Distortion of alpha-uranium structure in praseodymium metal to 311 GPA

Abstract

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V/V ₀ = 0.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P2₁2₁2₁ phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b-axis and the c-axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures.

Keywords:

• DAC
• Rare earth metals
• Phase transformations

Acknowledgements

We acknowledge support from the Division of Materials Research-Metals Program, National Science Foundation (NSF) under Grant No. DMR-0203779. This research was conducted (in part) at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the US Department of Energy, Division of Material Sciences and Division of Chemical Sciences.