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

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (alpha-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the a-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 alpha-U structure to a primitive orthorhombic P2(1)2(1)2(1) 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 alpha-U phase in a variety of f-band systems at high pressures.