Systematic structural study in praseodymium compressed in a neon pressure medium up to 185 GPa

Abstract

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal compressed in a soft pressure-transmitting medium at ambient temperature up to 185 GPa. We observe the previously reported high-pressure structural transition sequence up to 20 GPa and the coexistence of body-centered orthorhombic (bco) Pr and \ensuremath{\alpha}-U Pr from $\ensuremath{\sim}20$ up to $\ensuremath{\sim}38\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The \ensuremath{\alpha}-U structure of Pr is stable from 20 to 185 GPa, and no evidence of the proposed transition to a primitive orthorhombic phase $>147\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ was observed. With density functional theory (DFT), we calculated the lattice parameters and $y$ coordinate for \ensuremath{\alpha}-U Pr and found good agreement between our calculations and experimental measurements. The obtained DFT energies of the proposed primitive orthorhombic $(P{2}_{1}{2}_{1}{2}_{1})$ and the \ensuremath{\alpha}-uranium phases at $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ show that the \ensuremath{\alpha}-uranium phase is lower in energy. Hence, neither our experimental data nor our DFT results support the transition to a primitive orthorhombic phase $>150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. DFT suggests, however, that Pr may transform to the $P{2}_{1}{2}_{1}{2}_{1}$ phase above $\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We also compare the axial ratios and lattice parameters of praseodymium to \ensuremath{\alpha}-uranium structured Nd, Ce, and U.