Alloys electron compounds

Hume-Rothery s rule The statement that the phase of many alloys is determined by the ratio.s of total valency electrons to the number of atoms in the empirical formula. See electron compounds. 

R. Behrisch, ed., "Sputtering by Particle Bombardment II Sputtering of Alloys and Compound, Electron and Neutron Sputtering, Surface Topography," in Topics in Applied Physics, Vol. 52, Springer-Vedag, Berlin, 1983. 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

We shall first review the basic principles of VASP and than describe exemplary applications to alloys and compounds (a) the calculation of the elastic and dynamic properties of a metallic compound (CoSi2), (b) the surface reconstruction of a semiconducting compound (SiC), and (c) the calculation of the structural and electronic properties of K Sbi-j, Zintl-phases in the licpiid state. 

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. 

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. 

As a general rule, the 4/ spectrum of a rare earth in an alloy or compound is identical to that of the pure metal except for a shift in energy. SmCog, however, displays behavior not in accord with this rule. Its spectrum and those of Co and Sm metals (48) are shown in Fig. 21 Sm metal here was measured at resolution lower than that of the satellite line experiments. In Co and in the compound the conduction bands extend to — 4 eV. In SmCos the intensity of the 4/ spectrum is relatively low because the ratio of the number of 4/ to conduction electrons is only 5 48 while it is 5 3 in Sm. Although the quality of the SmCog spectrum does not allow a rigorous statement about the matter, it does appear that the structure in the Sm metal 4/ spectrum is not present in the spectrum of the compound. Perhaps... 

Semi-metallic behavior is not confined to the elements, but is also found in alloys and compounds. When involved in chemical bonding, the metalloids again exhibit intermediate qualities. They are capable of taking electrons from most metals and will readily lose electrons to most nonmetals. Their electronegativity values are also mid-range. Consequently, it is unlikely for them to be involved in ionic bonding when found in compounds, they will usually establish covalent bonding. 

Certain other alloys also with the body-centred cubic structure, LiHg, MgTl, etc., which have already been mentioned, have sometimes been regarded as exceptions to the Hume-Rothery rules. They fall into line with the other P structures only if we assume that Hg or T1 provides one valence electron. Since, however, the radii of the metal atoms in these alloys, as in those with the NaTl structure, are smaller than the normal values, it is probably preferable not to regard these as 3 electron compounds. 

The A2 metals and the elements of the earlier B subgroups (Bj metals) form the electron compounds already discussed. With the metals of the later B subgroups the A2 metals, like the Aj, tend to form intermetallic phases more akin to simple homopolar compounds, with structures quite different from those of the pure metals. The nickel arsenide structure has, like typical alloys, the property of taking up in solid solution a considerable excess of the transition metal. From Table 29.12... 

In the past few decades, research in the electrochemical and chemical deposition of metals, alloys, and compounds has brought about significant achievements that are important for the practical applications. The research in this area was related and/or supported with the developments in electronics, aerospace, automotive, energy conversion, and biomedical industries. 

If you can locate an element on the periodic table, you can predict its properties. Each element has imique characteristics because of its unique electron configuration. Together, the elements, their alloys, and compounds provide a wide variety of materials for coimtless applications. Compoimds of the elements range from ionic to covalent, from polar to nonpolar. They have size and shape. In Chapter 9, youU learn more about the formation of compounds and how to predict their shape and polarity. 

On the basis of this classification of the metallic elements we may consider four types of alloy containing a true metal and a B sub-group element, namely T1-Bv 7 -52, T2-B1 and T2-B2. We discuss these in turn, but systems of the type T2-B1 have been more widely studied than the others and it is therefore convenient to consider them first. For reasons which will appear later, they may be termed electron compounds. 

The simpler of these two structures is the caesium chloride arrangement, found in the phases LiHg, LiTl, MgTl, CaTl and SrTl. This is, of course, also the structure of the / phase in the silver-cadmium system and in other electron compounds (fig. 13.11), and for this reason the systems just mentioned are sometimes quoted as exceptions to Hume-Rothery s rule. Apart from this geometrical resemblance, however, these systems have little in common with the electron compounds, and it seems preferable to regard the Hume-Rothery rule as applicable only to alloys of the T2-B1 type. 

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,... 

PiG. 3.7.—Results of X-Rav Examination of Silver-Cadmium Alloys (Westgren), Showing Appearance of Xew Diffraction Pattern with Each New Electron Compound Phase. (Compare Cu-Zn Alloys, Pig. 34 (6)). 

The widespread use of metals for different kinds of application (e.g., pigments, coatings, alloys, electronic equipment) leads to the fact that some of the utilized metals (or their compounds) end up in wastes. Metals in wastes can cause severe environmental impacts, particularly with respect to ground-water pollution. 

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