Electron concentrated metallic systems

We have concentrated our discussion of solids on non-metallic systems where there are only paired electrons. In such systems, the excited electronic states are very much higher in energy than the ground state, typically in excess of... 

Most of the publications dedicated to the interaction between the RGMAs and a solid surface refer to the rare gas - metal system. The secondary electron emission that occurs in the system allows one to judge of the mechanism that deactivates metastable atoms on a metal surface, as well as to evaluate the concentration of metastable atoms in the gaseous phase. 

Marine organisms concentrate metals in their tissues and skeletal materials. Many of these trace metals are classified as micronutrients because they are required, albeit in small amounts, for essential metabolic functions. Some are listed in Table 11.4, illustrating the role of metals in the enzyme systems involved in glycolysis, the tricarboxylic acid cycle, the electron-transport chain, photosynthesis, and protein metabolism. These micronutrients are also referred to as essential metals and, as discussed later, have the potential to be biolimiting. 

The metallic nature of concentrated metal-ammonia solutions is usually called "well known." However, few detailed studies of this system have been aimed at correlating the properties of the solution with theories of the liquid metallic state. The role of the solvated electron in the metallic conduction processes is not yet established. Recent measurements of optical reflectivity and Hall coefficient provide direct determinations of electron density and mobility. Electronic properties of the solution, including electrical and thermal conductivities, Hall effect, thermoelectric power, and magnetic susceptibility, can be compared with recent models of the metallic state. 

The experimental results described in this review support the concept that, in certain reactions of the redox type, the interaction between catalysts and supports and its effect on catalytic activity are determined by the electronic properties of metals and semiconductors, taking into account the electronic effects in the boundary layer. In particular, it has been shown that electronic effects on the activity of the catalysts, as expressed by changes of activation energies, are much larger for inverse mixed catalysts (semiconductors supported and/or promoted by metals) than for the more conventional and widely used normal mixed catalysts (metals promoted by semiconductors). The effects are in the order of a few electron volts with inverse systems as opposed to a few tenths of an electron volt with normal systems. This difference is readily understandable in terms of the different magnitude of, and impacts on electron concentrations in metals versus semiconductors. 

Fig. 22. The nonmetal-to-metal transition a logarithmic plot of the effective radius afi of the localized-electron state versus the critical (electron) concentration for metallization, n,., in a variety of systems. [Adapted from Edwards and Sienko (68), and used with permission from the American Physical Society, The Physical Review (Solid Stale).)...

Thus the a phase will become unstable electronic concentration limits the region in which the structure of the solid solution can be identical with that of the solvent. The experimental values for JV/jVl at the limit of stability of the a phase for several systems is given in Table CXLL For two metals with the same number of valency electrons = i at all concentrations of the components, so that there... 

The value of is related to the molecular hyperpolarizability and depends on the electronic distribution in a given molecule. Only those molecules which have an inherent asymmetric electron density distribution, or an asymmetric distribution induced by adsorption, are capable of yielding an interfacial second-harmonic response. The value of may be calculated for both molecules and metals given their electronic properties [22]. In the case of molecules it depends on the number of electrons in the system, and therefore, is proportional to their surface concentration. For metals, is directly related to the surface concentration of free electrons. The intensity of the second-harmonic signal is proportional to the square of y ... 

Structures of Abnormal Valency or Electron Intermetallic Compounds. We have seen how in many alloy systems the / -. y- and e-phases are based on electron compounds the formula of which differ very widely but which have in common electron atom ratios of 8 2, 21 18 and 7 4. The range of existence of the particular phases is really a range of solid solution in the compound concerned, and this tends to decrease, as it does in primary solid solution, with increase of valency of the second metal. The j3-, y- and -phases have, however, more in common than mere electron concentration, for they have, in addition, the same lattice structure, although the atomic arrangement is usually a purely random one. Thus, the 8 2 / -compound phase is normally body-centred cubic, although it may have a modified cubic structure known as the /3-manganese one the 21 18 y-compound phase, known as the y-brass structure,... 

The factors determining the particular structure adopted hy an intermetallic compound or, indeed, whether such a compound exists at all as a single-phase material, have been the subject of much discussion for a considerable period of time. The Hume-Rothery rules for electron compound formation will he very familiar and are related physically to the size of the Fermi sphere in the appropriate Brillouin zone. For example, electron compounds are expected for valence electron concentrations of , fj and l for the bcc, y-brass and cph structures, respectively. The interplay of other factors such as the atomic size, solubility and crystal structure of the components on the formation of intermetallic compounds has been considered in considerable detail by many workers, including Yao (1962), who suggested that transition metal binary systems could be classified into groups according to an excess energy dE expressed as... 

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