Symmetry lowering under high pressure:: Structural evidence for f-shell delocalization in heavy rare earth metal terbium

Heavy rare earth metal terbium has been studied at synchrotron sources using both angular and energy dispersive x-ray diffraction to a maximum pressure of 155 GPa (volume fraction V/V-0=0.39). The complete regular trivalent rare earth structural sequence, hcp ->alpha-Sm -> double hcp -> fcc -> distorted fcc (hR24), is observed at low pressures except for the undistorted fcc. At 51 +/- 2 GPa, terbium undergoes a 5% volume collapse transitioning to C2/m monoclinic. The open, low symmetry structure is reminiscent of the 4f delocalization phenomenon in the light rare earth metals, particularly the alpha('') phase of cerium.