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 α-U Pr from ∼20 up to ∼38 GPa. The α-U structure of Pr is stable from 20 to 185 GPa, and no evidence of the proposed transition to a primitive orthorhombic phase > 147 GPa was observed. With density functional theory (DFT), we calculated the lattice parameters and y coordinate for α-U Pr and found good agreement between our calculations and experimental measurements. The obtained DFT energies of the proposed primitive orthorhombic (P2₁2₁2₁) and the α-uranium phases at ∼150 GPa show that the α-uranium phase is lower in energy. Hence, neither our experimental data nor our DFT results support the transition to a primitive orthorhombic phase > 150 GPa. DFT suggests, however, that Pr may transform to the P2₁2₁2₁ phase above ∼220 GPa. We also compare the axial ratios and lattice parameters of praseodymium to α-uranium structured Nd, Ce, and U.