The fusion curve, the fcc-bcc transformation, and the $\gamma$-to-$\alpha$ 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 $\gamma$-to-$\alpha$ 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 $\gamma$ to $\alpha$ Ce ($4f\to 5d$ 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 $\gamma$-to-$\alpha$ Ce transformation at a critical point, first proposed by Ponyatovskiĭ. The results of resistivity measurements suggest the coordinates 550°K and 17.5 kbar for the critical point.

• Received 3 August 1964

DOI:https://doi.org/10.1103/PhysRev.137.A179