Crystal grain growth at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>-uranium phase transformation in praseodymium

Abstract

Structural phase transformations under pressure are examined in praseodymium metal for the range $0--40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ 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)\ensuremath{\rightarrow}face centered cubic (fcc)\ensuremath{\rightarrow}distorted-fcc (d-fcc)\ensuremath{\rightarrow}$\ensuremath{\alpha}$-uranium $(\ensuremath{\alpha}\text{\penalty1000-\hskip0pt}\mathrm{U})$ 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 $\ensuremath{\alpha}\text{\penalty1000-\hskip0pt}\mathrm{U}$ phase was not confirmed. We observe dramatic crystal grain growth during the volume collapse concurrent with the symmetry-lowering transition to the $\ensuremath{\alpha}\text{\penalty1000-\hskip0pt}\mathrm{U}$ 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 $\ensuremath{\alpha}\text{\penalty1000-\hskip0pt}\mathrm{U}$ structure is a common occurrence in $f$-electron metals at high pressures.