High-pressure structure of praseodymium revisited: In search of a uniform structural phase sequence for the lanthanide elements

Abstract

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal to a pressure of 205 GPa. Between 20 and 165 GPa only the $oC4$ ($\ensuremath{\alpha}$-uranium) phase is observed, in agreement with previous studies. At 171(5) GPa we find a transition to a tetragonal $tI2$ phase which is isostructural with the high-pressure post-$oC4$ phase seen in the neighboring lanthanide cerium above 12 GPa, and with the high-pressure phase of the actinide thorium seen above 100 GPa. Electronic structure calculations determine the $oC4\ensuremath{\rightarrow}tI2$ transition to occur at 130 GPa at 0 K, but find another phase, with the $hP1$ (simple hexagonal) structure, to have a lower enthalpy than both the $oC4$ and $tI2$ structures above 20 GPa at 0 K.