Interatomic distances in liquid metals and alloys

Abstract

X-ray diffraction patterns of liquid tin and liquid mercury are unlike those of simple liquid metals such as Na, K, Cu and Au in that the intensity patterns show considerable asymmetry, or a distinct shoulder, on the high angle side of the main peak. The position of the main peak gives the average interatomic distance in the melt; for Sn and Hg the additional detail of the pattern gives a special close distance of approach. It is shown that this distance is related to that occurring in the low temperature allotropes of the solid metals. In diffraction patterns of liquids of the Au-Sn alloy system the main intensity peak is resolved into two distinct maxima. One component occurs at a position corresponding to a simple random mixture of the component elements, the other to a close interatomic distance. The relative heights of the peaks are composition dependent. The close-packed component persists across the diagram, being strongest at composition Au₃Sn and decreasing to a shoulder at pure Sn. As for Sn and Hg the short distance can be correlated with a short distance occurring in the crystalline solids of the system. A similar scheme may be applied to the systems Cu-Sn, Au-Ga and Au-In where double-headed diffraction patterns have been observed in the liquids. It is postulated that the origin of the close distance occurring in the liquid in each system is the same as that of the strong bond existing in the crystalline compounds.