Crystal grain growth at the α-uranium phase transformation in praseodymium -: art. no. 012108

Structural phase transformations under pressure are examined in praseodymium metal for the range 0-40 GPa at ambient temperature. Pressure was generated with a diamond-anvil cell, and data were collected using high-resolution synchrotron x-ray diffraction and the image plate technique. The structural sequence double hexagonal close packed (dhcp)-->face centered cubic (fcc)-->distorted-fcc (d-fcc)-->alpha-uranium (alpha-U) is observed with increasing pressure. Rietveld refinement of all crystallographic phases provided confirmation of the hR24 structure for the d-fcc phase while the previously reported monoclinic phase between the d-fcc and the alpha-U phase was not confirmed. We observe dramatic crystal grain growth during the volume collapse concurrent with the symmetry-lowering transition to the alpha-U structure. No preferred orientation axis is observed, and the formation process for these large grains is expected to be via a nucleation and growth mechanism. An analogous effect in rare earth metal cerium suggests that the grain growth during transformation to the alpha-U structure is a common occurrence in f-electron metals at high pressures.