The high-pressure phase diagram of ammonia dihydrate

Fortes, A. Dominic; Wood, Ian G.; Alfredsson, Maria; Vočadlo, Lidunka; Knight, Kevin S.; Marshall, W. G.; Tucker, Matthew G.; Fernandez-Alonso, Félix

doi:10.1080/08957950701265029

Cited by 59 publications

(82 citation statements)

References 18 publications

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“…Sample pressures in the gas pressure cell were determined directly from the He pressure (variance, dp Ӎ 100 bar, 0.01 GPa); those in the P-E press were obtained by fitting the measured Pb lattice parameter to the known equation of state for Pb (as described in ref. 40; dp Ӎ 0.15-0.2 GPa). Data were collected as a function of neutron time-of-flight at the ISIS Spallation Neutron Source, Rutherford Appleton Laboratory, U.K. by using the Polaris (41) and Pearl (42,43) diffractometers for measurements within the gas cell and P-E press, respectively.…”

Section: Methodsmentioning

confidence: 99%

Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Goodwin

Keen

Tucker

2008

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

233

266

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Silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], shows a large negative linear compressibility (NLC, linear expansion under hydrostatic pressure) at ambient temperature at all pressures up to our experimental limit of 7.65(2) GPa. This behavior is qualitatively unaffected by a transition at 0.19 GPa to a new phase Ag 3[Co(CN)6]-II, whose structure is reported here. The high-pressure phase also shows anisotropic thermal expansion with large uniaxial negative thermal expansion (NTE, expansion on cooling). In both phases, the NLC/NTE effect arises as the rapid compression/contraction of layers of silver atoms-weakly bound via argentophilic interactions-is translated via flexing of the covalent network lattice into an expansion along a perpendicular direction. It is proposed that framework materials that contract along a specific direction on heating while expanding macroscopically will, in general, also expand along the same direction under hydrostatic pressure while contracting macroscopically.negative linear compression ͉ negative thermal expansion ͉ high-pressure ͉ framework materials ͉ flexibility N egative linear compressibility (NLC), whereby a material expands along a specific direction on increasing hydrostatic pressure, is a very unusual effect. Indeed in a study of elastic constant data from 500 noncubic crystal phases, only 13 displayed NLC, and of those, 11 structures were of monoclinic or lower symmetry (1). Despite its rarity, NLC is a highly attractive mechanical property, with a key application being the development of effectively incompressible optical materials (1, 2). Such materials could be used in high-pressure environments, such as in optical telecommunications devices that must function at deep-sea pressures Ͼ1,000 atm. NLC also offers a means of producing ultrasensitive pressure detectors, such as interferometric optical sensors for sonar and aircraft altitude measurements. The effect is also often coupled to so-called ''auxetic'' behavior, which is itself being used to improve shock resistance in, e.g., body armor (3).Of the few known examples among inorganic materials, the most pronounced NLC effects have been reported for LaNbO 4 (4), elemental Se (5), the BXO 4 (X ϭ P, As) family (6), and the spin-Peierls compound ␣Ј-NaV 2 O 5 (7). In some other cases, transient NLC behavior may occur only at pressures just above a strain-induced phase transition to a structure of lower symmetry (e.g., refs. 8 and 9), or as a result of an uptake of additional interstitial molecules (10, 11).One fundamental barrier to the practical application of NLC is that its magnitude is generally much smaller than the ''normal'' (positive) compressibilities of standard materials.* By convention, linear compressibility is defined as the relative rate at which a given dimension ᐉ decreases with pressure (at constant temperature): K ᐉ ϭ Ϫ(Ѩlnᐉ/Ѩp) T . Typical values for crystalline materials lie in the range K ᐉ Ӎ 5-20 TPa Ϫ1 (12) (i.e., their linear dimensions decrease by Ϸ1% for each GPa increase in pressure). In cont...

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Section: Methodsmentioning

confidence: 99%

Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Goodwin

Keen

Tucker

2008

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

233

266

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“…AHH has arguably been studied the least among the ammonia hydrates, possibly because of its high ammonia content, far removed from the solar abundance ratio 1:7, and is therefore expected to be rare in nature. However, it is a crucial phase at high pressure and temperature, where both ADH and AMH decompose into AHH + ice-VII, around 3 GPa and at 280 K and 250 K, respectively (32,33). At slightly higher pressures (around 5 GPa to 20 GPa) and room temperature, all ammonia hydrates are found to form disordered molecular alloy (DMA) phases, which feature substitutional disorder of ammonia and water (maybe partially ionized into OH − /NH + 4 ) on a body-centered cubic (bcc) lattice (33)(34)(35)(36).…”

Section: Significancementioning

confidence: 99%

“…However, it is a crucial phase at high pressure and temperature, where both ADH and AMH decompose into AHH + ice-VII, around 3 GPa and at 280 K and 250 K, respectively (32,33). At slightly higher pressures (around 5 GPa to 20 GPa) and room temperature, all ammonia hydrates are found to form disordered molecular alloy (DMA) phases, which feature substitutional disorder of ammonia and water (maybe partially ionized into OH − /NH + 4 ) on a body-centered cubic (bcc) lattice (33)(34)(35)(36). The AHH DMA phase has been observed in two independent experiments (35,36) that found, at low temperatures, transitions from AHH phase II at 19 GPa to 30 GPa.…”

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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“…Several recent computational and experimental studies of ammonia hydrates have been carried out, including a number of high-pressure studies of ammonia dihydrate (Fortes et al 2003a(Fortes et al , 2003b(Fortes et al , 2007aFortes 2004). Most recently, the high-pressure neutron diffraction studies of ADH were summarized by Fortes et al (2007a), and new experimental analysis presented in Fortes et al (2009a) and in Loveday et al (2009).…”

Section: Figmentioning

confidence: 99%

Phase Behaviour of Ices and Hydrates

Fortes¹,

Choukroun

2010

Space Sci Rev

101

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The primary volatile 'rock-forming' minerals in the icy satellites of the outer solar system include water-ice and various hydrated crystals of methane and ammonia. The rich polymorphism of these substances as a function of pressure and temperature are described in this chapter. This polymorphism has a fundamental influence on the exchange of mass and energy between the core and the surface of icy satellites. We describe the current stateof-the-art in our understanding of the high pressure phase behaviour and the measurements of thermoelastic and transport properties of these substances. In addition we describe the structures and properties of hydrated phases of methanol, sulfuric acid, and various sulfate salts.

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The high-pressure phase diagram of ammonia dihydrate

Cited by 59 publications

References 18 publications

Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Stabilization of ammonia-rich hydrate inside icy planets

Phase Behaviour of Ices and Hydrates

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The high-pressure phase diagram of ammonia dihydrate

Cited by 59 publications

References 18 publications

Large negative linear compressibility of Ag 3 [Co(CN) 6 ]

Large negative linear compressibility of Ag 3 [Co(CN) 6 ]

Stabilization of ammonia-rich hydrate inside icy planets

Phase Behaviour of Ices and Hydrates

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Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]

Large negative linear compressibility of Ag ₃ [Co(CN) ₆ ]