Anomalous behavior of liquid K–Pb alloys: Excess stability, entropy, and heat capacity

Abstract

Electromotive force measurements of the potassium activity in liquid K–Pb alloys are reported as a function of composition for T=640, 723, and 879 K and also as a function of temperature for compositions ranging from 2–70 at.% potassium. Thermodynamic mixing properties derived from these measurements indicate anomalous behavior around the equiatomic composition. The Darken excess stability function (or its equivalent the Bhatia–Thornton concentration fluctuations) shows only one well-defined extremum, which occurs at XK=0.52. Contrary to the case of both the Li–Pb and Na–Pb systems, no peak is detected in the excess stability function of K–Pb at the composition corresponding to A4Pb, where A refers to the alkali metal. The variations of the total entropy of mixing with composition exhibit features characteristic of ordered solutions with a sharp negative minimum at about 52 at. % potassium. These results are in consonance with published measurements of electrical conductivity and its temperature coefficient which indicate ordering in the liquid phase around the potassium–lead equiatomic composition. At this composition, in a temperature interval close to the melting point of the corresponding compound K⋅Pb, the composition dependence of the changes in the average heat capacity upon mixing goes through a maximum with a magnitude of about 62 J mol−1 K−1, which is atypical of metallic systems. This unexpected result provides a surprising contrast to prior work on other alkali–lead alloys.