Copper intermetallic compound, with gold

Figure4.11 Comparison of experimental dissociation energies of gold intermetallic compounds with those of copper and silver from mass spectroscopic measurements by Cingerich and coworkers [18, 159, 173, 176] arranged in group order according to the periodic table.

Two metals that are chemically related and that have atoms of nearly the same size form disordered alloys with each other. Silver and gold, both crystallizing with cubic closest-packing, have atoms of nearly equal size (radii 144.4 and 144.2 pm). They form solid solutions (mixed crystals) of arbitrary composition in which the silver and the gold atoms randomly occupy the positions of the sphere packing. Related metals, especially from the same group of the periodic table, generally form solid solutions which have any composition if their atomic radii do not differ by more than approximately 15% for example Mo +W, K + Rb, K + Cs, but not Na + Cs. If the elements are less similar, there may be a limited miscibility as in the case of, for example, Zn in Cu (amount-of-substance fraction of Zn maximally 38.4%) and Cu in Zn (maximally 2.3% Cu) copper and zinc additionally form intermetallic compounds (cf. Section 15.4). 

In the Parkes desilvering process, 1—2% zinc is added to molten lead where it reacts with any gold, silver, and copper to form intermetallic compounds which float as cmsts or dross that is skimmed (see Lead and LEAD ALLOYS). 

Both silver and gold form ideal solid solutions with palladium. However, stoichiometric compositions with unique properties, such as in Hunter s preferred membrane composition of PdsAg, [31], might suggest the possibility of intermetallic compounds or ordered structures differing from that of the ideal solutions [35]. Palladium and copper also form ideal solid solutions, but in this system phase diagrams clearly show additional phases with crystal structures differing from the parent fee phase of the solid solutions. 

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