Ammonia as a case study for the spontaneous ionization of a simple hydrogen-bonded compound

Taras, P.; Troyan, I. A.; Eremets, M. I.; Drozd, Vadym; Medvedev, S. A.; Zaleski‐Ejgierd, Patryk; Magos-Palasyuk, Ewelina; Wang, Hongbo; Bonev, Stanimir; Dudenko, Dmytro; Naumov, Pavel G.

doi:10.1038/ncomms4460

Cited by 72 publications

(45 citation statements)

References 33 publications

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“…Indeed, the auto-ionization of NH 3 to form ammonium amide (NH 4 þ / NH 2 À ) was seen at pressures more than twice as high as our observations 27,28 . Mixtures of nitrogen and oxygen have also been shown to transform from molecular compounds to ionic solids (nitrosonium nitrate), though only at high temperatures 31 or when subjected to laser or X-ray irradiation 32 .…”

Section: Discussionmentioning

confidence: 51%

“…4a). These spectra bear a strong resemblance to recent experiments and theory on the ammonium amide phase (NH 4 þ /NH 2 À ), shown to result from the auto-ionization of NH 3 , albeit at much more extreme conditions [26][27][28] . The broadband centered near 1,500 cm À 1 is consistent with the bending modes of NH 4 þ and NH 2 À .…”

Section: Binary Phase Diagrammentioning

confidence: 80%

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Pressure-induced chemistry in a nitrogen-hydrogen host–guest structure

et al. 2014

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New topochemistry in simple molecular systems can be explored at high pressure. Here we examine the binary nitrogen/hydrogen system using Raman spectroscopy, synchrotron X-ray diffraction, synchrotron infrared microspectroscopy and visual observation. We find a eutectic-type binary phase diagram with two stable high-pressure van der Waals compounds, which we identify as (N 2 ) 6 (H 2 ) 7 and N 2 (H 2 ) 2 . The former represents a new type of van der Waals host-guest compound in which hydrogen molecules are contained within channels in a nitrogen lattice. This compound shows evidence for a gradual, pressure-induced change in bonding from van der Waals to ionic interactions near 50 GPa, forming an amorphous dinitrogen network containing ionized ammonia in a room-temperature analogue of the Haber-Bosch process. Hydrazine is recovered on decompression. The nitrogen-hydrogen system demonstrates the potential for new pressure-driven chemistry in high-pressure structures and the promise of tailoring molecular interactions for materials synthesis.

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Section: Discussionmentioning

confidence: 51%

Section: Binary Phase Diagrammentioning

confidence: 80%

Pressure-induced chemistry in a nitrogen-hydrogen host–guest structure

et al. 2014

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“…Furthermore both theoretical25262728 and experimental19293031 high-pressure studies on the N/H system (analogous to the N/F system studied here) indicate that a wealth of exotic N x H y structures should, and indeed does stabilize at HP conditions.…”

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confidence: 60%

Hexacoordinated nitrogen(V) stabilized by high pressure

Kurzydłowski

Zaleski‐Ejgierd

2016

Sci Rep

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In all of its known connections nitrogen retains a valence shell electron count of eight therefore satisfying the golden rule of chemistry - the octet rule. Despite the diversity of nitrogen chemistry (with oxidation states ranging from + 5 to −3), and despite numerous efforts, compounds containing nitrogen with a higher electron count (hypervalent nitrogen) remain elusive and are yet to be synthesized. One possible route leading to nitrogen’s hypervalency is the formation of a chemical moiety containing pentavalent nitrogen atoms coordinated by more than four substituents. Here, we present theoretical evidence that a salt containing hexacoordinated nitrogen(V), in the form of an NF6− anion, could be synthesized at a modest pressure of 40 GPa (=400 kbar) via spontaneous oxidation of NF3 by F2. Our results indicate that the synthesis of a new class of compounds containing hypervalent nitrogen is within reach of current high-pressure experimental techniques.

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“…It is the interplay of these interactions, and their relative emphasis as the molar volumes are reduced, that drive intriguing phase transitions, and the emergence of new structural features. Ammonia has been predicted to form an ionic crystal (NH4) + (NH2) − above 100 GPa (21), and this has recently been confirmed in experiment (22,23). The energetic cost of breaking the N-H bond is outweighed by ionic bonding NH + 4 · · · NH − 2 and more compact packing.…”

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confidence: 66%

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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Ammonia as a case study for the spontaneous ionization of a simple hydrogen-bonded compound

Cited by 72 publications

References 33 publications

Pressure-induced chemistry in a nitrogen-hydrogen host–guest structure

Pressure-induced chemistry in a nitrogen-hydrogen host–guest structure

Hexacoordinated nitrogen(V) stabilized by high pressure

Stabilization of ammonia-rich hydrate inside icy planets

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