High-Pressure Structural Studies of Hydroxides.

Abstract

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.

Hydrous Phases and Hydrogen Bonding at High Pressure

“…Decrease in O-H stretching frequency is commonly reported in materials studied at high pressures and this is interpreted as indicative of increased hydrogen bonding character (Kruger et al 1989, Duffy et al 1995b, Duffy et al 1995c. More recent structural studies (Besson et al 1992, Nelmes et al 1993b, Wilson et al 1995, Loveday et al 1997, Parise et al 1998b) using neutron powder diffraction suggest little change in the A-H distance to pressures as high as 16 GPa. This observation raises the possibility that the potential energy surface at high pressures ( Fig.…”

“…Many technical difficulties remain to be overcome, but rapid progress can be expected over the next decade with developments in new pressure cells for both single-crystal and powder neutron diffraction on the horizon. Many of these developments will mirror progress over the past decade in the ParisEdinburgh group (Besson et al 1992, Nelmes et al 1993a, Nelmes et al 1993b, Wilson et al 1995, Loveday et al 1996, Loveday et al 1997) and will depend on the availability of bright new sources such as the Spallation Neutron Source (www.sns.gov).…”