Trends in the formation capability of binary compounds

1 Trends in the formation capability of binary compounds. A few general comments about the formation of intermediate phases in binary alloy systems can be made by using maps similar to those previously employed in Fig. 2.8 in order to give a summary of the mutual solubility. 

In Figs. 2.22 and 2.23 all the binary combinations are mapped as a function of the Mendeleev numbers of the two elements involved. The compound formation capability is represented in Fig. 2.22 by means of a few codes, whereas in Fig. 2.23 an indication is given of the thermal stability of the intermediate phases. To this end, values correlated to the so-called Raynor Index (Raynor 1972, 1974) are coded in this figure. 

This index /R is defined as follows (for a generic phase or compound having the composition AXB  

The pairs of elements forming compounds have been mapped both those having at least one compound with /R 1 and those for which /R 1. An outline of the intermediate phase formation and behaviour may be obtained through a look at both maps. All the systems, for instance, shown in Figs. 2.1 (V-Mo, Cs-Rb, Sr-Ca), 2.9 (Ni-Au, Pt-Au, Ba-Ca), 2.11 (Au-Si, U-Cr) and 2.16 (Ga-Bi, Be-Ga, Na-Al) will be represented as non-compound systems in the map of Fig. 2.22. The system Au-Cu (Fig. 2.20(d)) corresponds to the formation of a certain number of intermediate phases (in Fig. 2.22) and low values of /R in the Fig. 2.23. The system Ca-Pb (Fig. 2.20(b)) on the contrary is represented by the existence of several intermediate phases and high values of /R, etc. 

As already suggested and discussed by several authors, such as Pettifor (1984, 1986), Villars etal. (1989), etc., these maps highlight a regular trend in the formation of binary compounds. In the lower-left corner of the map those systems are represented obtained by combinations among elements of the first four groups of the Periodic Table. 

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