Atomic Distribution in Molten Silver-Tin Alloy

Abstract

Molten silver-tin alloys have been studied by x-ray diffraction technique, using a focusing Θ--Θ diffractometer with Mo-Ka radiation (quartz crystal monochromator in the diffracted beam). The scattered intensities Icoheu in electron units and the interference function I (K)=Icoheu/ƒ2 being the scattering factor) have been measured in the range of K=4 π sin Θ/λ = 1.4 Å-1 to K = 15 Å~-1 for 6 different alloy compositions at temperatures 100 °C above the liquidus line. Four alloys (from 62% to 82% Ag) were measured at 825 ℃.The positions of the first maximum of Ieu or I(K) shift considerably with increasing silver content from K=2.25 Å-1 for pure Sn to K=2.64 Å-1 for pure Ag. The radial distribution function 4 π r2ϱ (r) and the probability function W (r) = ϱ (r)/ϱ0, where ϱ (r) and ϱ0 are the atomic and average atomic densities respectively, were obtained by FOURIER transform of I(K). The positions of the first maximum of 4 π r2ϱ (r), (i. e., r1) and of W(r), (i.e., r11) are a measure of the interatomic distances, and vary from r11 = 3.18 Å for Sn to r11= 2.86 Å for Ag. The plot of r11 vs. composition of the alloys shows a negative deviation from a linear law. A discontinuity appears in the curve in the range of 50-60% Ag. The coordination number Z increases rapidly on addition of Ag to Sn (Z = 8.5 for Sn) and reaches a value of about 11 beyond 60% Ag.