High-pressure studies of ammonia hydrates
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Chapter 6 Supplementary Material.zip (19.03Kb)
Date
28/06/2014Author
Wilson, Craig W.
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Abstract
Ammonia and water are major components of many planetary bodies, from
comets and icy moons such as Saturn's Titan to the interiors of the planets
Neptune and Uranus. Under a range of high pressures and/or low temperatures
known to occur in these planetary bodies, ammonia and water form a series
of compounds known as ammonia hydrates. Ammonia and water form three
stoichiometric compounds, ammonia hemihydrate, ammonia monohydrate and
ammonia dihydrate, which have ammonia-to-water ratios of 2:1, 1:1 and 1:2
respectively. Therefore a good understanding of the three stable ammonia
hydrates is required for modelling the interiors of these bodies.
Additionally, the ammonia hydrates are the simplest systems to incorporate
mixed (N-H O and O-H N) hydrogen bonds. Such bonds are important
biochemically, and along with O-H O H-bonds, mixed H-bonds are responsible
for the second-order structure of DNA, and they are also responsible for the proton
transfer reactions in enzymic processes. The understanding of these bonds and
processes rests on the knowledge of the relationship between bond strength and
geometry, and the ammonia hydrates provide a rich range of geometries against
which models of such mixed H-bonds can be tested.
X-ray and neutron diffraction techniques have been used to investigate the
behaviour of the ammonia-water complex and further the understanding of this
system. This includes solving the structure of a phase which was previously
thought to be an ammonia monohydrate phase, but has been shown here to be a
mixture of an ammonia hemihydrate phase and Ice VII. In addition to this, x-ray
and neutron diffraction experiments have been performed to explore how this
phase behaves under changing pressure and temperature conditions, and what
other implications that this has on the ammonia-water system. It has been found
that ammonia hemihydrate can also form a structural phase observed to form in
both ammonia monohydrate and ammonia dihydrate within the same pressure
and temperature regime, which opens the possibility of a solid solution existing
between all three stoichiometric ammonia hydrates.