merging datasets especially combined with collecting datasets with the DAC in
different orientations, i.e. mounting a triangular DAC along another side.
2.2.2 Mixing Product with PTM
For systems that show no phase transformations up to a maximum pressure where
they can be melted, there are other routes that can be explored. In the case of aniline
[22] and other systems such as acetone [26] and ethanol [14], these were loaded and
solidified from the pure liquid; hence, the experiments that can be performed are
limited. These limitations are either due to the melting point becoming too high for
successful melt and crystal growth or due to the compression of the sample becoming non-hydrostatic beyond a certain pressure. A method to circumvent some of
these issues is to mix the liquid or gas of choice with another to increase the pressure
at which the crystals can be grown or to ensure the hydrostaticity to higher pressures.
Much of the inspiration for this work comes from the work on gases and gas
clathrates where the gas of choice, e.g. nitrogen, is mixed with a second gas to act
as a pressure-transmitting medium (PTM), e.g. helium [27–30]. The PTM allows the
compound under study to dissolve rather than melt which can lower the barrier to
interconversion. A number of studies have used this approach to facilitate the growth
of new phases. We have investigated the high-pressure phase behaviour of acrylic
and methacrylic acid using this technique due to the chemical reaction (polymerisation) that occurred on heating these systems at high pressure and high temperature
[31–34]. Using a 50:50%v/v mixture with 4:1 methanol/ethanol, we were able to
grow crystals of acrylic acid at much higher pressures than could be accessed
through conventional routes (0.65 GPa acrylic acid; 1.5 GPa methacrylic acid).
The dissolution of the compound under study in the PTM helped to lower the barrier
to interconversion as the solvation energy helps to overcome the energy barrier [9];
this principle will be employed in Sect. 2.2.3. Other positive outcomes from
adopting this method are that phase transformations can be accelerated at ambient
temperature by use of the solution. Crystals of acrylic acid were observed to
transform when the pressure exceeded the Form I-Form II phase boundary
[33]. Katrusiak and co-workers have used this method to investigate o-xylene so
that higher pressures could be achieved [35]. Using a neat liquid, the authors could
only achieve a pressure of 0.31 GPa and still have the ability to melt the sample. By
mixing o-xylene with methanol in a 1:4 and 1:9 ratio, they were able to increase the
pressures to 1 and 3.5 GPa, respectively. This is a huge expansion of phase space that
can be explored. In this case, Marciniak and Katrusiak did not observe any new
polymorphs of o-xylene, but it does highlight the potential of the method for
polymorph discovery.
Crystallography Under High Pressures
147
different orientations, i.e. mounting a triangular DAC along another side.
2.2.2 Mixing Product with PTM
For systems that show no phase transformations up to a maximum pressure where
they can be melted, there are other routes that can be explored. In the case of aniline
[22] and other systems such as acetone [26] and ethanol [14], these were loaded and
solidified from the pure liquid; hence, the experiments that can be performed are
limited. These limitations are either due to the melting point becoming too high for
successful melt and crystal growth or due to the compression of the sample becoming non-hydrostatic beyond a certain pressure. A method to circumvent some of
these issues is to mix the liquid or gas of choice with another to increase the pressure
at which the crystals can be grown or to ensure the hydrostaticity to higher pressures.
Much of the inspiration for this work comes from the work on gases and gas
clathrates where the gas of choice, e.g. nitrogen, is mixed with a second gas to act
as a pressure-transmitting medium (PTM), e.g. helium [27–30]. The PTM allows the
compound under study to dissolve rather than melt which can lower the barrier to
interconversion. A number of studies have used this approach to facilitate the growth
of new phases. We have investigated the high-pressure phase behaviour of acrylic
and methacrylic acid using this technique due to the chemical reaction (polymerisation) that occurred on heating these systems at high pressure and high temperature
[31–34]. Using a 50:50%v/v mixture with 4:1 methanol/ethanol, we were able to
grow crystals of acrylic acid at much higher pressures than could be accessed
through conventional routes (0.65 GPa acrylic acid; 1.5 GPa methacrylic acid).
The dissolution of the compound under study in the PTM helped to lower the barrier
to interconversion as the solvation energy helps to overcome the energy barrier [9];
this principle will be employed in Sect. 2.2.3. Other positive outcomes from
adopting this method are that phase transformations can be accelerated at ambient
temperature by use of the solution. Crystals of acrylic acid were observed to
transform when the pressure exceeded the Form I-Form II phase boundary
[33]. Katrusiak and co-workers have used this method to investigate o-xylene so
that higher pressures could be achieved [35]. Using a neat liquid, the authors could
only achieve a pressure of 0.31 GPa and still have the ability to melt the sample. By
mixing o-xylene with methanol in a 1:4 and 1:9 ratio, they were able to increase the
pressures to 1 and 3.5 GPa, respectively. This is a huge expansion of phase space that
can be explored. In this case, Marciniak and Katrusiak did not observe any new
polymorphs of o-xylene, but it does highlight the potential of the method for
polymorph discovery.
Crystallography Under High Pressures
147