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SUPERionic, symmetric and ionic states in ICES mixtures (H2O, NH3 and CH4) under extreme conditions – SUPER-ICES

SUPERionic, symmetric and ionic states in ICES mixtures (H2O, NH3 and CH4) under extreme conditions

In this project, we will determine experimentally the physicochemical properties of binary mixtures of simple ices (H2O/NH3, <br />H2O/CH4 and NH3/CH4) over a wide range of pressure [1-100 GPa] and temperature [100-4000 K]. We will seek to determine if the <br />exotic states (called superionic, ionic and symmetric) observed in the pure components exist also in ice mixtures. The motivations <br />are highly multidisciplinary : fundamental physics, planetary sciences and materials sciences

The ices mixture : a new route to synthesize exotics ices (ionic, superionic and symmetric) at mild conditions ?

This project aims at investigating the mixtures of water, ammonia and methane ices over a wide range of pressure (1-100 GPa) and temperature (100-2000 K). Such mixtures are ubiquitous in the Universe and are present under extreme conditions of pressure and temperature inside giant icy planets (Neptune, Uranus), their satellites (e.g. Titan, Ganymede) and extra-solar planets. Although they are composed of simple molecules (H2O, NH3 and CH4), their properties remain largely unknown at very high density. Up to now, only the pure ices have been extensively investigated. These studies have highlighted interesting and unexpected properties: three exotic solid phases, called superionic, ionic and symmetric states, have been uncovered in pure water and ammonia ices around 1 Mbar (=100 GPa). In these states, the chemical bonds, whether covalent or hydrogen (H) bonds, are strongly modified. In particular, the superionic phase is a spectacular state of matter presenting simultaneously a crystalline (the fixed ion lattice) and a liquid (the diffusive ions) behavior. Superionic water and ammonia ices are predicted to be excellent proton conductors, and the existence of superionicity in ice mixtures, if demonstrated, could be a key element to explain the unusual magnetic field of the giant icy planets. <br />ice mixtures are the most simple and are thus ideal systems to study the four most important hydrogen bonds O-H..O, N-H..N, O-H..N and N-H..O, and the mechanisms of proton transfer along these bonds. In this context, high pressure investigations provide a unique tool to study these phenomena at it allows varying the strength of the H bonds through the reduction of the bond length, without the perturbation of changing chemistry. Moreover, understanding the structure-properties relationship and the mechanism of proton delocalization in these simple systems could be useful for the development of superionic compounds

During this project, we will focus on the three binary mixtures H2O/NH3, H2O/CH4 and NH3/CH4. The main goal will be to probe by experiment and computer simulations if the exotic states of matter found in the pure ices also exist in these ice mixtures and if, as predicted by recent ab initio calculations, they are stable at milder P-T conditions than in the pure components. Several in situ characterization techniques (Raman and IR spectroscopy, X-ray and neutron diffraction) will be used, as well as state-of-the-art ab initio theoretical methods. We will develop new tools to measure the electrical impedance of samples in diamond anvil cells in order to characterize the protonic conductivity under extreme P-T conditions.

This project is divide into three tasks :
Task 1 : Mixtures of H-bonded ices (NH3/H2O) as this system is an excellent candidate to observe ionicity and superionicity at mild P-T conditions.

Task 2 : mixtures of H-bonded ices with methane (NH3/CH4 and H2O/CH4). With these systems, we seek to understand the influence of a non H-bonded system, CH4, on the non molecular states resulting from proton transfer.

Task 3 is a transversal task devoted to technical developments. We aim at developing transport measurements under extreme P-T conditions in the diamond anvil cell (DAC), in order to finely characterize the superionic states.

We have starting by studying H2O/NH3 binary mixture as this system is the most promoising to observe ionicity/superionicity at mild conditions. At ambient pressure, ammonia and water are fully miscible in the liquid phase for any composition. Depending on the latter, this liquid crystallizes at low temperature into three different stoichiometric compounds: ammonia dehydrate (ADH) of composition (NH3)(H2O)2, ammonia monohydrate (AMH) of composition (NH3)(H2O), and ammonia hemihydrate (AHH) of composition (NH3)2(H2O).

We have starting by studying the ammonia monohydrate (AMH) compound forming from an equimolar mixture of water and ammonia. Our experiments demonstrate that relatively mild pressure conditions (7.4 GPa at 300 K) are sufficient totransform AMH from a prototypical hydrogen-bonded crystal into a form where the standard molecular forms of water and ammonia coexist with their ionic counterparts, hydroxide (OH-) and ammonium (NH4+) ions. Using ab initio atomistic simulations, we explain this surprising coexistence of neutral/charged species as resulting from a topological frustration between local homonuclear and long-ranged heteronuclear ionisation mechanisms. This work has been accepted for publications in Nature Communications (August 2017)

In our next studies, we will focus on the determination of high temperature properties ammonia / water mixtures as our calculations predict superionicity. To characterize it, we will develop conductivity measurements in diamond anvil cells. The comparison of the results on the three ammonia hydrates is important and can have large implications in the field of planetary sciences.

Later, we will study the influence of methane, a compound without hydrogen bonds, on these mixtures to see if it blocks or not the ionization / superionization

[1] C. Liu, A. Mafety, J.A. Queyroux, C. Wilson, H. Zhang, K. Beneut, G. Le Marchand, B. Baptiste, P. Dumas, G. Garbarino, F. Finocchi, J.S. Loveday, F. Pietrucci, A.M. Saitta, F. Datchi and S. Ninet, Topologically frustrated ionisation in a water-ammonia

This project aims at investigating the mixtures of water, ammonia and methane ices over a wide range of pressure (1-100 GPa) and temperature (100-2000 K). Such mixtures are ubiquitous in the Universe and are present under extreme conditions of pressure and temperature inside giant icy planets (Neptune, Uranus), their satellites (e.g. Titan, Ganymede) and extra-solar planets. Although they are composed of simple molecules (H2O, NH3 and CH4), their properties remain largely unknown at very high density. Up to now, only the pure ices have been extensively investigated. These studies have highlighted interesting and unexpected properties: three exotic solid phases, called superionic, ionic and symmetric states, have been uncovered in pure water and ammonia ices around 1 Mbar (=100 GPa). In these states, the chemical bonds, whether covalent or hydrogen (H) bonds, are strongly modified. In particular, the superionic phase is a spectacular state of matter presenting simultaneously a crystalline (the fixed ion lattice) and a liquid (the diffusive ions) behavior. Superionic water and ammonia ices are predicted to be excellent proton conductors, and the existence of superionicity in ice mixtures, if demonstrated, could be a key element to explain the unusual magnetic field of the giant icy planets.

Besides the interests in condensed-matter physics and planetary sciences, this project is expected to have an impact over a wide range of disciplines, including inorganic chemistry, materials science and biology. Indeed, ice mixtures are the most simple and are thus ideal systems to study the four most important hydrogen bonds O-H..O, N-H..N, O-H..N and N-H..O, and the mechanisms of proton transfer along these bonds. This topic has direct implications for our understanding of various phenomena, including the high melting point of water, the shape of proteins and photosynthesis. In this context, high pressure investigations provide a unique tool to study these phenomena at it allows varying the strength of the H bonds through the reduction of the bond length, without the perturbation of changing chemistry. Moreover, understanding the structure-properties relationship and the mechanism of proton delocalization in these simple systems could be useful for the development of superionic compounds as components of solid-state batteries, a topic currently under intensive investigation.

During this project, we will focus on the three binary mixtures H2O/NH3, H2O/CH4 and NH3/CH4. The main goal will be to probe by experiment and computer simulations if the exotic states of matter found in the pure ices also exist in these ice mixtures and if, as predicted by recent ab initio calculations, they are stable at milder P-T conditions than in the pure components. Several in situ characterization techniques (Raman and IR spectroscopy, X-ray and neutron diffraction) will be used, as well as state-of-the-art ab initio theoretical methods. We will develop new tools to measure the electrical impedance of samples in diamond anvil cells in order to characterize the protonic conductivity under extreme P-T conditions. The latter will be a strong asset for the high pressure community and should have many applications in experimental physics, geophysics and material sciences. The project coordinator and the team of people which has been assembled around this project present all the necessary expertise and abilities to reach the challenging but realistic goals.

Project coordinator

Madame sandra ninet (Institut de Minéralogie de Physique des Matériaux et de Cosmochimie)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IMPMC Institut de Minéralogie de Physique des Matériaux et de Cosmochimie

Help of the ANR 275,205 euros
Beginning and duration of the scientific project: September 2015 - 48 Months

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