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 T-M=359 degrees 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 371 degrees C. The crystalline phase observed for T>371 degrees C is stable up to at least 430 degrees C. Grazing-incidence x-ray diffraction data at T>371 degrees 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, 359 degrees C < T < 371 degrees C.