Multiple superionic states in helium–water compounds

Liu, Cong; Gao, Hao; Wang, Yong; Needs, R. J.; Pickard, Chris J.; Sun, Junqiang; Wang, Hui-Tian; Xing, D. Y.

doi:10.1038/s41567-019-0568-7

Cited by 91 publications

(73 citation statements)

References 49 publications

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“…It is of particular interest whether the melting line intersects the isentropes of Uranus or Neptune, which would suggest a partially solid lower mantle in these planets that retains efficient electrical and thermal conductivity through proton transport. This phase evolution has been reported for the individual ices of NH 3 and H 2 O [16], for mixtures of helium and water [43], ternary mixtures including methane [44] and, as already mentioned, has also been calculated for AMH and ADH [36,37].…”

Section: Introductionsupporting

confidence: 75%

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

Robinson

Hermann

2020

J. Phys.: Condens. Matter

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The interiors of giant icy planets depend on the properties of hot, dense mixtures of the molecular ices water, ammonia, and methane. Here, we discuss results from first-principles molecular dynamics simulations up to 500 GPa and 5000 K for four different ammonia-water mixtures that correspond to the stable stoichiometries found in solid ammonia hydrates. We show that all mixtures support the formation of plastic and superionic phases at elevated pressures and temperatures, before eventually melting into molecular or ionic liquids. All mixtures' melting lines are found to be close to the isentropes of Uranus and Neptune. Through local structure analyses we trace and compare the evolution of chemical composition and longevity of chemical species across the thermally activated states. Under specific conditions we find that protons can be less mobile in the fluid state than in the (colder, solid) superionic regime.

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Section: Introductionsupporting

confidence: 75%

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

Robinson

Hermann

2020

J. Phys.: Condens. Matter

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“…We used a variable-composition structure-prediction algorithm, as implemented in a machine-learning accelerated crystal structure search [59], in which we combined the Bayesian optimization and a Gaussian process model to improve the search efficiency and diversity. This method has been successfully applied in many systems, including helium water compounds [13], T-graphene [60], and metal pentazolate salts [61], etc. We have performed extensive variable-composition searches on different pressures, such as 50, 100, 300, and 500 GPa, as well as fixed stoichiometry (He∶NH 3 ¼ 1∶2, 1∶1, and 2∶1, etc.)…”

Section: A Crystal Structure Searchmentioning

confidence: 99%

Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature

et al. 2020

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Both helium and ammonia are main components of icy giant planets. While ammonia is very reactive, helium is the most inert element in the universe. It is of great interest whether ammonia and helium can react with each other under planetary conditions, and if so, what kinds of structures and states of matter can form. Here, using crystal structure prediction methods and first-principles calculations, we report three new stable stoichiometries and eight new stable phases of He-NH 3 compounds under pressures up to 500 GPa. These structures may exhibit perovskitelike structures for HeNH 3 and He 2 NH 3 , and a host-guest crystal structure for HeðNH 3 Þ 2. Superionic states are found in all these He-NH 3 compounds under high pressures and temperatures in which the hydrogen atoms are diffusive while the nitrogen and helium atoms remain fixed. Such dynamical behavior in helium ammonia compounds is quite different from that in helium water compounds, where weakly interacting helium is more diffusive than stronger bound hydrogen. The lowdensity host-guest phase of space group I4cm is found to be stable at very low pressures (about 3 GPa) and it enters into a plastic state, characterized by freely rotating ammonia molecules. The present results suggest that plastic or superionic helium ammonia compounds may exist under planetary conditions, and helium contributes crucially to the exotic physics and chemistry observed under extreme conditions.

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“…The propension of He to from compounds under high P without forming chemical bonds is explained by its insertion with ionic compounds and consequent stabilization of Coulomb interactions [94], besides the volume reduction associated with the reaction. Interestingly, like Xe 2 O 6 H 6 , He 2 H 2 O is superionic at high T above 40 GPa [94], with free mobility of He and protons. However, this occurs at 40 GPa only, 10 GPa lower than for pure ice.…”

Section: Noble Gas Compounds Relevant To Giant Planets Interiorsmentioning

confidence: 99%

Noble Gas Reactivity in Planetary Interiors

Sanloup

2020

Front. Phys.

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While the field of noble gas reactivity essentially belongs to chemistry, Earth and planetary sciences have brought a different perspective to the field. Indeed, planetary interiors are natural high pressure (P) and high temperature (T) laboratories, where conditions exist where bonding of the heaviest noble gases may be induced thermodynamically through volume reduction (Le Châtelier's principle). Earth and planetary sciences besides generate numerous and precise observations such as the depletion of the terrestrial and martian atmospheres in xenon, pointing to the potential for Xe to be sequestered at depth, potentially induced by its reactivity. More generally, this paper will review the advances on noble gas reactivity at the extreme P-T conditions found within planetary interiors from experiments and theoretical investigations. This review will cover the synthesis of cage compounds, stoichiometric oxides and metals, and non-stoichiometric compounds where noble gases are only minor or trace elements but could be essential in solving some Earth and planetary puzzles. An apparent trend in noble gas reactivity with P emerges. In the case of Xe which is the most documented, metals are synthesized above 150 GPa, i.e., at terrestrial core conditions, stoichiometric Xe-oxides between 50 and 100 GPa, i.e., in the P-T range of the Earth's lower mantle, but Xe-O high energy bonds may also form under the modest pressures of the Earth's crust (<1 GPa) in non-stoichiometric compounds. Most planetary relevant noble gas compounds found are with xenon, with only a few predicted helium compounds, the latter having no or very little charge transfer between helium and neighboring atoms.

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Multiple superionic states in helium–water compounds

Cited by 91 publications

References 49 publications

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

Plastic and superionic phases in ammonia–water mixtures at high pressures and temperatures

Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature

Noble Gas Reactivity in Planetary Interiors

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