties such as charge/carrier distribution and superconductivity ?6,8,9?. Such factors and their fine-tuning are shortly discussed in Chapter 5. © 2005 Springer-Verlag Berlin Heidelberg.