Crystalline surface phases of the liquid Au-Si eutectic alloy

A two-dimensional crystalline layer is found at the surface of the liquid eutectic Au₈₂Si₁₈ alloy above its melting point TM=3590.2em0ex^∘C. Underlying this crystalline layer, we find a layered structure, 6--7 atomic layers thick. This surface layer undergoes a first-order solid-solid phase transition occurring at 3710.2em0ex^∘C. The crystalline phase observed for T>3710.2em0ex^∘C is stable up to at least 4300.2em0ex^∘C. Grazing-incidence x-ray diffraction data at T>3710.2em0ex^∘C imply lateral order comprising two coexisting phases of different oblique unit cells, in stark contrast with the single phase with a rectangular unit cell found for low-temperature crystalline phase, 3590.2em0ex^∘C<T<3710.2em0ex^∘C.