Phase stability and cohesive properties of Au–Sn intermetallics: A first-principles study

The total energies and cohesive properties of 29 Au–Sn intermetallics (stable, metastable, and virtual) are calculated from first-principles density-functional theory (DFT) employing ultrasoft pseudopotentials (USPP) and both local-density approximation (LDA) and generalized gradient approximation (GGA) for the exchange-correlation functional. Among the intermetallics considered, the ground-state structures are found to be AuSn, AuSn₂, and AuSn₄. Another phase Au₅Sn, though present in the equilibrium diagram, lies slightly above the ground state convex hull. The formation energies of stable phases calculated using USPP–LDA and USPP–GGA are nearly the same. Except for AuSn, calorimetric data for enthalpies of formation show a good agreement with the calculated formation energies. Based on our first-principles results, it is argued that the structures of two metastable phases arecP52-type γ brass (isotypic with Al₄Cu₉) at Au–20.5 at.% Sn andhP1-type γ (isotypic with HgSn_6–10) at Sn–8 at.% Au. For the intermetallics considered in this study, we provide optimized values of lattice parameters and Wyckoff positions. The experimental lattice parameters show a better agreement with those calculated using USPP–LDA than with USPP–GGA. The results presented here form the basis for creating a reliable thermodynamic database to facilitate calculations of stable and metastable phase diagrams of binary and multicomponent systems containing Au and Sn, relevant to electronic packaging and many other joining applications.