Structural phase transition in NH4F under extreme pressure conditions.

Ammonium fluoride (NH4F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH4F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH4F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.

Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid.

According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm. The molecular self-diffusion coefficient was derived from the best Gaussian (at low pressures) or Lorentzian (at high pressures) fits to the neutron spectra. The Gaussian-to-Lorentzian crossover is progressive and takes place at about the Widom line intercept (59 bar). At considerably higher pressures, a liquid-like jump diffusion mechanism properly describes the supercritical fluid on both sides of the Frenkel line. The present observation of a gas-like to liquid-like crossover in the self dynamics of a simple supercritical fluid confirms emerging views on the unexpectedly complex physics of the supercritical state, and could have planet-wide implications and possible industrial applications in green chemistry.

Low-Temperature Dynamics of Water Confined in Unidirectional Hydrophilic Zeolite Nanopores.

The dynamical properties of water molecules confined in the unidirectional hydrophilic nanopores of AlPO4-54 are investigated with quasi-elastic neutron scattering as a function of temperature down to 118 K. AlPO4-54 has among the largest pores known for aluminophosphates and zeolites (about 1.3 nm), though they are small enough to prevent water crystallization due to the high degree of confinement. Water molecular diffusion into the pore is here measured down to 258 K. Diffusion is slower than in bulk water and has an activation energy of Ea = (20.8 ± 2.8) kJ/mol, in agreement with previous studies on similar confining media. Surprisingly, local hydrogen dynamics associated with water reorientation is measured down to temperatures (118 K), i.e., well below the expected glass transition temperature of bulk water. The reorientational time scale shows the well-known non-Arrhenius behavior down to the freezing of water mass diffusion, while it shows a feeble temperature dependence below. This fast local dynamics, of the order of fractions of nanoseconds, is believed to take place in the dense, highly disordered amorphous water occupying the pore center, indicating its possible plastic nature.

Temperature- and pressure-dependence of the hydrogen bond network in plastic ice VII.

We model, via classical molecular dynamics simulations, the plastic phase of ice VII across a wide range of the phase diagram of interest for planetary investigations. Although structural and dynamical properties of plastic ice VII are mostly independent on the thermodynamic conditions, the hydrogen bond network (HBN) acquires a diverse spectrum of topologies distinctly different from that of liquid water and of ice VII simulated at the same pressure. We observe that the HBN topology of plastic ice carries some degree of similarity with the crystal phase, stronger at thermodynamic conditions proximal to ice VII, and gradually lessening when approaching the liquid state. Our results enrich our understanding of the properties of water at high pressure and high temperature and may help in rationalizing the geology of water-rich planets.

Advances in the study of supercooled water.

In this review, we report recent progress in the field of supercooled water. Due to its uniqueness, water presents numerous anomalies with respect to most simple liquids, showing polyamorphism both in the liquid and in the glassy state. We first describe the thermodynamic scenarios hypothesized for the supercooled region and in particular among them the liquid-liquid critical point scenario that has so far received more experimental evidence. We then review the most recent structural indicators, the two-state model picture of water, and the importance of cooperative effects related to the fact that water is a hydrogen-bonded network liquid. We show throughout the review that water's peculiar properties come into play also when water is in solution, confined, and close to biological molecules. Concerning dynamics, upon mild supercooling water behaves as a fragile glass former following the mode coupling theory, and it turns into a strong glass former upon further cooling. Connections between the slow dynamics and the thermodynamics are discussed. The translational relaxation times of density fluctuations show in fact the fragile-to-strong crossover connected to the thermodynamics arising from the existence of two liquids. When considering also rotations, additional crossovers come to play. Mobility-viscosity decoupling is also discussed in supercooled water and aqueous solutions. Finally, the polyamorphism of glassy water is considered through experimental and simulation results both in bulk and in salty aqueous solutions. Grains and grain boundaries are also discussed.

Diffusion in dense supercritical methane from quasi-elastic neutron scattering measurements.

Methane, the principal component of natural gas, is an important energy source and raw material for chemical reactions. It also plays a significant role in planetary physics, being one of the major constituents of giant planets. Here, we report measurements of the molecular self-diffusion coefficient of dense supercritical CH4 reaching the freezing pressure. We find that the high-pressure behaviour of the self-diffusion coefficient measured by quasi-elastic neutron scattering at 300 K departs from that expected for a dense fluid of hard spheres and suggests a density-dependent molecular diameter. Breakdown of the Stokes-Einstein-Sutherland relation is observed and the experimental results suggest the existence of another scaling between self-diffusion coefficient D and shear viscosity η, in such a way that Dη/ρ=constant at constant temperature, with ρ the density. These findings underpin the lack of a simple model for dense fluids including the pressure dependence of their transport properties.

On the link between polyamorphism and liquid-liquid transition: The case of salty water.

We investigate a LiCl:6D2O water solution in the deep undercooled regime as a function of pressure by neutron diffraction, small angle neutron scattering, and molecular dynamics simulations. We probe the structure of the undercooled liquid and the existence of density fluctuations in the system along isotherms just above the observed first-order-like polymorphic transition in the solid between a high density amorphous (s-HDA) and a very high density amorphous (s-VHDA) form [L. E. Bove et al., Phys. Rev. Lett. 106, 125701 (2011)]. We observe a continuous pressure evolution of the structure factor of the liquid and the absence of density fluctuations when crossing the continuation line of the s-HDA and s-VHDA boundary. These results indicate that no liquid-liquid transition is observed in the system in correspondence with the solid polyamorphism.

Observation of methane filled hexagonal ice stable up to 150 GPa.

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4-D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at ∼40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III → MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

In Vivo Water Dynamics in Shewanella oneidensis Bacteria at High Pressure.

Following observations of survival of microbes and other life forms in deep subsurface environments it is necessary to understand their biological functioning under high pressure conditions. Key aspects of biochemical reactions and transport processes within cells are determined by the intracellular water dynamics. We studied water diffusion and rotational relaxation in live Shewanella oneidensis bacteria at pressures up to 500 MPa using quasi-elastic neutron scattering (QENS). The intracellular diffusion exhibits a significantly greater slowdown (by -10-30%) and an increase in rotational relaxation times (+10-40%) compared with water dynamics in the aqueous solutions used to resuspend the bacterial samples. Those results indicate both a pressure-induced viscosity increase and slowdown in ionic/macromolecular transport properties within the cells affecting the rates of metabolic and other biological processes. Our new data support emerging models for intracellular organisation with nanoscale water channels threading between macromolecular regions within a dynamically organized structure rather than a homogenous gel-like cytoplasm.

Salt- and gas-filled ices under planetary conditions.

In recent years, evidence has emerged that solid water can contain substantial amounts of guest species, such as small gas molecules-in gas hydrate structures-or ions-in salty ice structures-and that these 'filled' ice structures can be stable under pressures of tens of Gigapascals and temperatures of hundreds of Kelvins. The inclusion of guest species can strongly modify the density, vibrational, diffusive and conductivity properties of ice under high pressure, and promote novel exotic properties. In this review, we discuss our experimental findings and molecular dynamics simulation results on the structures formed by salt- and gas-filled ices, their unusual properties, and the unexpected dynamical phenomena observed under pressure and temperature conditions relevant for planetary interiors modelling. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

Calorimetric study of water's two glass transitions in the presence of LiCl.

A DSC study of dilute glassy LiCl aqueous solutions in the water-dominated regime provides direct evidence of a glass-to-liquid transition in expanded high density amorphous (eHDA)-type solutions. Similarly, low density amorphous ice (LDA) exhibits a glass transition prior to crystallization to ice Ic. Both glass transition temperatures are independent of the salt concentration, whereas the magnitude of the heat capacity increase differs. By contrast to pure water, the glass transition endpoint for LDA can be accessed in LiCl aqueous solutions above 0.01 mole fraction. Furthermore, we also reveal the endpoint for HDA's glass transition, solving the question on the width of both glass transitions. This suggests that both equilibrated HDL and LDL can be accessed in dilute LiCl solutions, supporting the liquid-liquid transition scenario to understand water's anomalies.

Fast methane diffusion at the interface of two clathrate structures.

Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are also expected to play a paramount role in future energy and environmental technologies. Here we report experimental observation of an extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures, namely clathrate structures I and II. Methane translational diffusion-measured by quasielastic neutron scattering at 0.8 GPa-is faster than that expected in pure supercritical methane at comparable pressure and temperature. This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Our results could have implications for understanding the replacement kinetics during sI-sII conversion in gas exchange experiments and for establishing the methane mobility in methane hydrates embedded in the cryosphere of large icy bodies in the Universe.

Dynamical Crossover in Hot Dense Water: The Hydrogen Bond Role.

We investigate the terahertz dynamics of liquid H2O as a function of pressure along the 450 K isotherm, by coupled quasielastic neutron scattering and inelastic X-ray scattering experiments. The pressure dependence of the single-molecule dynamics is anomalous in terms of both microscopic translation and rotation. In particular, the Stokes-Einstein-Debye equations are shown to be violated in hot water compressed to the GPa regime. The dynamics of the hydrogen bond network is only weakly affected by the pressure variation. The time scale of the structural relaxation driving the collective dynamics increases by a mere factor of 2 along the investigated isotherm, and the structural relaxation strength turns out to be almost pressure independent. Our results point at the persistence of the hydrogen bond network in hot dense water up to ice VII crystallization, thus questioning the long-standing perception that hydrogen bonds are broken in liquid water under the effect of compression.

X-ray and Neutron Scattering of Water.

This review article focuses on the most recent advances in X-ray and neutron scattering studies of water structure, from ambient temperature to the deeply supercooled and amorphous states, and of water diffusive and collective dynamics, in disparate thermodynamic conditions and environments. In particular, the ability to measure X-ray and neutron diffraction of water with unprecedented high accuracy in an extended range of momentum transfers has allowed the derivation of detailed O-O pair correlation functions. A panorama of the diffusive dynamics of water in a wide range of temperatures (from 400 K down to supercooled water) and pressures (from ambient up to multiple gigapascals) is presented. The recent results obtained by quasi-elastic neutron scattering under high pressure are compared with the existing data from nuclear magnetic resonance, dielectric and infrared measurements, and modeling. A detailed description of the vibrational dynamics of water as measured by inelastic neutron scattering is presented. The dependence of the water vibrational density of states on temperature and pressure, and in the presence of biological molecules, is discussed. Results about the collective dynamics of water and its dispersion curves as measured by coherent inelastic neutron scattering and inelastic X-ray scattering in different thermodynamic conditions are reported.

The preparation and structure of salty ice VII under pressure.

It is widely accepted that ice, no matter what phase, is unable to incorporate large amounts of salt into its structure. This conclusion is based on the observation that on freezing of salt water, ice expels the salt almost entirely as brine. Here, we show that this behaviour is not an intrinsic physico-chemical property of ice phases. We demonstrate by neutron diffraction that substantial amounts of dissolved LiCl can be built homogeneously into the ice VII structure if it is produced by recrystallization of its glassy (amorphous) state under pressure. Such 'alloyed' ice VII has significantly different structural properties compared with pure ice VII, such as an 8% larger unit cell volume, 5 times larger displacement factors, an absence of a transition to an ordered ice VIII structure and plasticity. Our study suggests that there could be a whole new class of 'salty' high-pressure ice forms.

Damping of sound waves in the terahertz range and strength of the boson peak.

By applying a new two-step line-shape analysis to inelastic neutron and x-ray scattering spectra of glassy systems, we were able to resolve the acoustic excitations from the low-frequency excess modes and to accurately estimate the damping of sound waves in the terahertz frequency range. Using this approach, we estimated the damping parameter for terahertz acoustic waves in a wide class of chemically different glasses and did a quantitative comparison of the results with prediction of theoretical models. By comparing the estimates of the mean-free path of the acoustic modes in different glasses and the corresponding boson peak strengths, we show the existence of a simple correlation between these two quantities. The relationship between attenuation of the terahertz acoustic modes, strength of the boson peak, and fragility is discussed.