Crystal structure of the distorted FCC high-pressure phase of praseodymium

Abstract

Using synchrotron radiation and an imaging plate the authors have performed a high-angular-resolution powder X-ray diffraction study of the 'distorted FCC' high-pressure phase of praseodymium. The crystal structure of this phase is identified as the trigonal space group R3m (D3d5) with eight atoms in the rhombohedral unit cell. The observed pressure dependence of atomic positional parameters provides quantitative evidence of an occurrence of the continuous distortion of the FCC lattice. The observed atomic displacement pattern implies that the softening and condensation of a zone-boundary transverse phonon mode at the L point of the Brillouin zone drives this phase transition.