X-ray reflectivity studies of atomic-level surface-segregation in a liquid eutectic alloy of AuSn

X-ray reflectivity studies reveal atomic-level surface-segregation at the free surface of the eutectic ${\mathrm{Au}}_{71}{\mathrm{Sn}}_{29}$ liquid alloy. The surface-segregation extends up to three layers, in which the top layer is almost a pure monolayer of Sn, the second layer is almost a pure monolayer of Au and the third layer appears to be slightly enhanced in Au. Although the surface-segregation concentration profiles can be qualitatively accounted for by the theories of Defay-Prigogine and Strohl-King, they cannot satisfactorily account for the measured surface tension.

Figure 1

Kinematics of the x-ray measurements from the sample. The incident and detected x-ray wave vectors are $k_{\mathrm{in}}$ and $k_{\mathrm{out}}$, respectively.

Figure 2

Off-specular diffuse scattering from the liquid surface for the purpose of measuring the surface tension. The peak corresponds to the specular condition $\beta =5.07°$ $(q_z=1.1{\mathrm{\AA }}^{-1})$. The profile calculated from the model is shown for the best fit value of surface tension $\gamma =615\mathrm{mN}∕\mathrm{m}$ (—). The inset is an expanded version of the best fit and confidence limits at the specular peak. Calculations shown for $\gamma =600\mathrm{mN}∕\mathrm{m}$ (----), $\gamma =640\mathrm{mN}∕\mathrm{m}$ (–-–) indicate the confidence limits. The line for $\gamma =750\mathrm{mN}∕\mathrm{m}$ (-.-) illustrates the calculation for average of the published values.

Figure 3

The background subtracted x-ray reflectivity, $R∕R_F$ and the one calculated from the best fit of the electron density model given by Eq. (3). The dashed line is the fit from the simplest DC model that is discussed in the text. The inset illustrates the same data after division by $CW(q_z,T)$ along with the fits from the corresponding models.

Figure 4

The surface-normal electron density profile of the AuSn liquid alloy surface as obtained from the best fit to the density model, Eq. (3). of the reflectivity data. The atomic densities of Au and Sn that contribute to the total density are also shown.