Strucural Phase Transitions in Praeseodymium to 205 GPa*

On compression, the trivalent lanthanides undergo a common sequence of structural transitions to phases comprising different stackings of hcp layers, with transition pressures that increase as Z increases across the series. Further compression results in transitions to "volume-collapsed" phases, typically with hexagonal (hP3 in Pearson notation), or orthorhombic (oF8, oF16. oC4) structures. oF8 and oC4 are also found in Pu and U, respectively, at ambient pressure, and also in Am, Cm and Cf at high pressure. Pr is the lowest-Z lanthanide in which the oC4 phase is observed, from 20 GPa to 147GPa, above which it is reported to transform to a distorted-oC4 structure, space group P2₁2₁2₁ (oP4), different to the tetragonal post-oC4 structure (tI2) reported in neighbouring Ce, and the distorted-oC4 structures, space group Pnma, reported in Am, Cm and Cf. As the trivalent lanthanide metals are expected to continue exhibiting a common transition sequence at pressures higher than they have been studied to date (typically 150-300 GPa), confirming the existence and structure of the oP4 phase in Pr is important, as it is currently seen only in that element. In this presentation we will describe the results of recent x-ray diffraction studies of Pr to 205 GPa, which focussed on determining its structural behaviour above 150 GPa.