Crystalline Surface Phases of the Liquid Au-Si Eutectic Alloy

Abstract

A two-dimensional crystalline layer is found at the surface of the liquid eutectic Au82Si18 alloy above its melting point TM=359 °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 °C. The crystalline phase observed for T>371 °C is stable up to at least 430 °C. Grazing-incidence x-ray diffraction data at T>371 °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 °C<T<371 °C.