Werner F. Kuhs

The structure of a new phase of ice

Ice has eleven known crystalline phases (Fig. 1), in which the watermolecules are linked through hydrogen bonds into tetrahedralframeworks. Thisuncommonly large number of different solid phases attests to thestructural versatility of the water molecule. Here we reportthe identification of a new, twelfth phase of crystalline ice inthe pressure range 0.2–0.6 GPa. Thetopology of this phase is unlike that of any of the knownphases, and contains a mixture of five- and seven-membered ringsof water molecules. It has a density similar to that of iceIV, which also occurs in this pressure range within thestability region of ice V. Both phases are likely to bemetastable with respect to the less-dense ice V. This region ofthe water phase diagram thus provides a potential model system forexperimental and theoretical studies ofmetastability.

Ice structures, patterns, and processes: A view across the icefields

European Science Foundation workshop Euroice 2008 held in Granada, Spain from 1–4 October 2008; Spanish national project, Hielocris, financed by the Consejo Superior de Investigaciones Cientificas; funding from FWF, Austria (No. P23027); MINCINN, Spain (No. FIS2010-16455, No. PR2010-0012, and No. FIS2010-22322-528C02-02); and SNSF, Switzerland (No. 200021121857)

Extent and relevance of stacking disorder in “ice Ic”

A solid water phase commonly known as “cubic ice” or “ice Ic” is frequently encountered in various transitions between the solid, liquid, and gaseous phases of the water substance. It may form, e.g., by water freezing or vapor deposition in the Earth’s atmosphere or in extraterrestrial environments, and plays a central role in various cryopreservation techniques; its formation is observed over a wide temperature range from about 120 K up to the melting point of ice. There was multiple and compelling evidence in the past that this phase is not truly cubic but composed of disordered cubic and hexagonal stacking sequences. The complexity of the stacking disorder, however, appears to have been largely overlooked in most of the literature. By analyzing neutron diffraction data with our stacking-disorder model, we show that correlations between next-nearest layers are clearly developed, leading to marked deviations from a simple random stacking in almost all investigated cases. We follow the evolution of the stacking disorder as a function of time and temperature at conditions relevant to atmospheric processes; a continuous transformation toward normal hexagonal ice is observed. We establish a quantitative link between the crystallite size established by diffraction and electron microscopic images of the material; the crystallite size evolves from several nanometers into the micrometer range with progressive annealing. The crystallites are isometric with markedly rough surfaces parallel to the stacking direction, which has implications for atmospheric sciences.

Ice perfection and onset of anomalous preservation of gas hydrates

Anomalous preservation is the well-established but little-understood phenomenon of a long-term stability of gas hydrates outside their thermodynamic field of stability. It occurs after some initial decomposition into ice in the temperature range between 240 and 273 K. In situ neutron diffraction experiments reveal that the low-temperature on-set of this effect coincides with the annealing of stacking faults of the ice formed initially. The defective, stacking-faulty ice below 240 K apparently does not present an appreciable diffusion barrier for gas molecules while the annealed ordinary ice Ih above this temperature clearly hinders gas diffusion. This is supported by further experiments showing that the so-called ice Ic formed from various high-pressure phases of ice, gas hydrates or amorphous ices does transform fully to ordinary ice Ih only at temperatures near 240 K, i.e. at distinctly higher temperatures than generally assumed. In this light, some quite disparate observations on the transformation process from ice Ic to ice Ih can now be better understood. The transformation upon heating is a multistep-process and its details depend on the starting material and the sample history. This ‘memory’ is finally lost at approximately 240 K for laboratory time-scale experiments.

Cage Occupancy and Compressibility of Deuterated N2-Clathrate Hydrate by Neutron Diffraction

This paper reports pressuredependent high resolution neutron diffraction work onN2-clathrates, which for the first time providesnumbers on the compressibility as well as the locationand degree of filling of the guest molecules in thesmall and large cages. N2-clathrates crystallize,at least at lower pressures and temperatures near0 °C, in the Stackelberg type II structure.However, during the diffraction experiments we haveobserved the transient and partial formation of thevon Stackelberg type I N2-clathrate at pressuresexceeding several hundred bar. The filling of thesmall cages in the type II clathrate roughly followsa Langmuir isotherm. In contrast to most previousassumptions there is strong evidence that the largecages are doubly occupied in both type I and type IIN2-clathrates. The observed filling can be fittedreasonably well by a two-constant Langmuir model.

The formation of meso‐ and macroporous gas hydrates

We present results of experimental studies on the formation of gas hydrates (clathrates) at conditions of geophysical interest. Clathrate hydrates formed by a reaction of gas at ice Ih surfaces are always found to be mesoporous to macroporous with pores sizes between 100 to 400 nm and pore volumes of approximately 25–40% for CH4, Ar and N2 hydrate, and smaller pores of 20 to 100 nm with a porosity of approximately 10–20% for CO2 hydrate. The three-dimensional sponge-like microstructure occurs in single crystalline grains of typically a few µm size and was observed by field-emission scanning electron microscopy. It forms over a wide range of p-T conditions below the ice Ih melting. The porous microstructure is stable for at least several months, even close to the clathrate decomposition, and is proposed to be formed by local differences in the energy balance between hydrate formation and ice decompositon. The results presented are considered of potential major importance for the understanding of the behaviour of natural gas hydrates found e.g. in polar ice sheets and permafrost regions, and also in some celestial bodies.

The structure and ordering of ices III and V

The structures of ices III and V have been studied under their thermodynamic conditions of stability by neutron diffraction. The results clearly indicate partial ordering of the water molecule orientations for both ice structures. For ice V the ordering is both pressure and temperature dependent, while no significant changes in ordering were noted for ice III within the small region of stability. No reduction in symmetry, necessary for complete orientational ordering, was observed for ice V at low temperatures. The ordering behavior of ice V at low temperatures (<150 K), when considered in conjunction with dielectric measurements at high temperatures, suggests that while relaxation is achieved predominantly through the diffusion of rotational defects at high temperatures, the mechanism at low temperatures appears to be the migration of ionic defects which require only a small activation energy for mobilization.

In situ structural properties of N2-, O2-, and air-clathrates by neutron diffraction

The crystal structures of type II N2-,O2-, and air-clathrate hydrates have been studied under their thermodynamic conditions of stability by neutron powder diffraction. The isothermal compressibilities were deduced from the refined lattice constants and found to be significantly different for N2 and O2 clathrate hydrates. The water host lattice structure rearranges as a function of pressure with a resulting change of the volume ratio of small and large cages. The cage fillings were established on an absolute scale for the first time. The large cages were found to be partly occupied by two molecules. The results are compared with predictions based on the statistical thermodynamic theory of van der Waals and Platteeuw. While the filling follows the predicted trend significant differences exist in detail.

Experimental studies on the formation of porous gas hydrates

Abstract Gas hydrates grown at gas-ice interfaces were examined by electron microscopy and found to have a sub-micrometer porous structure. In situ observations of the formation of porous CH4- and CO2- hydrates from deuterated ice Ih powders were made at different pressures and temperatures, using time-resolved neutron diffraction data from the high-flux D20 diffractometer (ILL, Grenoble) as well as in-house gas consumption measurements. The CO2 experiments conducted at low temperatures are particularly important for settling the open question of the existence of CO2 hydrates on Mars. We found that at similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. A phenomenological model for the kinetics of the gas hydrate formation from powders of spherical ice particles is developed with emphasis on ice-grain fracturing and sample-consolidation effects due to the outward growth of gas hydrate. It describes (1) the initial stage of fast crack-filling and hydrate film spreading over the ice surface and the two subsequent stages which are limited by (2) the clathration reaction at the ice-hydrate interface and/or by (3) the diffusive gas and water transport through the hydrate shells surrounding the shrinking ice cores. In the case of CO2-hydrate, the activation energies of the ice-surface coating in stage 1 are estimated to be 5.5 kJ/mol at low temperatures and 31.5 kJ/mol above 220 K, indicating that water molecule mobility at the ice surface plays a considerable role in the clathration reaction. Comparable activation energies of 42.3 and 54.6 kJ/mol are observed in the high temperature range for the reaction- and diffusion-limited stages 2 and 3, respectively.

Crystal structures with a challenge: high-pressure crystallisation of ciprofloxacin sodium salts and their recovery to ambient pressure

Two novel sodium salts of the antibiotic ciprofloxacin were crystallised at pressures of 0.25 and 0.6 GPa and subsequently recovered to ambient pressure. The structures are the first reported examples of ciprofloxacin chelating a Group IA monovalent cation. Ambient-pressure crystallisation of the same solution used for high-pressure experiments, yielded crystals of the known hexahydrate. In a parallel study, the previously unknown structure of anhydrous ciprofloxacin was determined from powder diffraction data. The structures are described and compared using the XPac method.