Solid solutions substitutional

This puts the 5.5% alloy into the single phase (a) field and all the Mg will dissolve in the A1 to give a random substitutional solid solution. 

When a pure metal A is alloyed with a small amount of element B, the result is ideally a homogeneous random mixture of the two atomic species A and B, which is known as a solid solution of in 4. The solute B atoms may take up either interstitial or substitutional positions with respect to the solvent atoms A, as illustrated in Figs. 20.37a and b, respectively. Interstitial solid solutions are only formed with solute atoms that are much smaller than the solvent atoms, as is obvious from Fig. 20.37a for the purpose of this section only three interstitial solid solutions are of importance, i.e. Fc-C, Fe-N and Fe-H. On the other hand, the solid solutions formed between two metals, as for example in Cu-Ag and Cu-Ni alloys, are always substitutional (Fig. 20.376). Occasionally, substitutional solid solutions are formed in which the... 

Fig. 20.37 (a) Interstitial solid solution, (b) random substitutional solution and (c) an ordered substitutional solid solution forming a superlattice... 

There are a number of differences between interstitial and substitutional solid solutions, one of the most important of which is the mechanism by which diffusion occurs. In substitutional solid solutions diffusion occurs by the vacancy mechanism already discussed. Since the vacancy concentration and the frequency of vacancy jumps are very low at ambient temperatures, diffusion in substitutional solid solutions is usually negligible at room temperature and only becomes appreciable at temperatures above about 0.5T where is the melting point of the solvent metal (K). In interstitial solid solutions, however, diffusion of the solute atoms occurs by jumps between adjacent interstitial positions. This is a much lower energy process which does not involve vacancies and it therefore occurs at much lower temperatures. Thus hydrogen is mobile in steel at room temperature, while carbon diffuses quite rapidly in steel at temperatures above about 370 K. 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

As noted, light-emitting diodes can be used to illustrate a variety of basic chemical concepts. Substitutional solid solutions like GaAsJPj (0 < x < 1) effectively extend the periodic table by providing a tunable band gap, which translates to tunability in the color of emitted light (4). 

Many alloys are substitutional solid solutions, well-studied examples being copper-gold and copper-nickel. In both of these examples, the alloy has the same crystal structure as both parent phases, and the metal atoms simply substitute at random over the available metal atom sites (Fig. 4.4a). The species considered to be the defect is clearly dependent upon which atoms are in the minority. 

The likelihood of forming a substitutional solid solution between two metals depends upon a variety of chemical and physical properties. A large number of alloy systems were investigated by Hume-Rothery, in the first part of the last century, with the aim of understanding the principles that controlled alloy formation. His findings with respect to substitutional solid solution formation are summarized in... 

Figure 4.4 (100) planes of substitutional solid solutions (a) substitutional alloys, typified by Ni-Cu and (b) inorganic oxides, typified by MgO-NiO. 

In multi-component systems, elements preferentially occupying the same site are generally allowed to substitute each other forming more or less extended substitutional solid solutions. Similar considerations have been found to be valid for the hP12-MgZn2 and hP24-Ni2Mg structures. 

Substitutional solid solution with a major component. The trace element substitutes for a major element in a regular position of the crystal lattice. interstitial solid solution. Similar to the preceding phenomenon, but here the trace element occupies an interstitial position in the crystal lattice. 

As outlined in section 10.1, the presence of trace elements in crystals is attributable to several processes, the most important one being the formation of substitutional solid solutions. The ease of substitution depends on the magnitude of interactions between trace element and carrier. We have already seen (section 3.8.4) that macroscopic interaction parameter W can be related to microscopic interactions in a regular solution of the zeroth principle ... 

The strain energy term is thus assumed to be independent of crystal structure for the three major types of substitutional solid solutions. 

Since many metal hexacyanometalates have very similar structures, the formation of solid solutions is possible. The positions and can be occupied not only by one kind of ion, but by a variety of similar ions, allowing the existence of substitutional solid solutions with a random distribution of the different ions on their specific positions. Table 2 gives an overview of the types of solid solutions that can be formed and some examples. 

In interstitial compounds, however, the nonmetal is conveniently regarded as neutral atoms inserted into the interstices of the expanded lattice of the elemental metal. Obviously, this is an oversimplification, as the electrons of the nonmetal atoms must interact with the modified valence and conduction bands of the metal host, but this crude picture is adequate for our purposes. On this basis, Hagg made the empirical observation that insertion is possible when the atomic radius of the nonmetal is not greater than 0.59 times the atomic radius of the host metal—there is no simple geometrical justification for this, however, as the metal lattice is concomitantly expanded by an unknown amount. These interstitial compounds are sometimes called Hagg compounds.9,10 They are, in effect, interstitial solid solutions of the nonmetal in the metal (as distinct from substitutional solid solutions, in which actual lattice atoms are replaced, as in the case of gold-copper and other alloys Section 4.3). 

R.W. Balluffi and B.H. Alexander. Development of porosity during diffusion in substitutional solid solutions. J. Appl. Phys., 23(11) 1237—1244, 1952. 

Azulene has weak absorption in the visible region (near 7000 A) and more intense band systems in the ultraviolet. The first ultraviolet system, which commences at about 3500 A, has been examined in substitutional solid solution in naphthalene (Sidman and McClure, 1956) and in the vapour state (Hunt and Ross, 1962), and can be observed in fluorescence from the vapour (Hunt and Ross, 1956). Theory predicts that the transition is 1Al<-lAl(C2K), i.e. allowed by the electronic selection rules with polarization parallel to the twofold symmetry axis (see, e.g., Ham, 1960 Mofifitt, 1954 Pariser, 1956b). The vibrational analysis shows that the transition is allowed but does not establish the axis of polarization. The intensity distribution among the vibrational bands indicates a small increase in CC bond distance without change in symmetry. 

The alloys just considered are substitutional solid solutions. Interstitial solid solutions are alloys with small atoms, for example, H, C, N, and O, in the interstitial sites, usually O and T sites. Some alloys have random distribution (disordered) if the melt is quenched but become ordered if heated and annealed or if cooled slowly. An example is the 1 1 alloy CuAu. The disordered structure is ccp, and the ordered structure is also ccp, except alternate layers parallel to a cell face contain Cu or Au. 

To demonstrate the viability of the synthesis and characterization approach in the present workflow, two binary Pt-Fe alloy libraries were designed, synthesized and characterized by XRD (Figs. 11.4 to 11.6) [19]. One library (Fig. 11.5) was characterized as synthesized, while the other (Fig. 11.6) was annealed at 400 °C for 12 h in a hydrogen/argon atmosphere. Pt-Fe is a well-known binary alloy system, exhibiting both substitutional solid solution compositional ranges and intermetal-lic compounds. 

As the amount of Fe is increased, the (111) peak shifts to smaller d-spacings, reflecting a contraction of the lattice. The (111) peak positions in Fig. 11.5 show a continuous shift from pure Pt to pure Fe. The Pt-Fe XRD patterns are consistent with a single-phase, substitutional solid solution (disordered alloy) over the entire compositional range. In contrast, Fig. 11.6 clearly displays diffraction from inter-metallic compounds of lower symmetry. Post-deposition annealing has resulted in an ordering of the Pt and Fe atoms, the effect of which is the crystallization of an ordered metal alloy of lower symmetry than 100% Pt. In essence, the applied vacuum deposition method is ideally suited for the preparation of multi-component,... 

Fig. 11.5 Eight XRD patterns taken along row C of the library in Fig. 11.4 with no post-deposition annealing. The XRD patterns of the synthesized alloys are consistent with a substitutional solid solution with a face-centered cubic (fee) structure. The black dots indicate diffraction from the fee (111) plane. The thin vertical lines indicate the expected positions of the diffraction peaks of 100% Pt.

M.A. Oliveira, M.L. Peterson, D. Klein, Continuously substituted solid solutions of organic cocrystals, Cryst. Growth Des. 8 (2008) 4487-4493. 

Ternary compounds have been observed for the R-Si, Ge, Sn, Ga, In, Se, Te-Sb systems. The systems with arsenic and bismuth are characterized by formation of substitutional solid solutions between isotypic binary pnictides. No compounds have been found in the partly investigated ternary La-Al-Sb system at 773 K (Muravjova, 1971). For the Yb-Al-Sb, the formation and crystal structure of the Yb AlSbn have been reported (Fisher et al., 2000). 

There are two kinds of solid solutions, namely, substitutional solid solutions and interstitial solid solutions. In substitutional solid solutions, the solute element occupies a position of one of the solvent elements in the solvent crystal. In interstitial solid solutions, on the other hand, the solute element occupies on of the vacant spaces between solvent elements in the solvent crystal lattice without displacing a solvent element. 

In Chapter 8, the simple case of totally immiscible solids, exhibiting a minimum melting eutectic, was discussed. There are a variety of other behaviors that can be demonstrated in solid-liquid equilibria. For example, a solid solution may be formed. In a solid solution, the arrangement of atoms shows some degree of randomness on the molecular level. This occurs in a substitutional solid solution, where the components are very similar and can substitute for each other in the solid lattice. Although the lattice is regular, which atoms in the lattice are substituted is random. (If the substitution were periodic, the system would be a compound.) Copper and nickel illustrate this behavior and form a substitutional solid solution at all concentrations. Another type of solid solution is an interstitial... 

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