Metastable phases and compounds in the ternary system Ag–Au–l

Abstract

The distribution of the connodes in the equilibrium phase diagram can reveal that there is a possibility of metastable phases within the system. In this paper this has been illustrated for the ternary Ag–Au–l system. It follows from vapour pressure data that up to 66 mol.% Au can dissolve in solid α-Agl. However, at temperatures > 525 K the pure compound Aul would assume the same crystallographic structure as α-Agl and could then form a continuous range of mixed AgAul crystals. The metastable α-AgAul phase is stabilized by Ag-atoms. It is expected to have the metal atoms in a molten sublattice, similar to α-Agl. Below 404 K the α-phase solid solution Ag_0.5Au_0.5l transforms into a stoichiometric compound AgAul₂. The thermodynamic data on this transformation reveal a strongly negative change in entropy. This would be expected for the transition from the highly disordered α-phase into a compound with different metal atoms in specific lattice sites.