Alloys electron band structure

Although bimetallic catalysts did not represent a totally new area of research in the early 1960s, my research emphasized entirely new aspects of this subject. Earlier work on metal alloy catalysts was dominated by efforts to relate the catalytic activity of a metal to its electron band structure. Very little attention had been given to other aspects of metal alloy catalysts, such as the possibility of influencing the selectivity of chemical transformations on metal surfaces and of preparing metal alloys in a highly dispersed state. These aspects were the basis for my work on bimetallic catalyst systems. 

Our ignorance concerning even the qualitative nature of catalyst surfaces can be illustrated by reference to alloy catalysis 403a). That a surface alloy can have structure quite different from the normal bulk phases has already been observed by LEED for the Ni-Mo system, in which the surface structures do not correspond at all to the ordinary bulk alloys Ni4Mo and NiaMo 404). In many experiments with alloys an abrupt change of catalytic behavior at a particular alloy composition has been correlated with a change in the electronic band structure of the solid. But what is the nature of the surface Average interior composition of a binary alloy is hardly affected if one kind of alloy atom... 

Early work on electron band structure by soft X-ray spectroscopy was concentrated on pure metals, and it was not until the advent of photoelectron spectroscopies that alloys started to be examined. It soon became clear that small additions of nickel to copper resulted in the appearance of electrons having energies close to the Fermi value there was no common d-band, but each component exhibited its own band structure (Figure 1.17). Many other kinds of physical measurement confirmed this, and corresponding behaviour was observed with the palladium-silver system (Figure 1.18). It became necessary to find a new and better theory. 

The minute particles, which a solid consists of, have the extraordinary quantum features. However, there is a gap between quantum theory on the one hand and engineering on the other hand. Even the principal notions and terms are different. The quantum physics operates with such notions as electron, nucleus, atom, energy, the electronic band structure, wave vector, wave function, Fermi surface, phonon, and so on. The objects in the engineering material science are crystal lattice, microstructure, grain size, alloy, strength, strain, wear properties, robustness, creep, fatigue, and so on. 

Dimmock, J.O., A.J. Freeman, and R.E. Watson, 1966, Electronic band structure and optical properties of rare earth metals, Abeles, F. ed.. Proceedings of the International Colloquium on Optical Properties and Electronic Structure of Metals and Alloys, Paris, 1965, (North-Holiand Publishing Co., Amsterdam), pp. 273-245. 

This is the first book devoted to the theoretical modelling of refractory carbides and nitrides and alloys based on them. It makes use of computational methods to calculate their spectroscopic, electric, magnetic, superconducting, thermodynamical and mechanical properties. Calculated results on the electronic band structure of ideal binary transition-metal carbides and nitrides are presented, and the influences of crystal lattice defects, vacancies and impurities are studied in detail. Data available on chemical bonding and the properties of multi-component carbide- and nitride-based alloys, as well as their surface electronic structure, are described, and compared with those of bulk crystals. 

Schematic presentation of the electronic band structure of product films formed on cast alloy and MC alloy in NaCI aqueous solution. 

Next, we focus on the general trend of electronic structure of transition metals. It is known that the electronic structure of transition metal alloys can be described by means of the so-called rigid band model as a first approximation, which states that the electronic band structure is unchanged upon substitutional alloying and the electronic structure is simply described... 

The electron—photon coupling that forms the microscopic basis of MOKE makes it possible, in principle, to determine the electron spin-dependent band structure of elements and alloys. This is done by examining the dependence of the Kerr response on the wavelength of the incident light. 

Experimentally it is found that the Fe-Co and Fe-Ni alloys undergo a structural transformation from the bee structure to the hep or fee structures, respectively, with increasing number of valence electrons, while the Fe-Cu alloy is unstable at most concentrations. In addition to this some of the alloy phases show a partial ordering of the constituting atoms. One may wonder if this structural behaviour can be simply understood from a filling of essentially common bands or if the alloying implies a modification of the electronic structure and as a consequence also the structural stability. In this paper we try to answer this question and reproduce the observed structural behaviour by means of accurate alloy theory and total energy calcul ions. 

Van der Woude and Miedema [335] have proposed a model for the interpretation of the isomer shift of Ru, lr, Pt, and Au in transition metal alloys. The proposed isomer shift is that derived from a change in boundary conditions for the atomic (Wigner-Seitz) cell and is correlated with the cell boundary electron density and with the electronegativity of the alloying partner element. It was also suggested that the electron density mismatch at the cell boundaries shared by dissimilar atoms is primarily compensated by s —> electron conversion, in agreement with results of self-consistent band structure calculations. 

---