Atomic-Scale Surface Demixing in a Eutectic Liquid BiSn Alloy

Resonant x-ray reflectivity of the surface of the liquid phase of the ${\mathrm{Bi}}_{43}{\mathrm{Sn}}_{57}$ eutectic alloy reveals atomic-scale demixing extending over three near-surface atomic layers. Because of the absence of an underlying atomic lattice which typically defines adsorption in crystalline alloys, studies of adsorption in liquid alloys provide unique insight on interatomic interactions at the surface. The observed composition modulation could be accounted for quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of the single-layer Gibbs model, revealing a near-surface domination of the attractive Bi-Sn interaction over the entropy.

Figure 1

Dispersion corrections (a) $f^{\prime }(E)$ and (b) $f^{\prime \prime }(E)$ of Bi near the L3 absorption edge at $E_{\mathrm{L}3}=13.418\mathrm{keV}$. The right scale of (b) is the electron density contrast $\Delta Z^{\prime }/Z_{\infty }^{\prime }=(Z_{\mathrm{Bi}}^{\prime }-Z_{\mathrm{Sn}}^{\prime })/Z_{\infty }^{\prime }$.

Figure 2

XR measured at the indicated energies, with fits by the three-layer model (lines). The dashed line is the XR of a uniform-composition surface. Inset: The $E=12.00\mathrm{keV}$ measured $R/R_F$ with fits by the three models discussed in the text (lines). Error bars are smaller than the symbols’ size.

Figure 3

Electron density profiles as derived from the fits to the reflectivities shown in Fig. 2. Inset: the bulk-normalized differences in electron density of Bi and Sn, $({\rho }_{\mathrm{Bi}}-{\rho }_{\mathrm{Sn}})/{\rho }_{\infty }$.