Metastable phase formation in the Au-Si system via ultrafast nanocalorimetry

We have investigated the stability and solidification of nanometer size Au-Si droplets using an ultrafast heating/cooling nanocalorimetry and in situ growth techniques. The liquid can be supercooled to very low temperatures for both Au-rich (ΔT∼95K) and Si-rich (ΔT∼220K) samples. Solidification of a unique metastable phase δ 1 is observed with a composition of 74±4 at. % Au and a b -centered orthorhombic structure ( a =0.92, b =0.72, and c =1.35nm; body-center in the a-c plane), which grows heteroepitaxially to Au s . Its melting temperature T m is 305±5°C. There is competition during formation between the eutectic and δ 1 phases but δ 1 is the only metastable alloy observed. For small size droplets, both the δ 1 and eutectic phases show considerable depression of the melting point (size-dependent melting).