18 valence electron superconductors

Valence electron density for the diamond structures of carbon and silicon. (Figure redrawn from Cohen M L i. Predicting New Solids and Superconductors. Science 234 549-553.)... 

The situation is very similar in the Chevrel phases. These are ternary molybdenum chalcogenides A,.[Mo6Xg] (A = metal, X = S, Se) that have attracted much attention because of their physical properties, especially as superconductors. The parent compound is PbMo6Sg it contains Mo6Sg clusters that are linked with each other in such a way that the free coordination sites of one cluster are occupied by sulfur atoms of adjacent clusters (Fig. 13.9). The electric properties of Chevrel phases depend on the number of valence electrons. With 24 electrons per cluster (one electron pair for each edge of the... 

In fact, this attraction between negative charges (that violates the principles of electrostatics) is mediated by the crystal structure of the superconductor. In every metal lattice there is a reciprocal stripping of valence electrons between metal sites which results in these metal sites, fixed at lattice positions, assuming a positive charge. As shown in Figure 7, when a moving electron crosses these positive metal sites the metal ions are attracted towards the electron trajectory and disturbed from its equilibrium position. 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

In the fully-tetravalent state CeN and CeP(p) have the same valence-electron configuration as the NaCl-type superconductors ZrN, ZrP(h), HfN, ThN, ThP, YS, LaS, LuS, etc. By analogy we argued that CeN (Hulliger, 1968) and CeP(p) (Hulliger and Hull, 1970) might also show a transition to the superconductive state near 1 K. On our CeN samples, however, we were not able to detect a transition neither under a pressure of 25 kbar down to 1.2 K nor at normal... 

The alkali metal fullerides, M3C60 (M = Na, K, Rb, and Cs), are a little different. The absence of the sign indicates that these are not endohedral fullerenes. Take K3C6O a representative example. It is prepared from stoichiometric amounts of solid C o potassium vapor. The three potassium atoms transfer their valence electrons to the fuUerene so that K3C60 more accurately written as (K )3(C o ) This special compound becomes a superconductor below the critical temperature, 7, of 19.3 K. (Below the 7, a superconductor is perfectly conducting—that is, it has zero resistance.) It has a face-centered cubic structure of C o anions with potassium cations in both the octahedral and tetrahedral holes as shown in Figure 15.9. (See p. 173 for more details on the positions and numbers of these holes.)... 

Materials can be classified into four broad categories metals, ceramics, semiconductors, and polymers. Metals, because of their detached electrons, are good reflectors of light, good conductors of heat and electricity, and tend to be ductile. Ceramics generally are poor conductors of electricity because their valence electrons are tied up in chemical bonds, although the recent discovery of ceramic superconductors is a notable exception. Their... 

The generally accepted theory of electric superconductivity of metals is based upon an assumed interaction between the conduction electrons and phonons in the crystal.1-3 The resonating-valence-bond theory, which is a theoiy of the electronic structure of metals developed about 20 years ago,4-6 provides the basis for a detailed description of the electron-phonon interaction, in relation to the atomic numbers of elements and the composition of alloys, and leads, as described below, to the conclusion that there are two classes of superconductors, crest superconductors and trough superconductors. 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

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. 

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