Binary systems intermediate compounds

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC>3 from the binary ones (BaO and CC>2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature,... 

Figure 2.22. Compound formation capability in binary systems. The different element combinations are mapped on Mendeleev number coordinates and those systems are indicated in which the formation of intermediate phases has been observed (either from the liquid or in the solid state). Blank boxes indicate systems for which no certain data are available. Notice that the compound-forming alloys are crowded in a region corresponding to a large difference in the Mendeleev numbers of the elements involved (for instance, basic metals with semi-metals).

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

The intermediate phases formed in the various binary systems have been represented, in a first approximation, as point compounds. The points, which in the different binaries correspond to phases having the same composition and structure, have then been connected, defining multi-component ternary stability fields (in this case, line fields). On each horizontal line of this multi-diagram triangle the same overall composition is found (the same Mg content and the same total... 

Figure 5.18. Ti, Zr, Hf binary alloys. Dashed boxes show selected metals which, at least at high temperature, form one field (and in some cases two, a and (3, fields) of continuous solid solutions with the 4th group metals. Notice that these metals are in a small region of the Periodic Table close to the 4th group. In the systems marked by an asterisk intermediate compounds are formed (with or without a continuous solid solution).

FIOURE 16.42 E hase diagram for the binary system La203-Nb205, containing four congruently melting intermediate compounds [114]. 

The binary tungsten-carbon system (Fig. 4.1) [4.13-4.15] is of high technical importance. It contains three intermediate compounds W2C(P), WCi ((y), and WC(5), the latter being the main constituent in most of the commercial cemented carbides (hardmetals see Chapter 9). Besides, ternary and even more complex compoimds exist which are of interest in alloyed steels and cemented carbides. Tungsten hexacarbonyl is an important precursor for organic synthesis. 

The binary phase diagram for MgO-Al203 is simpler than that for the Ca0-Al203 system (Fig. 2). There is only one stable intermediate compound that of the spinel phase (Mg2A104) [60]. Spinel melts at 2,105°C, but there is a eutectic at 1,995°C and a limited solid solution between stoichiometric spinel and MgO (periclase), up to 6wt% MgO, can be dissolved into the spinel structure without exsolution. This limited solid solution is an important property that is utilized in manufacture of spinels for use in reducing conditions [70]. 

Figure 17.26 Intermediate compound AB divides the binary system A-B into two similar parts. Note that if the composition axis were in mole fraction or mole %, AB would appear midway between A and B, but not if the axis Is In weight %.

The presence of and PO in silica also changes the pore shape. Supports of intermediate composition in the binary systems Si02 Al20 and Si02 AlP0 both conlmit open cylindrical and spherical pores while the pure compounds show only one pore type. 

Different polymorphs are just different crystalline forms of the same chemical compound having same chemical formula. Thus, they are all represented on the pure compound side of the phase diagram (Figure 3.21b). ContrarUy, solvates refer to the binary system of a compound and a solvent and, therefore, are represented as intermediate compounds between the pure compound and the solvent in the phase diagram (Figure 3.21a). Hence, they differ in chemical formula. Each solvate may have own polymorphic forms (of same chemical formula), which then again belong to the unary system of the intermediate compound. 

Figure 3.21 Solvates (a) and polymorphs (b) characterized by their solubility curves in water. The phase diagrams of Mg(N03)2/H20 and NH4NO3/H2O are depicted in (a) and (b), respectively. Solvates are indicated by vertical lines in the binary system, thus characterizing intermediate compounds that can melt congruently or incongruently. In the case shown, different hydrates of magnesium nitrate containing 2, 6, and 9 mol water in the crystal...

What is phase competition in the process of a solid-state reaction. Consider a diffusion couple of two mutually soluble components A/B, which diffuse into each other forming intermediate phases and, possibly, a solid solution, beta binary system A-B have three stable intermediate phases 1, 2, 3, a metastable compound 4, and an amorphous phase 5. The dependence of the Gibbs free energy on the composition of these phases is given in Figure 4.1. 

Figure 13.16 Binary system between pentadecanoic acid-heptadecanoic acid (adapted from ref. 39 intermediate non-stoichiometric compounds in grey, i.e. co-crystals)...

The phase diagrams of ternary systems are very useful for metallurgists and materials scientists. It is unfortunate that the phase diagrams of many ternary systems are not determined yet, so it is desirable to use thermodynamic method to calculate ternary phase diagrams based on the data of known relevant binary systems. The thermodynamic method used here, however, cannot confirm whether there is some ternary intermediate compound formed or not. Since it is well known that many ternary systems are ternary new phase formers, it is impossible to make a complete computerized prediction of a ternary phase diagram without the consideration of the possibility of the... 

Many binary systems consisting of halides of monovalent metals and halides of trivalent metals form intermediate compounds of MesMe Xe (X=F, Cl, Br, I) type. Some of them are congruently melting compounds and the others are incongruently melting compounds. Table 6.6 lists the melting types and relevant atomic parameters of 89 intermediate compounds of this valence type. 

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