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 Au${}_{82}$Si${}_{18}$ alloy above its melting point $T_M=359^{\circ }$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 $371^{\circ }$C. The crystalline phase observed for T$>$371 ${}^{\circ }$C is stable up to at least 430 ${}^{\circ }$C. Grazing Incidence X-ray Diffraction data at T$>$371 ${}^{\circ }$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 $359^{\circ }$C$<T<371^{\circ }$C.

10 pages, 8 figures

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