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The structural pressure dependence of the high-pressure ices VII and VIII has been measured up to 20 GPa by neutron powder diffraction. Measurements of the atomic thermal motion reveal that, while it is strongly reduced by pressure in both phases, the stepwise change in the oxygen thermal motion at the VIII/VII transition is pressure independent within the precision of the measurements. Also, the O-D bondlength in ice VIII changes by less 0.002 •ð to 20 GPa. These results suggest that the site-separation of the disordered oxygen atoms in ice VII and the direction of the disorder displacement remain closely similar to those found at 5 GPa.

The structures of the deuterated hydroxides (M(OD)2; M=Mg, Ca, Mn, Ni , Co) have been refined using the Rietveld method and neutron powder diffraction data collected at high pressure. There is no statistically significant increase in the O-D interatomic distance at increased pressure while the hydrogen bonding interaction D ...O increases as this distance decreases and the O-D...O angle increases. The intramolecular O-D bond valences , determined indirectly from the intermolecular D...O distances, decrease steadily for all materials as pressure is increased .

Hydrogen bonds involving hydroxyl groups appear to to be significantly different from the H-bonds found in molecu lar solids. High-pressure structural studies of hydroxides using neutron diffraction provide a means to explore the changes in geometry over a wide range of interatomic separations. The structure of NaOD-V is found to have a com plex geometry which appears on the basis of bond-valence-sum calculations to be at most weakly H-bonded. KOD-VI is found not to be structurally related to NaOD-V as previously thought. Studies of ammonia monohydrate have pro vided the first full determination of the ambient pressure structure and have revealed phase transitions under pressure.

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