High-Pressure Diffraction Studies of Rubidium Phase IV
Lundegaard, Lars Fahl
Rb-IV is the stable high-pressure phase of rubidium between 16 and 21 GPa. The structure of Rb-IV has long been known to be complex, but it is only recently that it has been solved as being an incommensurate host-guest composite structure, comprising a tetragonal host framework containing chains of "guest" atoms that form structures incommensurate with the host along their common c- axis. While similar composite structures have been observed in a number of other elemental metals, Rb-IV is unique in that on pressure decrease below 16.7 GPa at 300 K, the chains of guest atoms become disordered and liquid-like. This thesis is a detailed structural study of Rb-IV. High-pressure, combined with high-temperature powder diffraction techniques, have been used to map the P-T phase diagram of rubidium between 15 GPa and 20 GPa and between 298 K and 600 K. The results show that the guest order-disorder transition pressure is strongly temperature dependent, and that the disordered phase is observed to the highest temperatures. Technical developments, which have made it possible to extract reliable modulation reflection intensities from a Rb-IV single crystal, are described. The resulting data are used for a full modulated structure refinement of Rb-IV, revealing a saw-tooth shaped modulation of the guest structure, from which new information on the host-guest interactions has been extracted. Inelastic X-ray scattering techniques have been used to measure the longitudinal acoustic (LA) phonons in a Rb-IV single crystal. Two LA-like phonon branches, one for each of the two composite subsystems, are observed along the common c-axis. The sound velocities in the host and guest structures are determined and the pressure dependence is shown to differ by a factor of two. Finally, developments that will enable future combined high-pressure high- temperature single-crystal diffraction studies, and single-crystal diffraction studies at pressures above 100 GPa, will be presented.