Anomalous thermal contraction of the first coordination shell in metallic alloy liquids

Except for a few anomalous solids and liquids, materials expand upon heating. For liquids, this should be reflected as a shift in the peak positions in the pair correlation function, g(r), to higher r. Here, we present the results of a detailed study of the volume thermal expansion coefficients and the temperature dependences of g(r) for a large number of binary, ternary, and quaternary liquids in the equilibrium and supercooled (metastable liquid below the liquidus temperature) states. The data were obtained from x-ray scattering and volume measurements on levitated liquids using the electrostatic levitation technique. Although the volumes of all liquids expand with increasing temperature, the peak positions in g(r) for the first coordination shells contract for the majority of alloy liquids studied. The second and third peaks in g(r) expand, but at rates different from those expected from the volume expansion. This behavior is explained qualitatively in terms of changes in the coordination numbers and bond-lengths as clusters in liquids break up with increasing temperature.