discussion in relation to low-melting compounds: (1) the potential need for temperature to aid access to polymorphs and (2) the metastability of phases due to kinetics.
Annotated on the pressure/temperature phase diagram are the experiments of
Bridgman [18] (melting line, black squares) and Funnell et al. [22] (green data
points) as well as a tentative phase boundary between Forms I and II (our addition).
The solid green circle designates the single-crystal data for Form II from a crystal
that was nucleated at 0.8 GPa and annealed with temperature to reach the melting
line at ~390 K. On cooling, the crystal was stable (or sufficiently metastable) at
ambient temperature for a dataset to be collected. In this case, a temperature increase
was necessary to be able to access the new form. Had the crystal been grown around
the melting line at ambient temperature (0.15 GPa), the known Form I would have
been observed. This is quite common, and we have observed this with
3-chlorophenol and 3-fluorophenol where we determined the structures at very low
pressures of 0.1 and 0.12 GPa [23]. In cases like these, it is worth applying further
pressure until one is no longer able to reach the melting curve on heating. To access
pressures beyond this, recrystallisation methods will be required (Sect. 2.2.3). On a
practical note, the speed of melting and growth can be altered by how far away from
the melting curve the experiment is being conducted; the closer to the melting line,
the slower the crystal growth will be, and the growth can be accelerated by
performing at higher pressures due to being deeper into the phase stability region.
Small changes in the pressure can help in the crystal growth if, for example, many
crystallites continue to nucleate during the growth stage.
The second point of discussion is the metastability of phases beyond phase
boundaries. This behaviour is common where a large rearrangement of molecules
is required between the two phases. From neutron powder diffraction measurements,
Funnell et al. were able to isolate Form I of aniline at 0.65 GPa, which has allowed
the tentative assignment of the phase boundary, and compress Form I to 1.04 GPa
without any transformation taking place (hollow green triangles). From the structure
of the two phases, they were able to explain this metastability through the large
differences in structure which forms a large kinetic barrier to overcome. We have
also observed this with 2-methylphenol where it crystallises in P3 2 at low pressures
and can be compressed to 8 GPa without any indication of transition to the highpressure monoclinic phase (P2 1 /c) due to the substantial change in structure required
[21]. This has also been noted in high-melting point compounds such as glycine
[2, 8] and imidazole [24].
As a final note in this topic, in our experience of data processing of high-pressure
materials, we have been able to deconvolute three to four individual diffraction
patterns and merge the datasets to a satisfactory outcome [1, 25]. Our advice to those
starting out in the area of crystal growth at high pressure would be to err in the side of
caution and grow multiple crystals, collect data on them and try to process the
datasets derived from these crystallites before attempting to grow one single crystal.
The difficulty of nucleation means that the loss of a crystal can lead to further hours
of nucleation experiments to obtain the polycrystalline material again. In addition,
the multiple crystals can lead to a substantial increase in the data completeness after
146
S. A. Moggach and I. D. H. Oswald
Annotated on the pressure/temperature phase diagram are the experiments of
Bridgman [18] (melting line, black squares) and Funnell et al. [22] (green data
points) as well as a tentative phase boundary between Forms I and II (our addition).
The solid green circle designates the single-crystal data for Form II from a crystal
that was nucleated at 0.8 GPa and annealed with temperature to reach the melting
line at ~390 K. On cooling, the crystal was stable (or sufficiently metastable) at
ambient temperature for a dataset to be collected. In this case, a temperature increase
was necessary to be able to access the new form. Had the crystal been grown around
the melting line at ambient temperature (0.15 GPa), the known Form I would have
been observed. This is quite common, and we have observed this with
3-chlorophenol and 3-fluorophenol where we determined the structures at very low
pressures of 0.1 and 0.12 GPa [23]. In cases like these, it is worth applying further
pressure until one is no longer able to reach the melting curve on heating. To access
pressures beyond this, recrystallisation methods will be required (Sect. 2.2.3). On a
practical note, the speed of melting and growth can be altered by how far away from
the melting curve the experiment is being conducted; the closer to the melting line,
the slower the crystal growth will be, and the growth can be accelerated by
performing at higher pressures due to being deeper into the phase stability region.
Small changes in the pressure can help in the crystal growth if, for example, many
crystallites continue to nucleate during the growth stage.
The second point of discussion is the metastability of phases beyond phase
boundaries. This behaviour is common where a large rearrangement of molecules
is required between the two phases. From neutron powder diffraction measurements,
Funnell et al. were able to isolate Form I of aniline at 0.65 GPa, which has allowed
the tentative assignment of the phase boundary, and compress Form I to 1.04 GPa
without any transformation taking place (hollow green triangles). From the structure
of the two phases, they were able to explain this metastability through the large
differences in structure which forms a large kinetic barrier to overcome. We have
also observed this with 2-methylphenol where it crystallises in P3 2 at low pressures
and can be compressed to 8 GPa without any indication of transition to the highpressure monoclinic phase (P2 1 /c) due to the substantial change in structure required
[21]. This has also been noted in high-melting point compounds such as glycine
[2, 8] and imidazole [24].
As a final note in this topic, in our experience of data processing of high-pressure
materials, we have been able to deconvolute three to four individual diffraction
patterns and merge the datasets to a satisfactory outcome [1, 25]. Our advice to those
starting out in the area of crystal growth at high pressure would be to err in the side of
caution and grow multiple crystals, collect data on them and try to process the
datasets derived from these crystallites before attempting to grow one single crystal.
The difficulty of nucleation means that the loss of a crystal can lead to further hours
of nucleation experiments to obtain the polycrystalline material again. In addition,
the multiple crystals can lead to a substantial increase in the data completeness after
146
S. A. Moggach and I. D. H. Oswald