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High-pressure and high-temperature experiments were performed to investigate the stability of methane hydrate under 6–40 GPa and 298–573 K using synchrotron X-ray powder diffraction in combination with a diamond anvil cell. The results thus obtained revealed that methane hydrate decomposes into solid methane and ice VII at temperatures considerably lower than the melting curves of solid methane and ice VII in the pressure range of 6–40 GPa.

The sensing capabilities of gold nanorods under high-pressure conditions were investigated in methanol-ethanol mixtures (up to 13 GPa) and in water (up to 9 GPa) through their optical extinction. The longitudinal SPR band of AuNR exhibits a redshift with pressure which is the result of two main competing effects: compression of the conduction electrons which increases the bulk plasma frequency (blueshift) and increase in the solvent density (redshift). The variation in de SPR peak wavelength allows us to estimate the bulk modulus of the gold nanoparticles with a precision of 10 % and to obtain analytical functions providing the pressure dependence of the refractive index of water in three phases: liquid, ice VI and ice VII. Furthermore, the SPR band shows abrupt jumps at the liquid to ice phase VI and ice phase VII transitions, which are in accordance with the first-order character of these transitions. Financial support from Project PGC2018-101464-B-I00 (FEDER) and MALTA-Consolider Team (RED2018-102612-T) of the Ministerio de Ciencia, Innovación y Universidades is acknowledged.

The thermal conductivity of partially dissociated and ionised water is calculated in a large-scale study using density functional theory (DFT)-based molecular dynamics (MD) simulations. In doing so, the required heat current of the nuclei is calculated by mapping the effective particle interactions from the DFT-MD simulations onto classical pair potentials. It is demonstrated that experimental and theoretical thermal conductivity data for liquid heavy water and for ice VII are well reproduced with this efficient procedure. Moreover, the approach also allows for an illustrative interpretation of the characteristics of the thermal conductivity in the dense chemically reacting fluid. The thermodynamic conditions investigated here range from densities between 0.2 and 6 g cm−3 and temperatures between 600 and 50 000 K, which includes states highly relevant for understanding the interiors of water-rich planets like Uranus and Neptune and exoplanets of similar composition.

Three high-pressure structures of methane hydrate, a hexagonal structure (str. A) and two orthorhombic structures (str. B and str. C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str. I) decomposed into str. A and fluid at 0.8 GPa. Str. A transformed into str. B at 1.6 GPa, and str. B further transformed into str. C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The bulk moduli, K0, for str. I, str. A, and str. C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively.

Invariant, three-phase melting curves, of ice VI in equilibrium with solid CO2, of ice VII in equilibrium with solid CO2, and of solid CO2 in simultaneous equilibrium with a majority aqueous and a majority CO2 fluid, were explored in the binary system of carbon dioxide and water. Diamond-anvil cells were used to develop pressures of 5 GPa. Water exhibits a large melting temperature depression (73°C less than its pure melting temperature of 253°C at 5 GPa) indicative of large concentrations of CO2 in the aqueous solution. The melting point of water-saturated CO2 does not show a measureable departure from that of the pure system at temperatures lower than ~200°C and only 10°C at 5 GPa (from 327°C).

We determine the structural data of seven of the polymorphs of ice (ice Ih, ice Ic, ice IX, ice II, ice VI, ice VII and ice VIII) from ab initio calculations. The dynamical properties have been analysed within the harmonic approximation via a finite-difference evaluation of dynamical matrices from atomic forces. Supercells are used to model the various ordered and disordered phases considered. Calculations are done at zero pressure in order to compare directly with neutron scattering studies performed on recovered phases. The normal modes are resolved into projections chosen to display their intra- and inter-molecular character. Further projections are performed for ice VI, ice VII and ice VIII to probe the interactions between sub-lattices. Trends in the dynamical results are discussed in terms of changes in the structural complexity of the various phases considered.

In situ high-pressure measurements of the far-infrared absorption spectra in ice VII and VIII covering the region of translational lattice modes are made by utilizing an infrared beam line of synchrotron radiation at the Institute for Molecular Science (Okazaki). Although the lattice dynamics are almost same for both the phases, large spectral changes are observed on the occasion of the proton order-disorder transition. In the proton disordered phase (ice VII), absorption spectra reflecting a density of states of lattice modes are obtained. The proton ordered phase (ice VIII) gives rise to an absorption peak due to the optical phonon at the r point. The anomalous pressure dependence of the phonon energy in ice VIII indicates the occurrence of a lattice instability below 2.7 GPa with decreasing pressure, which is supported by the recent neutron inelastic scattering experiments by Besson et al.

Incoherent inelastic neutron-scattering spectra of ice VII and VIII have been obtained at an in situ pressure of 2.2 ± 0.1 GPa on the TFXA spectrometer at the pulsed neutron source, ISIS. Comparisons with recovered phases (at ambient pressure) show large shifts of the phonon modes. For instance, the optic peak for ice VIII is shifted from 25 to 32 meV (dω/ω = 0.20 ± 0.01). The coefficient of increase of the frequency of the mode with pressure, dω/dp, is 2.3 meV/GPa (or 18.5 cm−1/GPa). In ice VII, the proton-disordered form of ice VIII, the spectra, measured under the same pressure, show that: firstly, the sharp features of the librational band in the region 50–100 meV disappear, indicating that the protons are indeed disordered as is found in other proton-disordered forms of ice; secondly, a weak higher-energy optic mode at 49.5 meV appears due to the presence of the second proton configuration as seen in other proton-disordered ice Ic.

High brilliance and high stability make the European Synchrotron Radiation Facility an ideal instrument for structural studies at very high pressures, especially for low-Z materials were high compressibility and poor scattering power are associated with very small sample volumes. We review here measurements on hydrogen, oxygen and ice VII to pressures beyond 100 GPa using single crystal and powder techniques. A brief discussion of future prospects is also included.

High pressure experiments were performed with deuterated ice VII using diamond anvil cell in a pressure range of 0.1 MPa to 60 GPa at room temperature in order to further understanding a long-argued issue of phase change in ice VII at approximately 11 GPa. In-situ X-ray diffractometry revealed splitting of diffraction lines of cubic ice VII above 11GPa, which were indexed as a tetragonal structure. The tetragonal structure survived at least up to 60 GPa. Pressure versus volume curves of cubic and tetragonal structures were obtained and the bulk moduli were calculated. Raman spectroscopy showed that the full width at half maximum of O-D vibrational mode decreased with increasing pressure, and showed minimum value at 11 GPa, and then it became broader again above this pressure. The squared vibrational frequency changed linearly with pressure, and the slope changed at about 14 GPa, indicating existence of phase change. All experimental results evidently supported the existence of phase change of ice VII at approximately 11 GPa.

A new assembly for ultrasonic measurements of water and ice on multi-anvil apparatus has been designed, and the ultrasonic compressional wave velocities in water and ice up to 4.2 GPa and 500 K are achieved. The pressure of the sample is calibrated by the melting curve of ice VII and the transformation pressure of liquid to solid at ambient temperature. The continuous changing process of the sound velocity transforming from water into ice at high pressure is achieved, and the experimental results of sound velocities at high pressure at room temperature on the melting curve of water are consistent with the previous works by Brillouin scattering. It is believed that our new method of ultrasonic measurements of water is reliable, and worth being used for studying more liquids at high pressure.

The phase diagram of water is both unusual and complex, exhibiting a wide range of polymorphs including proton-ordered or disordered forms. In addition, a variety of stable and metastable forms are observed. The richness of H2O phases attests the versatility of hydrogen bonded network structures that include kinetically stable amorphous ices. In addition, natural gas hydrates are observed upon cooling and/or pressurization. The structures of gas hydrates are quite different from ices, yet guest molecules are enclathrated inside hydrogen bonded water cages. Hence, the presence of small impurity molecules can affect strongly the behaviours of hydrogen bonding and alter the phase diagram and phase boundaries of H2O. In this paper, we report the effect of methane impurity on solidification and melting of H2O under dynamic loading conditions. The presence of impurity mainly affects the growth rate of metastable ice VII, slowing down from 1.4 m/s in pure H2O to 0.26 m/s in impure H2O, neither the occurrence of metastable ice VII nor the melting of ice VI.

We investigated freezing of pure glycerol as well as glycerol-water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0-10 GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5 GPa for pure glycerol to 6.5 GPa for the 3:1 GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2 GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H(2)O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0-7 GPa range, including at low temperatures.

Phase transitions of high-pressure ices and water were observed using diamond-anvil pressure cells. It was discovered that ice VII and water can coexist at room temperature in the metastable state within the stable region of ice VI, and that single crystals of ice VII can be grown. It was confirmed by X-ray diffraction precession photography that these crystals are single crystals. The crystal habits of the single crystals in equilibrium with water were observed. Millerian indices could be assigned to some faces of these crystals. Twin mosaics of ice VIII were observed under crossed polarized nicols.

In the model each molecule has four tetrahedrally-directed bonding directions and can form asymmetrical bonds with suitable oriented molecules on nearest neighbour sites of a body centred cubic lattice. An open four-bonded structure, like ice I(c), with half the BCC lattice sites vacant and a close packed structure, like ice VII, with interpenetrating four-bonded networks are thus possible. In the present fluid phase theory all BCC lattice sites are equivalent and these structures are present only as short range order, together with disordered regions due to molecular disorientation and displacement. Energies of bonding and of interaction with nonbonded nearest neighbours and also a close packing energy are introduced. The state surface of the fluid is investigated using a generalized first order statistical approximation based on a group of four sites. The isothermal compressibility and the coordination number (i.e. mean number of nearest neighbours of a given molecule) are also calculated. For suitable energy values there are maxima on the curves of density against temperature at constant pressure both in the liquid and in the supercritical region. In agreement with experiment this anomalous behaviour of the density ceases at a certain supercritical pressure. The observed minimum in the isothermal compressibility, both in the liquid and supercritical regions, occurs in the model.

The ice VII-VIII phase transition was observed by the X-ray precession method using diamond-anvil pressure cells. Single crystals of ice VII–cubic, a=3.350±0.005 Å (sample A) and a=3.360±0.005 Å (sample B)–transformed into twin-mosaic crystals of ice VIII–tetragonal, a=4.68±0.01 Å and c=6.81±0.01 Å (sample A) and a=4.70±0.01 Å and c=6.84±0.01 Å (sample B)–. The formation of the twin mosaics caused some misorientation and distortion in the crystal. These disappeared during the VIII→VII transition. Thermal hysteresis of the transition temperature was observed.