CRYSTAL-STRUCTURE OF THE DISTORTED FCC HIGH-PRESSURE PHASE OF PRASEODYMIUM

Using synchrotron radiation and an imaging plate we have performed a h igh-angular-resolution powder x-ray diffraction study of the 'distorte d FCC' high-pressure phase of praseodymium. The crystal structure of t his phase is identified as the trigonal space group R3mBAR (D3d5) with eight atoms in the rhombohedral unit cell. The observed pressure depe ndence 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 co ndensation of a zone-boundary transverse phonon mode at the L point of the Brillouin zone drives this phase transition.