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.
No reliable structural data have been reported on ice II under pressure, earlier work in the literature relating either to samples recovered to ambient pressure or the helium hydrate that is formed when helium is used as the pressurizing medium. We report structural refinements of helium-free ice II at three points in the phase’s region of stability. The structural differences from the helium-affected structure are significant, and can be related to the mainly repulsive interaction between the helium and both the oxygen and hydrogen atoms of the ice framework. These repulsions explain, among other changes, the different behaviors of the a (expansion) and c (contraction) lattice parameters, and the change in compressibility on the inclusion of helium.
Neutron diffraction experiments can provide an extremely high-resolution structural picture of clay-fluid systems. Here we describe the application of time-of-flight neutron scattering to hydrated clays, including discussion of issues such as isotopic labelling, sample containment, and data analysis. Recent studies of hydrated vermiculites under ambient conditions are used as an example. We then describe a new high-pressure/high-temperature sample environment that is being used to study clay-fluid interactions, in situ under hydrostatic sedimentary basin conditions. This environment enables us to approximate conditions encountered during burial, at depths of up to 10 km.
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