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 I coh eu in electron units and the interference function I (K)=I coh eu /ƒ 2 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 I eu 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 r 2 ϱ (r) and the probability function W (r) = ϱ (r)/ϱ 0 , where ϱ (r) and ϱ 0 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 r 2 ϱ (r), (i. e., r 1 ) and of W(r), (i.e., r 1 1 ) are a measure of the interatomic distances, and vary from r 1 1 = 3.18 Å for Sn to r 1 1 = 2.86 Å for Ag. The plot of r 1 1 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.