Development of high-pressure single-crystal neutron diffraction on KOALA
Abstract
This thesis project has focused on the development of high-pressure single-crystal diffraction
experiments on the neutron Laue diffractometer KOALA at the OPAL reactor at
ANSTO, Australia. Over the course of this project several candidate systems have been
studied under conditions of high-pressure using X-ray diffraction with a view to their
use in developmental experiments on KOALA. The results of two high-pressure KOALA
experiments are presented as well as the notable results from X-ray diffraction on the
candidate systems.
The first experiment on hexamethylenetetramine provided valuable insights into how
reduced crystallite size and reciprocal-space access affects data collected on KOALA.
In addition, data treatment techniques were developed to deal with the unique and
challenging high-pressure Laue data, including corrections for attenuation due to the cell
body. The ability to collect data through the body of cell prompted a further experiment
on the complex, low-symmetry structure of the amino acid l-arginine dihydrate. Despite
the smaller crystal size and dominant parasitic scattering from the diamond-anvil cell,
the data collected allow a full anisotropic refinement of hexamethylenetetramine with
bond lengths and angles that agree with literature data within experimental error.
This technique is highly suited to low-symmetry crystals, as shown by the successful
refinement of data from a l-arginine dihydrate crystal. In such cases the transmission
of diffracted beams results in higher completeness values than are possible with X-rays.
The hydrogen-bonded ferroelectric rubidium hydrogensulfate was the subject of
ambient-pressure experiments on KOALA investigating the nature of the ferroelectric
transition. Further high-pressure X-ray diffraction studies were carried out to resolve
the structures of phases at high-pressure and to investigate the ferroelectric transition
under pressure.
The potassium cobalt citrate metal-organic framework UTSA-16 has shown a wide
variety of pressure-mediated framework-solvent interactions including negative linear
compressibility, the ordering of potassium ions, and coordination changes which were
investigated by high-pressure single-crystal and powder X-ray diffraction. These behaviors
are rationalised by examination of the structural changes occurring in the framework
under pressure.
Two members of the widely studied alkylammonium tetrachlorometallate family,
tetramethylammonium tetrachloroferrate(III) and tetramethylammonium tetrachlorogallate(III), display numerous phase transitions with temperature. The structures of
these phases have been determined for the first time, and the contrast between the two
materials explored with first-principles calculations.