<i>Ab initio</i>calculation of thermodynamic potentials and entropies for superionic water

French, Martin; Desjarlais, M. P.; Redmer, R.

doi:10.1103/physreve.93.022140

Cited by 58 publications

(64 citation statements)

References 80 publications

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“…Temperatures in the interiors of Uranus and Neptune are typically less than 6000 K, so that the thermal conductivity of water inside both planets is caused mainly by the interactions between the nuclei. This should also apply to possibly occurring superionic phases [51], which have similar proton diffusion coefficients as the fluid but strongly reduced electronic conductivity [30,32,52].…”

Section: Resultsmentioning

confidence: 99%

Thermal conductivity of dissociating water—anab initiostudy

French

2019

New J. Phys.

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The thermal conductivity of partially dissociated and ionised water is calculated in a large-scale study using density functional theory (DFT)-based molecular dynamics (MD) simulations. In doing so, the required heat current of the nuclei is calculated by mapping the effective particle interactions from the DFT-MD simulations onto classical pair potentials. It is demonstrated that experimental and theoretical thermal conductivity data for liquid heavy water and for ice VII are well reproduced with this efficient procedure. Moreover, the approach also allows for an illustrative interpretation of the characteristics of the thermal conductivity in the dense chemically reacting fluid. The thermodynamic conditions investigated here range from densities between 0.2 and 6 g cm −3 and temperatures between 600 and 50000 K, which includes states highly relevant for understanding the interiors of water-rich planets like Uranus and Neptune and exoplanets of similar composition. During the last two decades, ab initio calculations based on finite-temperature density functional theory (FT-DFT) in combination with molecular dynamics (MD) simulations have become a reliable tool for accurate predictions of thermodynamic properties of matter under extreme conditions [12,13]. The prefix FT emphasises that the electron system is consistently treated at a nonzero (finite) temperature [14], which is typically equal to the temperature of the nuclei.The calculation of the thermal conductivity with FT-DFT-MD techniques requires a distincted treatment of the contributions from electronic heat conduction and from heat transport via the nuclei. The former is gained by evaluating the electronic Onsager coefficients using the Kohn-Sham eigenvalues and orbitals from the FT-DFT [15], which was recently accomplished also for partially ionised water [16].

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Section: Resultsmentioning

confidence: 99%

Thermal conductivity of dissociating water—anab initiostudy

French

2019

New J. Phys.

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“…Gray (blue) dashed lines indicate the onset of ice melting (superionicity) from simulations [55,56]. of stability for P 2 1 /c-AlOOH is below estimates of the ice melting line but above the onset of superionicity in ice, as seen in calculations [56,57]. However, while the latter would certainly favor decomposition, it is possible that AlOOH itself forms a superionic phase at those conditions.…”

Section: A Aluminum Oxide Hydroxidementioning

confidence: 89%

“…Under those extreme conditions (400 GPa, 4000 K), monoclinic AlOOH could conceivably form a superionic hydrous phase, with mobile protons diffusing through the Al-O lattice. Such a phase would compete with superionic ice phases, and accurate calculations of thermodynamic potentials at finite temperatures are necessary to establish their relative free energies [57].…”

Section: Discussionmentioning

confidence: 99%

Monoclinic high-pressure polymorph of AlOOH predicted from first principles

2016

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Aluminum oxide hydroxide, AlOOH, is a prototypical hydrous mineral in the geonomy. The study of the high-pressure phase evolution of AlOOH is of fundamental importance in helping to understand the role of hydrous minerals in the water storage and transport in Earth, as in other planets. Here, we have systematically investigated the high-pressure phase diagram of AlOOH up to 550 GPa using the efficient crystal structure analysis by particle swarm optimization (CALYPSO) algorithm in conjunction with first principles calculations. We predict a peculiar monoclinic phase (space group P 2 1 /c, 16 atoms/cell, Z = 4 ) as the most stable phase for AlOOH above 340 GPa. The occurrence of this new phase results in the breakup of symmetric linear O-H-O hydrogen bonds into asymmetric, bent O-H-O linkages and in sevenfold coordinated metal cations. The new P 2 1 /c phase turns out to be a universal high-pressure phase in group 13 oxide hydroxides, and stable for both compressed GaOOH and InOOH. The formation of the new phase in all compounds is favored by volume reduction due to denser packing.

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“…hydrogen-bonded systems (27), while, at high temperatures, superionicity is predicted to occur in both water and ammonia: The heavy O/N atoms form crystalline lattices, while protons are free to diffuse through the crystal (28)(29)(30).…”

Section: Significancementioning

confidence: 99%

Stabilization of ammonia-rich hydrate inside icy planets

Robinson

Wang

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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The interior structure of the giant ice planets Uranus and Neptune, but also of newly discovered exoplanets, is loosely constrained, because limited observational data can be satisfied with various interior models. Although it is known that their mantles comprise large amounts of water, ammonia, and methane ices, it is unclear how these organize themselves within the planets-as homogeneous mixtures, with continuous concentration gradients, or as well-separated layers of specific composition. While individual ices have been studied in great detail under pressure, the properties of their mixtures are much less explored. We show here, using first-principles calculations, that the 2:1 ammonia hydrate, (H 2 O)(NH 3 ) 2 , is stabilized at icy planet mantle conditions due to a remarkable structural evolution. Above 65 GPa, we predict it will transform from a hydrogen-bonded molecular solid into a fully ionic phase O 2− (NH + 4 ) 2 , where all water molecules are completely deprotonated, an unexpected bonding phenomenon not seen before. Ammonia hemihydrate is stable in a sequence of ionic phases up to 500 GPa, pressures found deep within Neptune-like planets, and thus at higher pressures than any other ammoniawater mixture. This suggests it precipitates out of any ammoniawater mixture at sufficiently high pressures and thus forms an important component of icy planets.ammonia hydrate | pressure | phase transition | density functional theory A remarkable number of recently and currently discovered exoplanets can be classified as super-Earths or miniNeptunes, with masses up to 10 Earth masses and mean densities around 1 g/cm 3 (1-3). The uncertainty in classification hints at a substantial problem: Researchers cannot distinguish from afar whether these bodies are mostly rocky (like Earth) or mostly icy (like Neptune). Inside our own solar system, we have the "ice giants" Uranus and Neptune. We know their mantle region contains large amounts of water, ammonia, and methane ices, as well as hydrogen in various forms, while their cores are presumably formed by a small amount of heavy elements (4-7). However, even for those planets, observational data are limited to global observables such as mass, radius, and gravitational and magnetic moments. These provide only a loose set of constraints on interior composition, which can be satisfied by a variety of planetary models. The low luminosity of Uranus, for instance, could be due to a thermal boundary layer, which would suggest significant composition gradients inside its mantle (7-9). To constrain plausible interior models, more astrophysical data are needed (1). At the same time, laboratory experiments and accurate calculations are necessary to establish the equations of state of the planets' potential constituent materials. As these constituents experience extreme pressure and temperature conditions inside the planets, many of their defining properties such as viscosity and thermal or electrical conductivity are likely to change drastically from what we are used to at ambient c...

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Ab initiocalculation of thermodynamic potentials and entropies for superionic water

Cited by 58 publications

References 80 publications

Thermal conductivity of dissociating water—anab initiostudy

Thermal conductivity of dissociating water—anab initiostudy

Monoclinic high-pressure polymorph of AlOOH predicted from first principles

Stabilization of ammonia-rich hydrate inside icy planets

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