High pressure ionic and molecular crystals of ammonia monohydrate within density functional theory

Griffiths, Gareth I. G.; Misquitta, Alston J.; Fortes, A. Dominic; Pickard, Chris J.; Needs, R. J.

doi:10.1063/1.4737887

Cited by 24 publications

(39 citation statements)

References 38 publications

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“…34 However, the van der Waals corrected PBE+G06 functional provides reliable transition pressures for molecular solids. [52][53][54] In the present study, the PBE+G06 functional reproduces the transition pressure accurately in contrast to the standard CA-PZ and PBE functionals, and this in very good agreement with the experimental transition pressure. 34 The calculated lattice parameters a, b, and c of phase I and lattice parameter a of phase II are shown in Fig.…”

Section: B Monoclinic → Rhombohedral Structural Phase Transition Undsupporting

confidence: 76%

Pressure induced structural phase transition in solid oxidizer KClO3: A first-principles study

Yedukondalu

Ghule

Vaitheeswaran

2013

The Journal of Chemical Physics

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High pressure behavior of potassium chlorate (KClO3) has been investigated from 0 to 10 GPa by means of first principles density functional theory calculations. The calculated ground state parameters, transition pressure, and phonon frequencies using semiempirical dispersion correction scheme are in excellent agreement with experiment. It is found that KClO3 undergoes a pressure induced first order phase transition with an associated volume collapse of 6.4% from monoclinic (P2(1)/m) → rhombohedral (R3m) structure at 2.26 GPa, which is in good accord with experimental observation. However, the transition pressure was found to underestimate (0.11 GPa) and overestimate (3.57 GPa) using local density approximation and generalized gradient approximation functionals, respectively. Mechanical stability of both the phases is explained from the calculated single crystal elastic constants. In addition, the zone center phonon frequencies have been calculated using density functional perturbation theory at ambient as well as at high pressure and the lattice modes are found to soften under pressure between 0.6 and 1.2 GPa. The present study reveals that the observed structural phase transition leads to changes in the decomposition mechanism of KClO3 which corroborates with the experimental results.

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Section: B Monoclinic → Rhombohedral Structural Phase Transition Undsupporting

confidence: 76%

Pressure induced structural phase transition in solid oxidizer KClO3: A first-principles study

Yedukondalu

Ghule

Vaitheeswaran

2013

The Journal of Chemical Physics

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“…[34, 37-40] Moreover, first-principles calculations predict the appearance of ionic phases, where proton transfer leads to the formation of hydroxyl and ammonium groups, in all hydrates at elevated pressures. [41][42][43][44] The highest pressure any hydrate has been studied in experiment is 41 GPa. [45] Here, we begin by discussing individually the calculated high-pressure and -temperature phase evolution of the three known ammonia hydrates.…”

Section: Resultsmentioning

confidence: 99%

“…[31,46,47] A computational prediction by Griffiths et al suggests that a tetragonal ionic ammonium hydroxide phase, (OH − )(NH + 4 ), becomes more stable than AMH-II above 2.8 GPa. [43] Only an incomplete transformation into this phase has been observed experimentally, which is possibly frustrated due to the substitutional disorder in the DMA phase. [40] The high-pressure phase evolution of AMH has recently been studied computationally by Bethkenhagen et al, who used crystal structure prediction to identify relevant solid phases, which were then used as initial configurations for molecular dynamics (MD) simulations.…”

Section: Resultsmentioning

confidence: 99%

“…The discrepancies between our calculations and the experimental phase diagrams for both ADH and AHH hydrates in the low-pressure regions might also be contributed to by the semilocal exchange-correlation functional that has been argued to overstabilise ionic structures. [43] This could lead to spurious stabilisation of ADH-I * over ADH-II, and of AMH-P 4/nmm over both ADH and AHH. However, the relative stabilities of the different hydrates are qualitatively unaffected for various other exchange-correlation functionals; in the SM we show phase diagrams equivalent to Figure 5 obtained from the LDA functional as well as from dispersion-corrections of the Grimme (D2), Tkatchenko-Scheffler (TS), and many-body dispersion (MBD) type.…”

Section: Resultsmentioning

confidence: 99%

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Novel phases in ammonia-water mixtures under pressure

Robinson

Marqués

Wang

et al. 2018

The Journal of Chemical Physics

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While ammonia and water readily form hydrogen-bonded molecular mixtures at ambient conditions, their miscibility under pressure is not well understood, yet crucial e.g. to model the interior of icy planets. We report here on the behaviour of ammonia-water mixtures under extreme pressure conditions, based on first-principles calculations of 15 stoichiometries in the pressure range of 1 atm to 10 Mbar. We show that compression facilitates proton transfer from water to ammonia in all relevant mixtures. This favors ammonia-rich hydrates above 1 Mbar, stabilized by complete de-protonation of water and the formation of the unusual structural motifs O 2− •(NH + 4) 2 and O 2− •(N 2 H + 7) 2. The hydronitrogen cations persist to the highest pressures studied. We predict a new ammonia-rich 4:1-hydrate at intermediate pressures and find that by 5.5 Mbar, close to the core-mantle boundary of Neptune, all cold ammonia-water mixtures are unstable against decomposition into their constituents.

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“…In AMH, calculations predicted that an ionic phase transition should stabilize NH + 4 ·OH − over NH3·H2O around 6 GPa (39). A more stable ionic structure was proposed recently (40), and subsequent crystal structure searches on AMH uncovered higher-pressure ionic phases, which were then used as starting points for ab initio molecular dynamics (MD) simulations to investigate the superionic regime of AMH (41). These studies attempted to explore molecular mixtures at conditions present deep within icy planetary bodies using electronic structure calculations, but there is an inherent assumption that AMH is indeed a relevant stoichiometry at elevated pressures in the waterammonia phase diagram.…”

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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High pressure ionic and molecular crystals of ammonia monohydrate within density functional theory

Cited by 24 publications

References 38 publications

Pressure induced structural phase transition in solid oxidizer KClO3: A first-principles study

Pressure induced structural phase transition in solid oxidizer KClO3: A first-principles study

Novel phases in ammonia-water mixtures under pressure

Stabilization of ammonia-rich hydrate inside icy planets

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