lie. This has been demonstrated with the ε-phase of RDX [68], magnesium sulphate
pentahydrate [69] and glycolide [70]. The epsilon phase of RDX is a hightemperature high-pressure polymorph. Miller et al. used the Paris-Edinburgh press
to compress and heat RDX to 4.3 GPa and 448 K where they were able to identify a
new phase through neutron diffraction. By quickly quenching the sample to ambient
temperature, they were able to retain the phase and monitor the phase behaviour as a
function of decompression to 0.75 GPa. Below this pressure, the polymorph was not
stable and converted, but through quenching the sample to 150 K, they were able to
recover it to ambient pressure. The phase persisted to 230 K before reverting to the
stable α-phase.
More bespoke piston-cylinder equipment has been developed by Wang et al. that
can be used to 2 GPa over a temperature range of 80–300 K in a laboratory setting
[69]. This development has enabled the isolation and quenching of the high-pressure
pentahydrate of magnesium sulphate to ambient pressure for characterisation using
X-ray powder diffraction. Again, a low-temperature device (PheniX-FL) [71] was
necessary to trap the high-pressure phase, but this is another example of a different
technological solution to high-pressure recovery.
There are several other systems, such as 3-hydroxy-4,5-dimethyl-1phenylpyridazin-6-one [72], cinchomeronic acid [73] and mefenamic acid [74],
that have high-pressure phases that can be quenched to ambient pressure, but the
longevity of the phase at ambient pressure was either short or not explored to any
extent. These instances of quenchable phases are limited at present; however, as the
technologies advance to aid stabilisation, this number will increase and the applicability to industrial processes will become more apparent.
3 Organic Materials Under Pressure
This section will provide an overview of the types of purely organic materials that
have been investigated at high pressure. It is split into the various molecular types
that have been our areas of interest such as alcohols, halogenated compounds, amino
acids and pharmaceutically relevant materials. These have been chosen as the
examples reflect some of the challenges that have been overcome by the use of
techniques highlighted in Sect. 2.
3.1 Alcohols
Some of the simplest systems to be investigated have been the alcohols. The relative
simplicity of the molecular structure and limited hydrogen bonding capability have
provided an ideal set of systems to explore the interplay between hydrogen bonding
and packing forces. In the 1990s there was an extensive body of work that investigated the solidification of liquids at low temperatures due to the development of
Crystallography Under High Pressures
159
pentahydrate [69] and glycolide [70]. The epsilon phase of RDX is a hightemperature high-pressure polymorph. Miller et al. used the Paris-Edinburgh press
to compress and heat RDX to 4.3 GPa and 448 K where they were able to identify a
new phase through neutron diffraction. By quickly quenching the sample to ambient
temperature, they were able to retain the phase and monitor the phase behaviour as a
function of decompression to 0.75 GPa. Below this pressure, the polymorph was not
stable and converted, but through quenching the sample to 150 K, they were able to
recover it to ambient pressure. The phase persisted to 230 K before reverting to the
stable α-phase.
More bespoke piston-cylinder equipment has been developed by Wang et al. that
can be used to 2 GPa over a temperature range of 80–300 K in a laboratory setting
[69]. This development has enabled the isolation and quenching of the high-pressure
pentahydrate of magnesium sulphate to ambient pressure for characterisation using
X-ray powder diffraction. Again, a low-temperature device (PheniX-FL) [71] was
necessary to trap the high-pressure phase, but this is another example of a different
technological solution to high-pressure recovery.
There are several other systems, such as 3-hydroxy-4,5-dimethyl-1phenylpyridazin-6-one [72], cinchomeronic acid [73] and mefenamic acid [74],
that have high-pressure phases that can be quenched to ambient pressure, but the
longevity of the phase at ambient pressure was either short or not explored to any
extent. These instances of quenchable phases are limited at present; however, as the
technologies advance to aid stabilisation, this number will increase and the applicability to industrial processes will become more apparent.
3 Organic Materials Under Pressure
This section will provide an overview of the types of purely organic materials that
have been investigated at high pressure. It is split into the various molecular types
that have been our areas of interest such as alcohols, halogenated compounds, amino
acids and pharmaceutically relevant materials. These have been chosen as the
examples reflect some of the challenges that have been overcome by the use of
techniques highlighted in Sect. 2.
3.1 Alcohols
Some of the simplest systems to be investigated have been the alcohols. The relative
simplicity of the molecular structure and limited hydrogen bonding capability have
provided an ideal set of systems to explore the interplay between hydrogen bonding
and packing forces. In the 1990s there was an extensive body of work that investigated the solidification of liquids at low temperatures due to the development of
Crystallography Under High Pressures
159