Dennis D. Klug

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, Saturns 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 Titans 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 Titans 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 Titans atmospheric methane.

Molecular-dynamics study of structure II hydrogen clathrates

Molecular-dynamics simulations are used to study the stability of structure II hydrogen clathrates with different H2 guest occupancies. Simulations are done at pressures of 2.5 kbars and 1.013 bars and for temperatures ranging from 100 to 250 K. For a structure II unit cell with 136 water molecules, H2 guest molecule occupancies of 0-64 are studied with uniform occupancies among each type of cage. The simulations show that at 100 K and 2.5 kbars, the most stable configurations have single occupancy in the small cages and quadruple occupancy in the large cages. The optimum occupancy for the large cages decreases as the temperature is raised. Double occupancy in the small cages increases the energy of the structures and causes tetragonal distortion in the unit cell. The spatial distribution of the hydrogen guest molecules in the cages is determined by studying the guest-water and guest-guest radial distribution functions at various temperatures.

The mechanisms for pressure-induced amorphization of ice Ih

There has been considerable interest in the structure of liquid water at low temperatures and high pressure following the discovery of the high-density amorphous (HDA) phase of ice Ih (ref. 1). HDA ice forms at a pressure close to the extrapolated melting curve of ice, leading to the suggestion that it may have structure similar to that of dense water. On annealing, HDA ice transforms into a low-density amorphous (LDA) phase with a distinct phase boundary,. Extrapolation of thermodynamic data along the HDA–LDA coexistence line into the liquid region has led to the hypothesis that there might exist a second critical point for water and the speculation that liquid water is mixture of two distinct structures with different densities,. Here we critically examine this hypothesis. We use quasi-harmonic lattice-dynamics calculations to show that the amorphization mechanism in ice Ih changes from thermodynamic melting for T > 162 K to mechanical melting at lower temperatures. The vibrational spectra of ice Ih, LDA ice and quenched water also indicate a structure for LDA ice that differs from that of the liquid. These results call into question the validity of there being a thermodynamic connection between the amorphous and liquid phases of water.

Molecular-dynamics simulations of binary structure II hydrogen and tetrahydrofurane clathrates

The binary structure II hydrogen and tetrahydrofurane (THF) clathrates are studied with molecular-dynamics simulations. Simulations are done at pressures of 120 and 1.013 bars for temperatures ranging from 100 to 273 K. For the small cages of the structure II unit cell, H2 guest molecule occupancies of 0, 16 (single occupancy), and 32 (double occupancy) are considered. THF occupancies of 0-8 in the large cages are studied. For cases in which THF does not occupy all large cages in a unit cell, the remaining large cages can be occupied with sets of four H2 guest molecules. The unit-cell volumes and configurational energies are compared in the different occupancy cases. Increasing the small cage occupancy leads to an increase in the unit-cell volume and thermal-expansion coefficient. Among simulations with the same small cage occupancy, those with the large cages containing 4H2 guests have the largest volumes. The THF guest molecules have a stabilizing effect on the clathrate and the configurational energy of the unit cell decreases linearly as the THF content increases. For binary THF + H2 clathrates, the substitution of the THF molecules in the large cages with sets of 4H2 molecules increases the configurational energy. For the binary clathrates, various combinations of THF and H2 occupancies have similar configurational energies.

The uncoupled O–H stretch in ice VII. The infrared frequency and integrated intensity up to 189 kbar

The integrated absorptivity of the O–H stretching vibration of a dilute solution of HDO in D2O ice VII and the peak frequencies of the O–H stretching, bending, and rotational vibrations of HDO have been measured in diamond anvils in the range 23.6–189 kbar. The integrated absorptivity of the O–H stretching vibration increases from 55 to 153 cm μmol−1 in this range, i.e., by a factor of 2.78, and the effective charge increases from 0.61 to 0.94 electronic charges, i.e., by the factor 1.54. The effective charge of the O–H stretch as a function of pressure can be correlated within the experimental precision by a simple model in which the dipole moment of an ice crystal changes sinusoidally as the protons are moved in phase along the O––O lines. It predicts that the change of dipole moment with oxygen–proton distance, when the oxygen–proton distance is small, is 1.1 D A−1. The theory could be tested by calculating the dipole moment of a neon atom caused by adding a small dipole moment to the nucleus. The unco...

Pressure-induced phase transitions in clathrate hydrates

Ice I transforms to a high‐density amorphous phase when pressed to 10 kbar at 77 K. Similar transformations in structure I and structure II clathrate hydrates have been studied by pressing samples in a piston‐cylinder apparatus and by molecular dynamics simulations. The simulations were also carried out on structure I and II empty lattices. The hydrates and the empty lattices were found to transform to high‐density phases under pressure at 77 K. The high‐density phases of the empty lattices could be recovered at zero pressure, as is possible in the case of high‐density amorphous phase of ice. However, it was not possible to recover the high‐density phases of the hydrates at zero pressure. Instead, they reverted back to their original crystalline structures when the pressure was released. The molecular dynamics results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible...

High-pressure polymeric phases of carbon dioxide

Understanding the structural transformations of solid CO2 from a molecular solid characterized by weak intermolecular bonding to a 3-dimensional network solid at high pressure has challenged researchers for the past decade. We employ the recently developed metadynamics method combined with ab initio calculations to provide fundamental insight into recent experimental reports on carbon dioxide in the 60–80 GPa pressure region. Pressure-induced polymeric phases and their transformation mechanisms are found. Metadynamics simulations starting from the CO2-II (P42/mnm) at 60 GPa and 600 K proceed via an intermediate, partially polymerized phase, and finally yield a fully tetrahedral, layered structure (P-4m2). Based on the agreement between calculated and experimental Raman and X-ray patterns, the recently identified phase VI [Iota V, et al. (2007) Sixfold coordinated carbon dioxide VI. Nature Mat 6:34–38], assumed to be disordered stishovite-like, is instead interpreted as the result of an incomplete transformation of the molecular phase into a final layered structure. In addition, an α-cristobalite-like structure (P41212), is predicted to be formed from CO2-III (Cmca) via an intermediate Pbca structure at 80 GPa and low temperatures (<300 K). Defects in the crystals are frequently observed in the calculations at 300 K whereas at 500 to 700 K, CO2-III transforms to an amorphous form, consistent with experiment [Santoro M, et al. (2006) Amorphous silica-like carbon dioxide. Nature 441:857–860], but the simulation yields additional structural details for this disordered solid.

Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2 guest molecules in order to investigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O–N2 radial distribution function, and therefore the structure in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs o...

The structure and dynamics of silica polymorphs using a two‐body effective potential model

The performance of a recently proposed two‐body potential model for SiO2 [van Beest et al., Phys. Rev. Lett. 64, 1995 (1990)] was critically evaluated through the calculation of the static and dynamical properties of several polymorphs of SiO2 using molecular dynamics methods. It was found that the calculated static structures are in excellent agreement with experiments. In particular, the pressure–volume equations of state for α‐quartz, cristobalite, and stishovite, the pressure‐induced amorphization transformation in α‐quartz and thermally induced the α→β transformation in cristobalite are well reproduced by this model. The calculated vibrational spectra are in fair agreement with experiments. The strengths and the weaknesses of the potential will be presented and discussed.

Transformation of ice VIII to amorphous ice by ‘‘melting’’ at low temperature

The melting curve of ice VIII near 25 kbar, which has a positive slope, has been estimated from various thermodynamic data and extrapolated approximately to 60 K at zero pressure. When ice VIII is heated from 77 K and ambient pressure it should, therefore, ‘‘melt,’’ presumably below the glass transition. It has been shown to do so, and transforms to low‐density amorphous ice when heated to about 125 K at ambient pressure. Ice I, whose melting curve has a negative slope, is already known to transform to a high‐density amorphous ice at 77 K and 10 kbar, and so this and the present transformation are symmetrical equivalents.