Alloys, electron theory

Understanding alloys in terms of electron theory. The band theory of solids had no impact on the thinking of metallurgists until the early 1930s, and the link which was eventually made was entirely due to two remarkable men - William Hume-Rothery in Oxford and Harry Jones in Bristol, the first a chemist by education and the second a mathematical physicist. 

Pettifor, D.G. and Cottrell, A.FI. (1992) Electron Theory in Alloy Design (The Institute of Materials, London). 

Cottrell, A (1998) Concepts in the Electron Theory of Alloys (lOM Communications, London). 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

Turchi, P.E.A. (1994) Electronic Theories of Alloy Phase Stability. In Intermetallic Compounds. Vol. 1 Principles, eds. Westbrook, J.H. and Fleischer, R.L. (J. Wiley Sons, Chichester), p. 21. 

Research on alloy catalysts started in the 1950s with attempts to investigate the role in catalysis of the electronic structure of metals. This research was initiated by several papers of Dowden which, measured by their response in the literature, rank among the most important papers ever written on catalysis. However, it appeared later (for reviews, see 1-5) that two basic ideas, on which the so-called electronic theory of catalysis was built up, were not correct. These ideas were as follows ... 

The most essential progress from the point of view of application of this theory in catalysis and chemisorption has actually been achieved by the very first papers (48-50), where the so-called coherent potential approximation (CPA) was developed and applied. By means of this, photoemission data were explained in a quite satisfying way and the catalytic research got full theoretical support for some of the ideas introduced in catalysis earlier on only semiempirical grounds (5) namely, individual components are distinguishable for molecules from the gas phase and the alloy atoms preserve very much of their metallic individuality also in alloys—something that was impossible according to the RBT and the early electronic theory of catalysis. 

How was this development reflected by the theory of catalysis on alloys An early and very important paper (9) discussed the selectivity and activity effects fully in terms of the old electronic theory of catalysis. Another paper (5), which appeared simultaneously with (9), turned attention to the fact that one must also consider effects other than only the changes in the electronic structure. The results on alloys should be rationalized on the basis of two aspects of alloying (5) ... 

The compounds discussed are mainly metal oxides, metal sulfides, and metal hydrides. Non-stoichiometry of alloys, which can be interpreted using the electron theory, is not treated. The author hopes that readers will inform him if they find any part of this book misleading or erroneous. 

L. Pauling, The electronic structure of metals and alloys, in Theory of Alloy Phases, American Society for Metals, Cleveland, OH, 1956, pp. 220-242. 

Modem Electron Theory by M. W. Einnis in Electron Theory in Alloy Design edited by D. G. Pettifor and A. H. Cottrell, Institute of Materials, London England, 1992. This is my favorite article on first-principles calculations. Einnis spells out all of the key points involved in carrying out first-principles calculations of the total energy. 

There are many theories on the mechanism of the segregation effect that suggest either a chemical or an electronic mechanism or both types of mechanisms. However, it seems that the most reliable mechanism is electronic as proposed by Mukheijee and Moran [35]. This electronic model calculates the chemical properties of the pure constituents from their physical parameters and then estimates those of the alloys. It employs the tight-binding electronic theory, the band filling of the density of states, and the bandwidth of the pure components for the calculations. However, it seems that the 2D Monte Carlo simulations produce better results by using the embedded atom and superposition methods. The latter allows for the calculation of the compositions from the relative atom positions, and the strain and the vibrational energies has been reviewed for 25 different metal combinations in [36]. It was also possible to predict composition oscillations as a consequence of the size mismatch. 

The Interstitial-Electron Theory has been applied to the structure of metals, alloys and interstitial compounds, to their magnetic and superconducting properties, as well as to a range of surface phenomena. This work has seemingly not come to the attention of the wider scientific community perhaps because it was published only in Japanese journals. It merits wider recognition and a critical evaluation. 

The nearly free electron theory developed by Faber and Ziman (1964) is an obvious starting point for discussing liquid alloys of type I. For those cases in which information is available about the three partial interference functions which characterize the structure of binary alloys, close quantitative agreement between theory and experiment has been obtained. We emphasize that a positive da/dT is entirely consistent with metaUic behaviour in Hquid alloys on account of the temperature dependence of the partial interference functions. For this reason many liquid alloys which have in the past been thought of in terms of a semiconducting framework should more properly be regarded as metallic. (It may, in certain cases, be necessary to introduce the Mott g factor but there is little evidence either way on this important point at the present time). Alloys of the second type will form the subject for section 7.7. 

However, PtsCo/C, despite showing the good performance for the ORR, has been reported as a less tolerant electrocatalyst for MOR [130]. This has been attributed to the enhanced metallic character of Ft in the PtaCo due to intra-alloy electron transfer from Co to Ft and to the adsorption of oxygen species on the more electropositive element (Co) that promotes MOR according to the bifunctional theory. 

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