Stability of the high-pressure monoclinic phases in Ce and Pr metals: Comparative diffraction study and phenomenological theory

Abstract

Pressure-induced structural phase transformations in rare-earth metals $\mathrm{Ce}$ and $\mathrm{Pr}$ were studied at different temperatures with an externally heated diamond anvil cell by angle dispersive x-ray diffraction using a synchrotron source. Evidence for the presence of the monoclinic $C2∕m$ $(Z=8)$ phase, $\mathrm{Pr}\text{\ensuremath{-}}\mathrm{VII}$, in the pressure range $10--25\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ has been observed. Theoretical models are proposed and the corresponding phase diagrams are worked out. They are compared to the experimental phase diagrams of cerium and praseodymium. The theory explains consistently the contradictory data on the stability of monoclinic and orthorhombic phases of $\mathrm{Ce}$.