Structure and dynamics of simple MOlecular FLuids under EXtreme conditions of pressure and temperature – MOFLEX

To reach our objectives, we are setting up three in situ experimental probes, distributed over the three partner’s sites: x-ray diffraction, time-resolved Raman spectroscopy, and Brillouin scattering. Each of these experiments will be optimized for the laser-heated diamond anvil cell, the only technology that is able to produce the targeted extreme conditions. Their combination will facilitate the data analysis and will enable to obtain a physical, chemical and thermodynamic description as complete as possible of the samples. We propose new solutions to overcome the great technical challenges that the production and characterization of fluid samples of low Z compounds under extreme P-T conditions present. The several preliminary studies made over the last two years demonstrate the high chance of success of this project. The experimental efforts will be complemented by theoretical investigations using state-of-the-art ab initio molecular dynamics, in order to help in interpreting the observations, to provide information difficult to access from experiment alone, and to possibly suggest new experiments.

The first scientific highlights of this project are :
1. Pseudo-transition in liquid hydrogen under pressure: The structure factor of liquid hydrogen has been measured for the first time in a diamond anvil cell up to 5 GPa at low temperatures (50-300 K), by x-ray diffraction. These measurements extend previous ones by a factor 1000 in pressure, or a 300% increase in density. The data quality obtained from our micrometric samples is comparable to that previously obtained by neutron diffraction on millimetric samples. This study has evidenced two different compression regimes in liquid H2/D2.
2. Pressure-induced structural transformations in fluid CO2: The structure of fluid CO2 has been determined for the first time up to 10 GPa, 300<T<700 K. Thanks to the new techniques developed in this project, the quality of the structural data is unprecedented. The experimental results are combined to simulations, showing a large anisotropy in the local structure of CO2 and a progressive transformation with density similar to those observed in the solid.<br />3. Liquid sodium at megabar : from simple to complex liquid: At ambient, solid sodium is a textbook example of a simple metal with cubic structure. This simplicity disappear at high pressure: several complex structures have been observed near 1 Mbar. Here, we have investigated the structure of fluid sodium in a wide P-T range, up to 120 GPa and 1000 K. This shows a drastic change in the structure factor, from a simple compact fluid to a complex one.

The new methods developped the first period of this project will be put to profit to collect experimental data on the various systems targeted by this project.

1. G. Weck, G. Garbarino, P. Loubeyre, F. Datchi, T. Plisson, and M. Mezouar. Liquid hydrogen structure factor to 5 GPa and evidence of a crossover between two density evolutions. Phys. Rev. B (Rapid Comm.) 91, 180204(R) (2015)
2. F. Datchi, G. Weck, A. M. Saitta, Z. Raza, G. Garbarino, S. Ninet, D. K. Spaulding, J. A. Queyroux, and M. Mezouar. Pressure-induced structural transformations in fluid carbon dioxide. Submitted

The objective of this project is to investigate the warm dense fluid regime of some fundamental constituents of Jovian and icy planets (H2, He, N2, O2, CO2, H2O, NH3 and CH4), when submitted to extreme conditions of pressure (~ 100 GPa range) and temperature (~900-4000 K). This P-T regime, situated in-between the molecular fluid and the fully dissociated plasma phases, has not yet been accessed by static compression experiments. Our aim is to experimentally observe and characterize the intriguing novel states of matter predicted by recent first-principles calculations, such as ionic water and ammonia, metallic oxygen and hydrogen, conductive helium or polymeric nitrogen and carbon dioxide. We also expect to answer fundamental questions regarding the insulator-to-metal, molecular-to-polymeric, or molecular-to-ionic phase transitions in these fluids: What are their structural and dynamical fingerprints? Are they liquid-liquid first-order transitions? Are they preceded by intermediate ordering and/or chemical bonding states? We will also determine the equation of state of these fluids which is a crucial ingredient for the modelling of planet interiors and will complement the information obtained by shock wave experiments in a density range which is not accessible by these methods.

To reach our objectives, we will set-up three in situ experimental probes, distributed over the three partner’s sites: x-ray diffraction, time-resolved Raman spectroscopy, and Brillouin scattering. Each of these experiments will be optimized for the laser-heated diamond anvil cell, the only technology that is able to produce the targeted extreme conditions. Their combination will facilitate the data analysis and will enable to obtain a physical, chemical and thermodynamic description as complete as possible of the samples. We propose new solutions to overcome the great technical challenges that the production and characterization of fluid samples of low Z compounds under extreme P-T conditions present. The several preliminary studies made over the last two years demonstrate the high chance of success of this project. The experimental efforts will be complemented by theoretical investigations using state-of-the-art ab initio molecular dynamics, in order to help in interpreting the observations, to provide information difficult to access from experiment alone, and to possibly suggest new experiments.

This proposal builds up on the internationally recognized expertise of the three partners on the studies of simple molecular systems under high pressure, and their long-standing, successful collaboration. We believe that the entirely new experimental platform proposed here will define a standard for the study of warm dense matter and will pave the way to breakthroughs in the understanding of fluids and amorphous solids at high P-T, which is a rapidly growing and competitive research field. Besides the fundamental impact in condensed-matter and planetary sciences, this project should be of great practical significance for the description of detonations reactions or the synthesis of novel materials under extreme environments.