soutenance de thèse de Mallick Bidyut Vendredi 25 Mai à 14H

 IMPMC,Université P. et M. Curie, 4 Place Jussieu, 75005 Paris

Salle de Conférence, 4ème étage, Tour 22-23

"A new experimental approach to study solid CO2 at extreme conditions"

Abstract:
CO2 is one of the most studied elements at extreme conditions. Despite its simplicity, at high pressure and high temperature its phase diagram is unusually complex. Above 40 GPa, when molecular solid CO2 submitted to high temperatures, it has revealed the existence of a new class of CO2 solids. These high P-T phases of CO2 have attracted a formidable interest in the recent years, both for their fundamental and geochemical importance. Based on experimental results, polymeric CO2 solids have been reported to have structures similar to the group IV dioxides (SiO2, GeO2, SnO2) with interesting properties like optical nonlinearity and superhardness. Several theoretical investigations devoted to these phases contested the experimental results. Obtaining high quality structural data at the extreme conditions of formation of these phases are indeed experimentally challenging. Huge pressure and temperature gradient inside CO2 sample chamber makes it difficult to produce a pure high pressure phase of CO2 and hence to probe it accurately. To overcome this problem, we did the first experimental studies of the solid phases of CO2 under hydrostatic conditions, using helium or neon as pressure medium. We are able to experimentally solve the structure of polymeric solid CO2-V and study its compressibility. Investigations on phase-II and phase-VI have also been done by using Raman and X-ray diffraction measurements in hydrostatic medium of neon. Our results are in very good agreement with theoretical investigations. The first experimental determination of the binary phase diagrams of CO2-He and CO2-Ne mixtures at high pressure has also been reported in this thesis.