The Paris-Edinburgh cell (PEC) is a widely used opposed-anvil device for neutron
scattering. Since its development, it has been used to study a number of samples
loaded as solids or liquids. However, studying gases at room temperature has
not yet been possible. Up until now only a few gases could be loaded as liquids,
in cryogenic conditions. Thus, it was impossible to study many gases and gas
mixtures and also it was difficult to use gases as pressure-transmitting media
(PTM). In order to overcome these limitations, a technique that would enable
loading of gases into the PEC was required.
The work described in this thesis was focused on the design and use of a
gas-loading system for the PEC. The challenge of designing such a system comes
from the fact that the gases need to be loaded into the gasket at sufficient density
in order to achieve any significant pressure during further compression in the cell.
This can be achieved by using a separate pressure vessel. Because the whole PEC
is too large to be placed inside the vessel, a technique of loading gas into the anvils
separated from the rest of the cell had to be devised. Designing the holder for the
anvils, which would make this possible, presented a major challenge as it should
allow the anvils to be transferred between the vessel and the PEC, with the gasket
filled with high-pressure gas. Then it needs to allow further compression of the
gasket inside the PEC.
The developed system consists of a pressure vessel and a locking clamp for
the anvils. The pressure vessel is a closed-end thick-walled cylinder with a top
cover which has an opening for a piston. The vessel is placed on the table of a
hydraulic press and the piston, sealed by a high-pressure reciprocating seal, is
used to transmit the force from hydraulic ram onto the anvils which are held by
the clamp and placed inside the vessel.
One of the anvils is fixed to the clamp and the other one is supported by
spring-loaded latches - the latches engage when the anvils are pushed towards
each other. Thus, when the force is applied onto the anvils to compress the
gasket, latches lock the anvils in their positions stopping them from retracting and
maintaining the gasket compressed after the force is released. The clamp allows
the gasket to be filled with the gas and then deformed to seal the compressed gas.
The locking mechanism keeps the gasket compressed, which enables the clamp to
be transferred from the vessel to the PEC.
After the system was built and tested, it was installed at ISIS neutron source
(Oxfordshire, UK), where it has been used in several experiments. The first experiment prepared with the gas-loading system was a neutron diffraction study
of nitrogen at high pressure. Nitrogen was chosen as a sample material because its
high-pressure structural phase diagram is well established. Nitrogen was loaded
into the gasket using the gas loader and then it was compressed in increments
to 6 GPa in the PEC. β and δ phases of solid nitrogen were clearly seen in the
collected neutron diffraction data. The experiment proved the usability of the
gas-loading system and verified its expected performance.
The second experiment utilizing the gas-loading system was to study singlecrystal
and powder samples of sodium chloride (NaCl) and squaric acid (H2C4O4).
For these studies argon was used as a PTM, replacing the conventionally used
methanol-ethanol mixture (ME). Up until this experiment the highest pressure
reported in single-crystal neutron-scattering experiments was 12 GPa. This limit
was set by the solidi cation pressure of ME. With argon as the PTM, the samples
were compressed to 15 GPa without any damage to the crystals. Another advantage
of replacing ME with argon is improved hydrostaticity. The highest pressure
that ME remain hydrostatic to is 11 GPa. Compressing beyond this point causes
sheer stress acting on the sample which affects the quality of the neutron scattering
data manifested in the appearance of peak-broadening in the diffraction
patterns. With use of argon, the powder samples have been compressed to 18
GPa while maintaining quasi-hydrostatic pressure conditions, resulting in clean
and sharp diffraction patterns without any noticeable peak-broadening
