The Structure of Metals

These three structures are the predominant structures of metals, the exceptions being found mainly in such heavy metals as plutonium. Table 6.1 shows the structure in a sequence of the Periodic Groups, and gives a value of the distance of closest approach of two atoms in the metal. This latter may be viewed as representing the atomic size if the atoms are treated as hard spheres. Alternatively it may be treated as an inter-nuclear distance which is determined by the electronic structure of the metal atoms. In the free-electron model of metals, the structure is described as an ordered array of metallic ions immersed in a continuum of free or unbound electrons. A comparison of the ionic radius with the inter-nuclear distance shows that some metals, such as the alkali metals are empty i.e. the ions are small compared with the hard sphere model, while some such as copper are full with the ionic radius being close to the inter-nuclear distance in the metal. A consideration of ionic radii will be made later in the ionic structures of oxides. 

The self-diffusion coefficients of metals, which describe the movement of atoms within a pure metal, vary over a very wide range of values at any 

The total electrical resistance at room temperature includes the contribution from scattering of conduction electrons by the vacancies as well as by ion-core and impurity scattering. If the experiment is repeated at a number of high temperature anneals, then the effects of temperature on the vacancy contribution can be isolated, since the other two terms will be constant providing that 

Typical values of the energy to form vacancies are for silver, 108kJmol 1 and for aluminium, 65.5 kJ mol-1. These values should be compared with the values for the activation enthalpy for diffusion which are given in Table 6.2. It can also be seen from the Table 6.2 that the activation enthalpy for selfdiffusion which is related to the energy to break metal-metal bonds and form a vacant site is related semi-quantitatively to the energy of sublimation of the metal, in which process all of the metal atom bonds are broken. 

At high temperatures there is experimental evidence that the Arrhenius plot for some metals is curved, indicating an increased rate of diffusion over that obtained by linear extrapolation of the lower temperature data. This effect is interpreted to indicate enhanced diffusion via divacancies, rather than single vacancy-atom exchange. The diffusion coefficient must now be represented by an Arrhenius equation in the form 

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V. K. Grigorovich, The Metallic Bond and the Structure of Metal.s Nova Science, Huntington, NY (1989). 

Hume-Rothery (1936) The Structure of Metals and Alloys (The Institute of Metals, London). 

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

The corrosion of metals invariably involves some kind of interaction between a metal and its environment, and in many cases the corrosion (location, form and rate) is significantly affected or even caused by some structural feature of the metal. It is essential, therefore, for the corrosion engineer to have some appreciation of the structure of metals, and an elementary survey is provided in this section which provides a basis for an account of metal structure in relation to corrosion that is the subject of Section 1.3. 

A crystal may be defined as an orderly three-dimensional array of atoms, and all metals are aggregates of more or less imperfect crystals. In considering the structure of metals, therefore, it is convenient to start with the arrangement of atoms in a perfect metal crystal and then to proceed to the imperfections which are always present in the crystal structure. 

X-ray diffraction studies on the structures of metal complexes in solution. H. Ohtaki, Rev. Inorg. Chem., 1982,4,103-177 (223). 

The analysis in this chapter has shown that during the past 10-15 years there have been only marginal modifications in our understanding of the structure of metal/solution interfaces based on the potential of zero charge. The general picture for the relative behavior of the various metals seems well established. In particular, new, more reliable data, where available, have confirmed trends already identifiable in a more ambiguous situation. 

In recent years it has become clear that the structure of metals and alloys may be described in terms of covalent bonds that resonate among the alternative interatomic positions in the metals, and that this resonance is of greater importance for metals than for substances of any Other class, including the aromatic hydrocarbons. Moreover, the phenomenon of metallic resonance of the valency bonds must be given explicit consideration in the discussion of metallic valency it is necessary in deducing the metallic valency from the number of available electrons and bond orbitals to assign to one orbital a special r le in the metallic resonance. 

The indication from interatomic distances that less than 4 bonding electrons per atom are operating in white tin has been recognized by W. Hume-Rothery, The Structure of Metals and Alloys/ The Institute of Metals Monograph and Report Series No. 1, p. 26... 

In the course of the further investigation of resonating valence bonds in metals the nature and significance of this previously puzzling unstable orbital have been discovered, and it has become possible to formulate a rational theory of metallic valence and of the structure of metals and intermetallic compounds. 

The task of predicting a reasonable structure for this alloy was carried out with no information about the powder X-ray diffraction pattern except that one group of investigators had said that it could not be indexed by any Bravais lattice. The prediction of the structure was made entirely on the basis of knowledge of the effective radii of metal atoms and the principles determining the structure of metals and intermetallic compounds. 

The structure of metallic deposits is determined primarily by the size, shape (faceting), type of arrangement, and mutual orientation of the crystallites. Two factors may influence the orientation and spatial alignment of the microcrystals in electrocrystallization the field direction (or direction of the electric current) and the nature of the substrate. The deposits are said to have texture when the crystallites are highly oriented in certain directions. Epitaxy implies that the lattice is altered under the influence of the substrate. 

Different ways of the structural classification of deposits exist. In one system, the following structures are distinguished arbitrarily (1) fine-crystalline deposits lacking orientation, (2) coarse-crystalline deposits poorly oriented, (3) compact textured deposits oriented in field direction (prismatic deposits), and (4) isolated crystals with a predominant orientation in the field direction (friable deposits, dendrites). The structure of metal deposits depends on a large number of factors solution composition, the impurities present in the solntion, the current density, surface pretreatment, and so on. 

The above effects are more familiar than direct contributions of the metal s components to the properties of the interface. In this chapter, we are primarily interested in the latter these contribute to M(S). The two quantities M(S) and S(M) (or 8% and S m) are easily distinguished theoretically, as the contributions to the potential difference of polarizable components of the metal and solution phases, but apparently cannot be measured individually without adducing the results of calculations or theoretical arguments. A model for the interface which ignores one of these contributions to A V may, suitably parameterized, account for experimental data, but this does not prove that the neglected contribution is not important in reality. Of course, the tradition has been to neglect the metal s contribution to properties of the interface. Recently, however, it has been possible to use modern theories of the structure of metals and metal surfaces to calculate, or, at least, estimate reliably, xM(S) and 5 (as well as discuss 8 m, which enters some theories of the interface). It is this work, and its implications for our understanding of the electrochemical double layer, that we discuss in this chapter. 

Figure 8.4 The structure of metal-phthalocyanine complexes, with increased complexity adapting to the requirement of different metal ionic species.

Flinn, R. A., and Trojan, P. K. (1981). Engineering Materials and Their Applications, 2nd ed. Houghton Mifflin, Boston. Chapters 2, 5, and 6. This book presents an excellent discussion of the structures of metals and the properties of alloys. 

The atomic structure of the nuclei of metal deposits, which have the simplest form since they involve only one atomic species, appear to be quite different from those of the bulk metals. The structures of metals fall mainly into three classes. In the face-centred cubic and the hexagonal structures each atom has 12 co-ordination with six neighbours in the plane. The repeat patterns obtained by laying one plane over another in the closest fit have two alternative arrangements. In the hexagonal structure the repeat pattern is A-B-A-B etc., whereas in the face-centred cubic structure the repeat pattern is A-B-C-A-B-C. In the body-centred cubic structure in which each atom is eight co-ordinated, the repeat pattern is A-B-A-B. (See Figure 1.4.)... 

While the structure of metals and metal surfaces belongs to solid state physics, a basic understanding is essential for many electrochemical processes, particularly those involving adsorption. A thorough treatment of this topic is beyond the scope of this book. Many metals that are used in electrochemistry (Au, Ag, Cu, Pt, Pd, Ir) have a face-centered cubic (fee) lattice, so we will consider this case in some detail. For other lattice structures we refer to the pertinent literature [1] and to Problem 1. 

The structures of metal-complex dyes, which must exhibit a high degree of stability during synthesis and application, is limited to certain elements in the first transition series, notably copper, chromium, iron, cobalt and nickel. The remaining members of the transition series form relatively unstable chelated complexes. The following description of the influence of electronic structure, however, is applicable to all members of the series. 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

Further studies are necessary to clarify how different treatments affect not only the size but also the structure of metal crystallites. 

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