Crystal structure of the high pressure phases of bismuth Bi III and Bi III' by high energy synchrotron X-ray diffraction

This paper reports the results of a synchrotron X-ray diffraction study on the crystal structures of Bi III and Bi III' which have been known to form under high pressure but have, for a long time, been unsolved. Powdered samples were compressed in a cubic-type multi-anvil press, MAX80, and diffraction data were collected using an Imaging Plate with monochromatized radiation of an energy of 49.7 keV. It was possible to identify at 3.8 GPa forty-eight reflections for Bi III in the sin theta/lambda range from 1.6 nm(-1) to 5.6 nm(-1), which were indexed in terms of a tetragonal unit cell with a=0.8659 nm and c=0.4238 nm (Z=10). Analysis based on the observed intensities of the reflections led to a structure in which atoms form a distorted body-centered cubic lattice. It is of the same type as the structure of the high pressure phase of antimony Sb II. When pressure was increased across the suggested transition pressure 4.3 GPa between Bi III and Bi III' to 6.6 GPa, no change in the diffraction pattern was observed, indicating that there is no distinction between the two phases as long as the crystal structure is concerned. Discussion is given on the sequence of high pressure phase transitions in the Group Vb elements.