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 $\ensuremath{\gamma}$-to-$\ensuremath{\alpha}$ Ce transformation have been investigated at high pressures. The fusion curve exhibits a broad minimum located at about 33 kbar and 662\ifmmode^\circ\else\textdegree\fi{}C and has an initial slope of -4.7\ifmmode^\circ\else\textdegree\fi{}/kbar. The fcc-bcc boundary has an initial slope of -1.4\ifmmode^\circ\else\textdegree\fi{}/kbar and meets the fusion curve at a triple point near 26 kbar and 674\ifmmode^\circ\else\textdegree\fi{}C. The $\ensuremath{\gamma}$-to-$\ensuremath{\alpha}$ Ce boundary was delineated by following the resistance discontinuity and has a slope of 26.5\ifmmode^\circ\else\textdegree\fi{}/kbar. The discontinuous resistance drop associated with this transformation progressively diminishes at higher temperatures, and above 545\ifmmode^\circ\else\textdegree\fi{}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 $\ensuremath{\gamma}$ to $\ensuremath{\alpha}$ Ce ($4f\ensuremath{\rightarrow}5d$ electronic promotion) along the extrapolated transition line, and reflects a $P\ensuremath{-}V$ 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 $\ensuremath{\gamma}$-to-$\ensuremath{\alpha}$ Ce transformation at a critical point, first proposed by Ponyatovski\ifmmode \breve{\imath}\else \u{\i}\fi{}. The results of resistivity measurements suggest the coordinates 550\ifmmode^\circ\else\textdegree\fi{}K and 17.5 kbar for the critical point.