Y. P. Handa

Laboratory analysis of a naturally occurring gas hydrate from sediment of the Gulf of Mexico

Intact natural gas hydrate of structure II made up more than 80% of the water present in nearbottom core material recovered from the Gulf of Mexico. Solid-state carbon-13 nuclear magnetic resonance with magic-angle spinning gave resolved lines from ethane, propane and isobutane and apparently from methane in the two sizes of cage in the hydrate lattice. Low-temperature dielectric loss peaks were assigned to reorientation of encaged propane, isobutane, H2S and CO2 molecules.

The structure of deuterated methane–hydrate

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, and 150 K. Precise crystallographic parameters of the ice-like D 2 O lattice and the thermal parameters of the encaged methane molecules have been obtained. The parameters of the host lattice differ only slightly from values found for hydrates with asymmetric guests included, which leads to the conclusion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperatures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages. The orientational scattering length density of the CD 4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Huller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane molecules. The large and asymmetric cage leads to a somewhat more pronounced modulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly unchanged from 2 to 150 K. At 200 K we observed the time-resolved decomposition of the hydrate structure I into hexagonal ice Ih.

Crystallographic Studies of Clathrate Hydrates. Part I

Abstract Low-temperature neutron and X-ray diffraction studies show the gas hydrates of oxygen and nitrogen to be structure II (Fd3m), as recently found also for the hydrates of the small argon and krypton molecules. New lattice parameters of three structure I and 14 structure II hydrates from powder X-ray diffraction at 170 K are reported. The thermal expansion coefficient of tetrahydrofuran hydrate was determined from X-ray diffraction at some 50 temperatures between 18 and 263 K and found to be three times as great as for ice near 100 K and 30% higher near 250 K. Lattice parameters qf 40 type II clathrate hydrates are compared at 0°C and found to lie within 0.10 A of 17.30 A.

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...

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.

Nature of the transformations of ice I and low‐density amorphous ice to high‐density amorphous ice

The changes of enthalpy at the irreversible transformation of high‐density amorphous ice to low‐density amorphous ice and of low‐density amorphous ice to ice Ih have been measured in a Tian–Calvet heat‐flow calorimeter. The equilibrium pressures of the two transformations have been estimated from the measured enthalpy and volume changes, neglecting the unknown, but small, entropy changes, as about 5 and 2 kbar, respectively. The course of the transformations has been followed in a piston‐cylinder apparatus having an internal diameter of either 50 or 12 mm. The transformations are extraordinarily sharp when they are done slowly, the transformation pressure being constant to ±50 bar in 5 or 10 kbar in several transformations, although sometimes a sample transformed in two steps that were a few hundred bars apart. It seems that the activation volumes of the transformations are large and negative, and so the mechanism of the transformations is highly cooperative.

Phase transitions in solid C60

Abstract Thermal analysis of solid C 60 (buckminsterfullerene) has been conducted from 150 to 1000 K with a differential scanning calorimeter. The onsets of two endothermic transitions are observed at 221 and 252 K. Low temperature X-ray powder diffraction experiments show little change in the lattice structures associated with these transformations. Our results also show that solid C 60 is stable up to 1000 K, contrary to a recent report.

The lattice dynamics of clathrate hydrates. An incoherent inelastic neutron scattering study

Abstract The vibrational densities of states in the lattice translation region below 130 cm −1 (3 THz) for structure I xenon hydrate and structure II krypton hydrate have been determined from incoherent inelastic neutron scattering experiments. The main features in the lattice vibrations are found to shift to higher frequencies when compared to ice I h . The shift is due to repulsive interactions between the water molecules and the guests which help to stabilize the hydrate structure. The experimental observation is in good accord with that predicted from earlier molecular dynamics calculations.

Some structural and thermodynamic studies of clathrate hydrates

X-ray and neutron diffraction studies show argon and krypton to preferentially form clathrate hydrates of structure II, rather than structure I as previously assumed; methane and hydrogen sulphide do form structure I. Re-examination of solid-solution thermodynamic theory shows that structure II is basically the more stable; structure I is generally formed only when the guest molecule is in the size range that favours occupancy of the 14-hedral over the 12-hedral cages. For molecules too large to enter the 12-hedra the relative stability of structure II is greatest at 0°C, in agreement with the observed sequence of change of stability of cyclopropane hydrate: I to II at -16° and II to I at 1.5°. Carbon dioxide hydrate is observed to decompose on prolonged standing at 105K in accord with the low-temperature instability predicted by Miller.

Some physical and thermophysical properties of clathrate hydrates

Oxygen, nitrogen and air, like argon and krypton (1), are found to preferentially form gas hydrates of structure II, rather than structure I as previously expected for gas hydrates of small guest molecules. Lattice parameters from X-ray diffraction are given in the Table.