Structure of the liquid-vapor interface of a dilute ternary alloy: Pb and Sn in Ga

Abstract

We report the results of grazing incidence x-ray diffraction and x-ray reflectivity studies of the liquid-vapor interfaces of three dilute ternary alloys, composed of a small constant amount of Pb (0.072 at. %) and a small variable amount of Sn (0.12, 3.4, and 7.7 at. %) dissolved in Ga. The solutes Pb and Sn compete to segregate in the liquid-vapor interfaces of these alloys; in each case, over the temperature range studied (30--120 \ifmmode^\circ\else\textdegree\fi{}C), the composition of the outermost layer of the interface is dominated by excess Pb. In the case of the alloy with Sn concentration of 0.12 at. %, the excess Pb in the liquid-vapor interface forms a crystalline monolayer that melts at 67.8 \ifmmode^\circ\else\textdegree\fi{}C. In the case of the alloy with Sn concentration of 3.4 at. %, the excess Pb in the liquid-vapor interface again forms a crystalline monolayer, but that crystalline monolayer melts at 40.5 \ifmmode^\circ\else\textdegree\fi{}C. In the case of the alloy with Sn concentration of 7.7 at. %, the excess Pb in the liquid-vapor interface forms a two-dimensional liquid over the entire accessible temperature range. When the excess Pb in the liquid-vapor interface forms a two-dimensional crystal, it fully covers the surface of the liquid; at higher temperatures, when the Pb monolayer is liquid, it only partially covers the liquid alloy surface, with the remainder covered by a partial monolayer of liquid Sn. Two-dimensional liquid Pb and two-dimensional liquid Sn appear to be immiscible under the conditions of our experiments. In the case of the sample with the smallest concentration of Sn, the correlation length obtained from the width of the diffraction peak drops almost discontinuously at 67.8 \ifmmode^\circ\else\textdegree\fi{}C to a smaller value. It then further decreases continuously until about 90 \ifmmode^\circ\else\textdegree\fi{}C, thereafter becoming constant. These data suggest the existence of an intermediate disordered phase stable between 67.8 and 90 \ifmmode^\circ\else\textdegree\fi{}C. Our experiments fall short of providing unambiguous evidence for the existence and character of this disordered phase. We speculate that between 67.8 and about 90 \ifmmode^\circ\else\textdegree\fi{}C the Pb monolayer has a hexatic structure and that above 90 \ifmmode^\circ\else\textdegree\fi{}C it is liquid, as is implied by the results of recent simulations of the temperature dependence of the structure of a quasi-two-dimensional layer of Pb.