Gold, mixed crystals with copper

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). 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

That this ratio of atoms to electrons is indeed determinative of the properties in many metallic systems is also seen from the solubility in the melt and likewise in the solid state. Zinc dissolves in the solid state in copper up to 38.4 % as a homogeneous mixed crystal (a-brass, the normal brass contains about 35 % zinc) almost the same limit of about 40 % holds for the combinations of the elements copper, silver and gold with zinc and cadmium. 

In some cases, however, the two metals do not form an unbroken series of solid solutions. In the case of zinc and silver, for example the addition of silver raises the freezing-point of the mixture, until a transition point is reached. This corresponds with curve I, Fig. 48. Silver and copper, and gold and copper, on the other hand, do not form unbroken series of mixed crystals, but the freezing-point curve exhibits an eutectic point, as in curve IL, Fig. 48. In these cases, liquid mixtures having the composition of the point C deposit, on freezing, a mixture of two solid solutions the composition of which is represented diagrammatically by the points D and E. 

A wide range of structural motifs are found for the copper-triad xanthates, ranging from monomeric species to layer structures. This variety of structures can be ascribed to the flexible coordination modes adopted by the xanthate ligands in this group, in particular for the copper and silver xanthates. However, there is a dearth of binary xanthate structures with only two available for copper and one each for silver and gold. The paucity of data can be ascribed directly to the inability to obtain suitable crystals for structure determination. Phosphine adducts are known for all three elements of this group and in the case of both copper and silver, mixed-metal species are also known. 

After mixing with limestone and coke the roasted ore is smelted in blast furnaces to yield crude lead. Major impurities are gold, silver, copper, antimony, arsenic and bismuth. At this stage the lead is hard, due to the arsenic and antimony present. The lead is now melted and kept molten at a temperature below the melting point of copper, whereby the copper present crystallizes and can be skimmed out. If air is now blown into the molten lead, arsenic and antimony float out as oxides, and after this process the lead is referred to as soft leadi. 

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