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 ${\mathrm{Au}}_{82}{\mathrm{Si}}_{18}$ alloy above its melting point ${T}_{M}=359\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. 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 $371\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The crystalline phase observed for $T>371\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is stable up to at least $430\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Grazing-incidence x-ray diffraction data at $T>371\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ 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, $359\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}<T<371\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$.