Phase transitions in praseodymium up to 23 GPa: An x-ray powder diffraction study

The lanthanide metal Pr has been studied by x-ray powder diffraction from 5 to 23 GPa with the aim of resolving the long-standing disagreements about the identity of the high-pressure phases, transitions, and structures of this element. We find that the distorted-fcc (d-fcc) phase observed above 7.4 GPa has the rhombohedral hR24 structure reported previously by Hamaya et al. and can definitely rule out the monoclinic structure proposed by other authors. At 13.7 GPa, the d-fcc phase starts to transform to another phase, Pr-VII, the structure of which is body-centered orthorhombic or a very slight distortion thereof. Pr-VII transforms to the alpha-uranium phase at 20.5(5) GPa. In addition to identifying the phase transitions and structures, we have also resolved many of the disagreements between previous diffraction studies and show that almost all previous work is in agreement with the present study. The likely existence of the Pr-VII phase in a second lanthanide, Nd, suggests that it might be an additional member of the sequence of phases observed across the lanthanide elements.