Structure and nature of ice XIX Salzmann, Christoph G.; Loveday, John S.; Rosu-Finsen, Alexander ...
Nature communications,
05/2021, Letnik:
12, Številka:
1
Journal Article
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Abstract
Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature ...of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a
Pbcn
-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials.
Methane and water demix under normal (ambient) pressure and temperature conditions because of the polar nature of water and the apolar nature of methane. Recent experimental work has shown, though, ...that increasing the pressure to values between 1 and 2 GPa (10–20 kbar) leads to a marked increase of methane solubility in water, for temperatures which are well below the critical temperature for water. Here, we perform molecular dynamics simulations based on classical force fieldswhich are well-used and have been validated at ambient conditionsfor different values of pressure and temperature. We find the expected increase in miscibility for mixtures of methane and supercritical water; however, our model fails to reproduce the experimentally observed increase in methane solubility at large pressures and below the critical temperature of water. This points to the need to develop more accurate force fields for methane and methane–water mixtures under pressure.
We have performed a series of neutron scattering experiments on supercritical krypton. Our data and analysis allow us to characterize the Frenkel line crossover in this model monatomic fluid. The ...data from our measurements was analyzed using Empirical Potential Structure Refinement to determine the short- and medium-range structure of the fluids. We find evidence for several shells of neighbors which form approximately concentric rings of density about each atom. The ratio of second to first shell radius is significantly larger than in any crystal structure. Modeling krypton using a Lennard-Jones potential is shown to give significant errors, notably that the liquid is overstructured. The true potential appears to be longer ranged and with a softer core than the 6–12 powerlaws permit.
Methane and water demix under normal (ambient) pressure and temperature conditions because of the polar nature of water and the apolar nature of methane. Recent experimental work has shown, though, ...that increasing the pressure to values between 1 and 2 GPa (10-20 kbar) leads to a marked increase of methane solubility in water, for temperatures which are well below the critical temperature for water. Here, we perform molecular dynamics simulations based on classical force fields-which are well-used and have been validated at ambient conditions-for different values of pressure and temperature. We find the expected increase in miscibility for mixtures of methane and supercritical water; however, our model fails to reproduce the experimentally observed increase in methane solubility at large pressures and below the critical temperature of water. This points to the need to develop more accurate force fields for methane and methane-water mixtures under pressure.
We have performed a neutron scattering experiment on supercritical fluid nitrogen at 160 K (1.27 T C) over a wide pressure range (7.8 MPa/0.260 g/mL–125 MPa/0.805 g/mL). This has enabled us to study ...the process by which nitrogen changes from a fluid that exhibits gaslike behavior to one that exhibits rigid liquidlike behavior at a temperature close to, but above, the critical temperature by crossing the Widom lines followed by the Frenkel line on pressure (density) increase. We find that the Frenkel line transition is indicated by a transition to a regime of rigid liquidlike behavior in which the coordination number remains constant within error, in agreement with our previous work at 300 K. The Frenkel line transition takes place at approximately the same density at 160 and 300 K. The data do not conclusively show an additional transition at the location of the known Widom lines. We find that behavior remains gaslike until the Frenkel line is crossed and our data support the hypothesis that Widom line transitions are density increase-driven.
The molecular structure of dense homogeneous fluid water-methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa. A mixed state ...with a fully H-bonded water network is revealed. The hydration shell of the methane molecules is, however, revealed to be pressure-dependent with an increase in the water coordination between 1.7 and 2.2 GPa. In parallel,
molecular dynamics simulations have been performed to provide insight into the microscopic mechanisms associated with the phenomenon of mixing. These calculations reproduce the observed phase change from phase separation to mixing with increasing pressure. The calculations also reproduce the experimentally observed structural properties. Unexpectedly, the simulations show mixing is accompanied by a subtle enhancement of the polarization of methane. Our results highlight the key role played by fine electronic effects on miscibility and the need to readjust our fundamental understanding of hydrophobicity to account for these.
The well-known expansion of water on cooling below 277K is one of several peculiar properties that could signal a second critical point near 220K and 0.1GPa in pressure, deep in the supercooled ...liquid phase. Evidence for this would be a first-order transition line between two distinct supercooled liquids at temperatures below the critical point. As that lies below the minimum crystallization temperature, experimental tests have instead used low- and high-density amorphous ices--LDA and HDA--as proxies for the supercooled liquids. But numerous studies over the past decade have not yielded a clear consensus about the nature of the HDA/LDA transition. Here we identify a previously uncharacterized state of high-density amorphous ice obtained if HDA is annealed at pressures near 2 kbar. The transition between this annealed HDA and LDA is strikingly different from the behaviour found in earlier work, in a way that favours the two-liquid model. PUBLICATION ABSTRACT
We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C 0 at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common ...water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move. It has a network that is unrelated to any experimentally known ice, silica, or zeolite network but is instead related to two Zintl compounds. Both hydrates are found to be stable in electronic structure calculations, with hydration ratios in very good agreement with experiment.
We present simulated x-ray diffraction patterns (XRD) from molecular dynamics studies of phase transformations in hydrogen at room temperature. Phase changes can be easily identified in simulation, ...by directly imaging the atoms and measuring correlation functions. We show that the room-temperature XRD patterns for hydrogen phases I, III, IV, and V are very similar. The signatures of the transformations in XRD are weak peaks and superlattice reflections denoting symmetry breaking from the hexagonal-closed-packed (hcp) phase I, and a pronounced change in the c/a ratio. The XRD patterns implied by molecular dynamics calculations are very different from those arising from the static minimum enthalpy structures found by structure searching. Simulations also show that within phase I, the molecules become increasingly confined to the basal plane and suggest the possibility of an unusual critical point terminating the phase I-III boundary line. With these results, we propose a paradigm shift, i.e., that the predictions from density functional theory calculations should be seen as the most likely hypothesis. Specifically, we show that recent experimental results support the picture advanced by molecular dynamics simulations, and are inconsistent with the interpretation of an isostructural hcp transformation.
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