Alloys electron concentration

In many semiconductors employed for LEDs, and especially in mixed alloys, the direct and indirect minima ate separated by smaller energies than those of the purely direct and indirect semiconductors. As a result, finite electron concentrations exist within both minima. The total electron concentration, n, is given by equation 5 ... 

The fee lattice of the coinage metals has 1 valency electron per atom (d °s ). Admixture with metals further to the right of the periodic table (e.g. Zn) increases the electron concentration in the primary alloy ( -phase) which can be described as an fee solid solution... 

Lithium has been alloyed with gaUium and small amounts of valence-electron poorer elements Cu, Ag, Zn and Cd. like the early p-block elements (especially group 13), these elements are icosogen, a term which was coined by King for elements that can form icosahedron-based clusters [24]. In these combinations, the valence electron concentrations are reduced to such a degree that low-coordinated Ga atoms are no longer present, and icosahedral clustering prevails [25]. Periodic 3-D networks are formed from an icosahedron kernel and the icosahedral symmetry is extended within the boundary of a few shells. 

Hume-Rothery phases (brass phases, electron compounds ) are certain alloys with the structures of the different types of brass (brass = Cu-Zn alloys). They are classical examples of the structure-determining influence of the valence electron concentration (VEC) in metals. VEC = (number of valence electrons)/(number of atoms). A survey is given in Table 15.1. 

Because of the permitted composition ranges, alloys with rather different compositions can adopt the same structure, as can be seen by the examples in Table 15.1. The determining factor is the valence electron concentration, which can be calculated as follows ... 

Parthe, E. (1969) The concept of a partial electron concentration value and its application to problems in crystal chemistry. In Developments in the Structural Chemistry of Alloy Phases, ed. Giessen, B.C. (Plenum Press, New York), p. 49. 

According to Girgis (1983) the existence field of the electron phases may be especially related to the combinations of d elements with the elements of the Periodic Table columns from 11 to 14 (from the Cu to Si groups). It can also be observed that, for several alloy systems, the dependence of the structures (structure types) on the electron concentration (instead of on the composition) may be clearly illustrated by well-known diagrams such as those shown in Fig. 4.39. 

Notice now that, on the assumption that the alloying behaviour is essentially dependent on the value of the electron concentration, the multi-diagram may be considered to contain a representation (and a simulation and a prediction ) not only of the binary diagrams but also of the ternary ones. 

Remarks on the alloy crystal chemistry of the 11th group metals. A selection of the phases formed in the binary alloys of Cu, Ag and Au and of their crystal structures is shown in Tables 5.54a and 5.54b. For a number of these phases, more details (and a classification in terms of Hume-Rothery Phases ) are given in 4.4.5 and in Table 4.5 (structure types, valence electron concentration, etc.). Table 5.54a and 5.54b show the formation of several phases having a high content... 

Hume-Rothery (12,13) has pointed out that in some alloys the structure of the intermetallic phases are determined by the electron concentration (E.C.). The work of Hume-Rothery and others has shown that the series of changes (i.e. a phase —> (3 phase —> 7 phase — phase), which occurs as the composition of an alloy is varied continuously, takes place at electron-atom ratios of 3/2, 21/13, and 7/4, respectively. The interpretation of these changes in terms of the Brillouin zone theory has been made by H. Jones (14) and can be understood from the A (E)-curves for typical face centered cubic (a) and body centered cubic (6) structures as... 

The effect of increasing the electron concentration (E.C.) of the alloy catalyst within a phase domain on the rate of the test reaction was studied. Using silver and gold as solid solvents and adding the multivalent metals of Groups II-V, the electron concentration can be increased up to an E.C. of 1.33 in the period Vb (Cd, In, Sn, Sb) and to an E.C. of... 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

At the critical Ni2.30Mn0.70Ga composition Tm and Tc are no longer coupled, which results in a drastic increase of the martensitic transformation temperature up to 530 K, and in a decrease of Tc down to 350 K. In the alloys with the higher Ni excess, the martensitic transformation occurs at temperatures above 600 K, whereas the Curie temperature continues to decrease. Considering the empirical correlation between the electron concentration e/a and the martensitic transformation temperature Tm [2], it can be suggested that further increase in Tm of the 0.30 < x < 0.36 alloys can be attained by the substitution of Ga for Ni or Mn. 

The reasons for the superior catalytic properties of these bimetallic catalysts are not adequately understood even after 30 years of active research in this area. Many of the explanations for the superior properties of the bimetallic catalysts are based on a structural point of view. Many argue that the bimetallic components form an alloy which has better catalytic properties than Pt alone. For example, alloy formation could influence the d-band electron concentration, thereby controlling selectivity and activity (3). On the other hand, the superior activity and selectivity may be the result of high dispersion of the active Pt component, and the stabilization of the dispersed phase by the second component (4). Thus, much effort has been expended to define the extent to which metallic alloys are formed (for example, 5-18). These studies have utilized a variety of experimental techniques. 

Figure 4. Magnetic moment in Bohr magnetons of an iron atom dissolved in various second row transition metals and alloys (one atomic per cent iron in each metal and alloy) as a function of electron concentration (11)...

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