High-pressure structural systematics in samarium up to 222 GPa

Abstract

Angle-dispersive x-ray powder diffraction experiments have been performed on samarium metal up to 222 GPa. Up to 50 GPa we observe the Sm type ($hR9$) $\ensuremath{\rightarrow}$ dhcp ($hP4$) $\ensuremath{\rightarrow}$ fcc ($cF4$) $\ensuremath{\rightarrow}$ distorted fcc ($hR24$) $\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}hP3$ transition sequence reported previously. The structure of the high-pressure phase above 93 GPa, previously reported as having a monoclinic structure with space group C2/m, is found to be orthorhombic, space group Fddd, with eight atoms per unit cell ($oF8$ in Pearson notation). This structure is the same as that found in Am, Cm, and Cf at high pressures. Analysis of samarium's equation of state reveals marked changes in compressibility in the $hP3$ and $oF8$ phases, with the compressibility of the $oF8$ phase being that of a ``regular'' metal.