Superconductors chemical properties

In this chapter on chemically modified fullerenes, I will discuss only a few representative molecules which incorporate fullerenes, and focus attention on the best-characterized compounds. A comprehensive review of chemical reactions with fullerenes has just been published by Taylor and Walton.[Ta93] It is not appropriate here to attempt a synthesis of the emerging field of the chemical properties of the fullerenes. Covered elsewhere in this book are ioni-cally bonded fullerides, such as the superconductor KsCeo (Chapter IV) and endohedral complexes, in which various atoms are captured inside the hollow carbon shell (Chapter VI). 

Growth of molecular inorganic conductors and superconductors as films or nanowires is mainly conducted by electrodeposition due to the physico-chemical properties of the precursors and the necessary electrochemical activation to produce the conductive phase. 

Gross stoichiometry variations in these modular systems are often acconunodated by the formation of other series members or the incorporation of planar faults, often equivalent to isolated lamellae of other series members, into the structure. However, many individual phases also show composition variation which is sometimes accommodated by random populations of point defects and in other phases by ordering and the generation of new structures. In both cases composition flexibility is important in influencing the physical and chemical properties of these materials. In particular this effect has been well studied in the cuprate superconductors, where changes in composition have repercussions for the superconducting transition temperature, r, of the phase. 

Possibly the most important single advance in the field of synthetic inorganic polymers in recent years has been the discovery, in 1973, that polymeric sulphur nitride (SN)n (Polythiazyl) is a metallic-type conductor, which at low temperatures becomes superconducting. Polythiazyl is the first known example of such a polymeric superconductor, which does not contain metal atoms. A history of (SN)r research to 1973, and a review of the chemical properties and preparative... 

Ternary ABO3 perovskite materials have numerous technological applications as these materials display a wide range of useful physical and chemical properties. They are important as catalysts, as ceramics, ferroelectrics, superconductors, as materials for fuel cells, fusion reactors, and optical and piezoelectric devices. 

In recent years, the scope of application of lanthanides and their compounds has significantly expanded. They are widely used in the production of optical elements (plasma display panels, optic fibers in telecommunications), metal-halogen lamps, superconductors, highly selective catalysts, hydrogen batteries, magnetic alloys, fuel cells, etc. Scientific investigations of their magnetic, nuclear, optical, thermal, and chemical properties are realized for successful achievements in these fields. 

The initial attempts to relate this language to quantum mechanics were understandably done through the orbital model that underlies the valence bond and molecular orbital methods employed to obtain the approximate solutions to Schrodinger s equation. The one-electron model, as embodied in the molecular orbital method or its extension to solids, is the method for classifying and predicting the electronic structure of any system (save a superconductor whose properties are a result of collective behavior). The orbital classification of electronic states, in conjunction with perturbation theory, is a powerful tool in relating a system s chemical reactivity and its response to external fields to its electronic structure and to the symmetry of this abstract structure. The conceptual basis of chemistry is, however, a consequence of structure that is evident in real space. 

Various Ru-oxides, YBa2Cu307, c (I), Ba Ru2/3Gdi/303 (II) as well as Ru-doped a-Fe203 (III), to probe the local chemical structure around the Ru atoms. Compound (I) has interesting properties with x < 0.2 it is a superconductor and with x 1 a semiconductor. Ru oxidation state and coordination are discussed on the basis of measured isomer shifts and quadrupole splittings Ru(IV) ions exclusively occupy Cu-1 sites which form one-dimensional chains... 

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 materials normally used in the construction of working electrodes are platinum, gold, mercury and carbon. However, there have been recent attempts to use more sophisticated materials such as superconductors (as will be discussed in Chapter 10, Section 1), but at moment, due to their poor chemical and mechanical properties, they are not very promising electrode materials. 

In electrochemical terms, one of the expected employment of superconductors is as electrode materials. However, before considering the eventual benefits offered to electrochemistry by such materials we must introduce the physical, physico-chemical and structural properties of superconductors.1... 

Before studying the properties of superconductors one must have efficient electrodes of these materials available. However, their fragile, porous and chemically non-inert nature makes them unsuitable in principle for use as electrodes. 

There are several comprehensive textbooks and review articles devoted to organic superconductors [3,4,182-186] which describe design and preparation, crystal and band structures, chemical, transport, magnetic, optical, and thermal properties, and theory. 

Saito G, Urayama H, Yamochi H, Oshima K (1988) Chemical and physical properties of a new ambient pressure organic superconductor with higher than 10 K. Synth Met 27 A331-A340... 

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