Ammonium fluoride's analogy to ice: Possibilities and limitations

Ammonium fluoride, NH₄F, is often seen as an analog to ice, with several of its solid phases closely resembling known ice phases. While its ionic and hydrogen-ordered nature puts topological constraints on the ice-like network structures it can form, it is not clear what consequences these constraints have for NH₄F compound formation and evolution. Here, we explore computationally the reach and eventual limits of the ice analogy for ammonium fluoride. By combining data mining of known and hypothetical ice networks with crystal structure prediction and density functional calculations, we explore the high-pressure phase diagram of NH₄F and host-guest compounds of its hydrides. Pure NH₄F departs from ice-like behavior above 80 GPa with the emergence of close-packed ionic structures. The predicted stability of NH₄F hydrides shows that NH₄F can act as a host to small guest species, albeit in a topologically severely constraint configuration space. Finally, we explore the binary NH₃-HF chemical space, where we find candidate structures for several unsolved polyfluoride phases; among them is the chemical analog to H₂O₂ dihydrate.

Transition from Gas-like to Liquid-like Behavior in Supercritical N2

We have studied in detail the transition from gas-like to rigid liquid-like behavior in supercritical N₂ at 300 K (2.4 T_C). Our study combines neutron diffraction and Raman spectroscopy with ab initio molecular dynamics simulations. We observe a narrow transition from gas-like to rigid liquid-like behavior at ca. 150 MPa, which we associate with the Frenkel line. Our findings allow us to reliably characterize the Frenkel line using both diffraction and spectroscopy methods, backed up by simulation, for the same substance. We clearly lay out what parameters change, and what parameters do not change, when the Frenkel line is crossed.

Magnetic and structural changes in LaCo0.9Mn0.1O3 at high pressure

The structural and magnetic properties of LaCo_0.9Mn_0.1O₃ have been studied as a function of pressure by neutron powder diffraction and DC magnetometry. The material is confirmed to exhibit rhombohedral R [Formula: see text] c symmetry between ambient pressure and 6 GPa. We have determined the bulk modulus B ₀ of the sample using a second-order Birch-Murnaghan equation of state which yielded: B ₀  =  140(9) GPa and V [Formula: see text]. We report a non-linear increase of the Curie temperature T _C from an ambient pressure value of 224.7 K to  ∼236 K at a pressure of 4 GPa. Finally, we confirm the glassy-like nature of the magnetism in LaCo_0.9Mn_0.1O₃, which is maintained throughout the pressure range explored.

Ostwald's rule of stages and metastable transitions in the hydrogen–water system at high pressure

The hydrogen water system has been extensively studied above 0.5 GPa and below 0.2. We present neutron diffraction studies in the intermediate pressure range.

Topologically frustrated ionisation in a water-ammonia ice mixture

Water and ammonia are considered major components of the interiors of the giant icy planets and their satellites, which has motivated their exploration under high P–T conditions. Exotic forms of these pure ices have been revealed at extreme (~megabar) pressures, notably symmetric, ionic, and superionic phases. Here we report on an extensive experimental and computational study of the high-pressure properties of the ammonia monohydrate compound forming from an equimolar mixture of water and ammonia. Our experiments demonstrate that relatively mild pressure conditions (7.4 GPa at 300 K) are sufficient to transform ammonia monohydrate from a prototypical hydrogen-bonded crystal into a form where the standard molecular forms of water and ammonia coexist with their ionic counterparts, hydroxide (OH⁻) and ammonium \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {{\rm{NH}}_{\rm{4}}^{\rm{ + }}} \right)$$\end{document}NH4+ ions. Using ab initio atomistic simulations, we explain this surprising coexistence of neutral/charged species as resulting from a topological frustration between local homonuclear and long-ranged heteronuclear ionisation mechanisms.

Water and ammonia are major constituents of icy planet interiors, however their phase behaviour under extreme conditions remain relatively unknown. Here, the authors show that ammonia monohydrate transforms under pressure into an alloy composed of molecules as well as ions, owing to a topological frustration.

Radiation attenuation by single-crystal diamond windows

As artificial diamond becomes more cost effective it is likely to see increasing use as a window for sample environment equipment used in diffraction experiments. Such windows are particularly useful as they exhibit exceptional mechanical properties in addition to being highly transparent to both X-ray and neutron radiation. A key application is in high-pressure studies, where diamond anvil cells (DACs) are used to access extreme sample conditions. However, despite their utility, an important consideration when using single-crystal diamond windows is their interaction with the incident beam. In particular, the Bragg condition will be satisfied for specific angles and wavelengths, leading to the appearance of diamond Bragg spots on the diffraction detectors but also, unavoidably, to loss of transmitted intensity of the beam that interacts with the sample. This effect can be particularly significant for energy-dispersive measurements, for example, in time-of-flight neutron diffraction work using DACs. This article presents a semi-empirical approach that can be used to correct for this effect, which is a prerequisite for the accurate determination of diffraction intensities.

Urea and deuterium mixtures at high pressures

Urea, like many network forming compounds, has long been known to form inclusion (guest-host) compounds. Unlike other network formers like water, urea is not known to form such inclusion compounds with simple molecules like hydrogen. Such compounds if they existed would be of interest both for the fundamental insight they provide into molecular bonding and as potential gas storage systems. Urea has been proposed as a potential hydrogen storage material [T. A. Strobel et al., Chem. Phys. Lett. 478, 97 (2009)]. Here, we report the results of high-pressure neutron diffraction studies of urea and D2 mixtures that indicate no inclusion compound forms up to 3.7 GPa.

On the stability of the disordered molecular alloy phase of ammonia hemihydrate

The disordered-molecular-alloy phase (DMA) of ammonia hydrates [J. S. Loveday and R. J. Nelmes, Phys. Rev. Lett. 83, 4329 (1999)] is unique in that it has substitutional disorder of ammonia and water over the molecular sites of a body centred cubic lattice. Whilst this structure has been observed in ammonia di- and mono-hydrate compositions, it has not been conclusively observed in the ammonia hemihydrate system. This work presents investigations of the structural behaviour of ammonia hemihydrate as a function of P and T. The indications of earlier studies [Ma et al. RSC Adv. 2, 4290 (2012)] that the DMA structure could be produced by compression of ammonia hemihydrate above 20 GPa at ambient temperature are confirmed. In addition, the DMA structure was found to form reversibly both from the melt, and on warming of ammonia hemihydrate phase-II, in the pressure range between 4 and 8 GPa. The route used to make the DMA structure from ammonia mono- and di-hydrates--compression at 170 K to 6 GPa followed by warming to ambient temperature--was found not to produce the DMA structure for ammonia hemihydrate. These results provide the first strong evidence that DMA is a thermodynamically stable form. A high-pressure phase diagram for ammonia hemihydrate is proposed which has importance for planetary modelling.

The crystal structure of methane B at 8 GPa--an α-Mn arrangement of molecules

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) Å at 8.3(2) GPa, which is a factor of √2 larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter 14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of α-manganese at ambient conditions.

Strength analysis and optimisation of double-toroidal anvils for high-pressure research

We used the finite element method for stress and deformation analysis of the large sample volume double-toroidal anvil and gasket assembly used with the Paris-Edinburgh press for neutron scattering, in order to investigate the failure of this assembly observed repeatedly in experiments at a load of approximately 240 tonnes. The analysis is based on a new approach to modelling an opposed anvil device working under extreme stress conditions. The method relies on use of experimental data to validate the simulation in the absence of the material property data available for high pressure conditions. Using this method we analysed the stress distribution on the surface and in the bulk of the double-toroidal anvils, and we conclude that the failure occurs on the surface of the anvil and that it is caused by the tensile stress. We also use the model to show possible ways of optimising the anvil design in order to extend its operational pressure range.

Note: Achieving quasi-hydrostatic conditions in large-volume toroidal anvils for neutron scattering to pressures of up to 18 GPa

We present developments that allow neutron-scattering experiments to be performed, with both single-crystal and powder samples, under quasi-hydrostatic conditions to pressures beyond previous limits. Samples of sodium chloride and squaric acid (H(2)C(4)O(4)) have been loaded with argon as the pressure-transmitting medium in encapsulated gaskets redesigned for double-toroidal anvils, using a gas-loading method at ambient temperature. These samples have been compressed up to 18 GPa in a Paris-Edinburgh press, and no evidence of peak broadening in either the single-crystal or the powder experiments was observed.

A rotator for single-crystal neutron diffraction at high pressure

We present a modified Paris-Edinburgh press which allows rotation of the anvils and the sample under applied load. The device is designed to overcome the problem of having large segments of reciprocal space obscured by the tie rods of the press during single-crystal neutron-scattering experiments. The modified press features custom designed hydraulic bearings and provides controls for precision rotation and positioning. The advantages of using the device for increasing the number of measurable reflections are illustrated with the results of neutron-diffraction experiments on a single crystal of germanium rotated under a load of 70 tonnes.

Gas loading apparatus for the Paris-Edinburgh press

We describe the design and operation of an apparatus for loading gases into the sample volume of the Paris-Edinburgh press at room temperature and high pressure. The system can be used for studies of samples loaded as pure or mixed gases as well as for loading gases as pressure-transmitting media in neutron-scattering experiments. The apparatus consists of a high-pressure vessel and an anvil holder with a clamp mechanism. The vessel, designed to operate at gas pressures of up to 150 MPa, is used for applying the load onto the anvils located inside the clamp. This initial load is sufficient for sealing the pressurized gas inside the sample containing gasket. The clamp containing the anvils and the sample is then transferred into the Paris-Edinburgh press by which further load can be applied to the sample. The clamp has apertures for scattered neutron beams and remains in the press for the duration of the experiment. The performance of the gas loading system is illustrated with the results of neutron-diffraction experiments on compressed nitrogen.

Recrystallisation of HDA ice under pressure by in-situ neutron diffraction to 3.9 GPa

We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3–3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between ~0.4–0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1–1.2 GPa to a mixture of ice VI and XII. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5–1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.

Structure of dense liquid water by neutron scattering to 6.5 GPa and 670 K

We present a neutron diffraction study of liquid water to 6.5 GPa and 670 K. From the measured structure factors we determine radial and angular distributions. It is shown that with increasing density water approaches a local structure common to a simple liquid while distorting only a little the tetrahedral first-neighbor coordination imposed by hydrogen bonds that remain intact.

Nature of the polyamorphic transition in ice under pressure

We present a neutron diffraction study of the transition between low-density and high-density amorphous ice (LDA and HDA, respectively) under pressure at approximately 0.3 GPa, at 130 K. All the intermediate diffraction patterns can be accurately decomposed into a linear combination of the patterns of pure LDA and HDA. This progressive transformation of one distinct phase to another, with phase coexistence at constant pressure and temperature, gives direct evidence of a classical first-order transition. In situ Raman measurements and visual observation of the reverse transition strongly support these conclusions, which have implications for models of water and the proposed second critical point in the undercooled region of liquid water.

Structure of high-density amorphous ice under pressure

We report in situ neutron diffraction studies of high-density amorphous ice (HDA) at 100 K at pressures up to 2.2 GPa. We find that the compression is achieved by a strong contraction ( approximately 20%) of the second neighbor coordination shell, so that at 2.2 GPa it closely approaches the first coordination shell, which itself remains intact in both structure and size. The hydrogen bond orientations suggest an absence of hydrogen bonding between first and second shells and that HDA has increasingly interpenetrating hydrogen bond networks under pressure.

Structure of Rb-III: novel modulated stacking structures in alkali metals

The crystal structure of Rb-III, stable between 13 and 17 GPa, has been determined from quasi-single-crystal x-ray diffraction data. It is orthorhombic, space group C222(1), with 52 atoms in the unit cell, and has an 8-10-8-8-10-8 stacking of 8- and 10-atom layers. The recently reported 84-atom structure of Cs-III can be understood as an 8-8-10-8-8-8-8-10-8-8 stacking of the same layers. These represent a new class of modulated elemental structures.

Transition from cage clathrate to filled ice: the structure of methane hydrate III

The structure of a new methane hydrate has been solved at 3 GPa from neutron and x-ray powder diffraction data. It is a dihydrate in which a 3D H-bonded network of water molecules forms channels surrounding the methane molecules. The network is closely related to that of ice-Ih and the methane-water system appears to be the first in which a cage clathrate hydrate is transformed into an ice-related hydrate (a "filled ice").

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.

Pressure-induced hydrogen bonding: structure of D2S phase I'

The full structure of the high-pressure cubic phase I' of hydrogen sulfide has been solved using neutron diffraction data. The molecules are partially rotationally disordered about the <111> axes, as in phase II at ambient pressure but with markedly greater nonuniformity of the toroidal D distribution. The changes in structure at the II-->I' transition signal the onset of significant pressure-induced hydrogen bonding.