title: Local structure of solid Rb at megabar pressures.
abstract: We have investigated the local and electronic structure of solid rubidium by means of x-ray absorption spectroscopy up to 101.0 GPa, thus doubling the maximum investigated experimental pressure. This study confirms the predicted stability of phase VI and was completed by the combination of two pivotal instrumental solutions. On one side, we made use of nanocrystalline diamond anvils, which, contrary to the more commonly used single crystal diamond anvils, do not generate sharp Bragg peaks (glitches) at specific energies that spoil the weak fine structure oscillations in the x-ray absorption cross section. Second, we exploited the performance of a state-of-the-art x-ray focussing device yielding a beam spot size of 5 × 5 μm(2), spatially stable over the entire energy scan. An advanced data analysis protocol was implemented to extract the pressure dependence of the structural parameters in phase VI of solid Rb from 51.2 GPa up to the highest pressure. A continuous reduction of the nearest neighbour distances was observed, reaching about 6% over the probed pressure range. We also discuss a phenomenological model based on the Einstein approximation to describe the pressure behaviour of the mean-square relative displacement. Within this simplified scheme, we estimate the Grüneisen parameter for this high pressure Rb phase to be in the 1.3-1.5 interval.
authors.affiliation: Centre for Life Nano Science IIT@Sapienza, Istituto Italiano di Tecnologia, I-00161 Roma, Italy.
authors.forename: S
authors.initials: S
authors.lastname: De Panfilis
authors.affiliation: INO-CNR and LENS, I-50019 Sesto Fiorentino, Firenze, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Gorelli
authors.affiliation: INO-CNR and LENS, I-50019 Sesto Fiorentino, Firenze, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Santoro
authors.affiliation: ISC-CNR, I-50019 Sesto Fiorentino, Firenze, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Ulivi
authors.affiliation: School of Physics and Astronomy, Centre for Science Under Extreme Conditions, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
authors.forename: E
authors.initials: E
authors.lastname: Gregoryanz
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Irifune
authors.affiliation: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Shinmei
authors.affiliation: European Synchrotron Radiation Facility, F-38043 Grenoble, France.
authors.forename: I
authors.initials: I
authors.lastname: Kantor
authors.affiliation: European Synchrotron Radiation Facility, F-38043 Grenoble, France.
authors.forename: O
authors.initials: O
authors.lastname: Mathon
authors.affiliation: European Synchrotron Radiation Facility, F-38043 Grenoble, France.
authors.forename: S
authors.initials: S
authors.lastname: Pascarelli