Electrodeposition of Intermetallics of Ag, Cu and Au with Sn

Intermetallic phases obtained by electrodeposition can be different from those obtained by metallurgical methods. A typical case regards IB metals with tin. We studied the electrodeposition of silver, copper and gold alloys with tin from different baths, in order to understand the rules under which the different intermediate phases crystallize. In the case of the Ag-Sn system only one intermetallic phase is obtained from different baths, of approximate composition Ag₃Sn, interpreted as an hexagonal close packed structure with c/a different from the metallurgical stable phase. For Cu and Au with Sn more intermediate phases are obtained: of these one is rich in Cu or Au, and may be attributed to an HCP phase; near equiatomic composition, a phase of the NiAs type is obtained, similar to the well known NiSn intermetallic phase; in the case of gold more intermetallics rich in tin are also obtained. The different behavior of these system is simply related to the sign of the enthalpy change for alloy formation. The differences from metallurgical system are related to the disappearing influence of long range order and to the vanishing effect of atomic size factor for electrochemical systems, giving as a result phases of higher symmetry.