Fusion curve of cerium to 70 kilobar and phenomena associated with supercritical behavior of fcc cerium

Abstract

The fusion curve, the fcc-bcc transformation, and the γ-to-α Ce transformation have been investigated at high pressures. The fusion curve exhibits a broad minimum located at about 33 kbar and 662°C and has an initial slope of -4.7°/kbar. The fcc-bcc boundary has an initial slope of -1.4°/kbar and meets the fusion curve at a triple point near 26 kbar and 674°C. The γ-to-α Ce boundary was delineated by following the resistance discontinuity and has a slope of 26.5°/kbar. The discontinuous resistance drop associated with this transformation progressively diminishes at higher temperatures, and above 545°K the resistivity is a smooth function of pressure. The fusion-curve minimum is the result of a rapid density increase in the neighboring fcc Ce with pressure, due to a continuous transition from γ-to-α Ce (4f→5d electronic promotion) along the extrapolated transition line, and reflects a P-V relationship in the solid typical of supercritical behavior. Thus the fusion as well as the resistivity data lend strong support to the termination of the γ-to-α Ce transformation at a critical point, first proposed by Ponyatovskii. The results of resistivity measurements suggest the coordinates 550°K and 17.5 kbar for the critical point.