this was not considered. The structure itself demonstrates a 4% compression over the
phase transition which together with the stabilisation of the host interactions provides the driving force for the phase transition to occur. The nature of host-guest
effects in hydroquinone was explored further using methanol and acetonitrile as the
guest molecules with reference to the unsolvated β-form of hydroquinone [52]. In
this case, the solvents play a large role in stabilising the geometry of the host
structure much like the helium PTM did in the previous example. The unsolvated
form is unstable on compression at fairly low pressures (0.32(5) GPa) where it
collapses into the α-form over a 10-h period. Supercompression of the phase to 1.2
(5) GPa was achieved for the limited time of a single-crystal X-ray diffraction data
collection before it collapsed into a polycrystalline product. Kinetics obviously
played its part as different crystals were used to construct the compression dataset
using silicone oil albeit the transformation was also observed in pentane/isopentane
mixture. The exploration of the host-guest structure with methanol or acetonitrile in
the cavity proved that the ambient pressure structure was stable to much higher
pressures using pentane/isopentane as the PTM (greater than tenfold increase in
stability). In each of these cases, the structure undergoes a phase transition to new
high-pressure phases at 6.2(1) and 4.0(1) GPa for methanol and acetonitrile, respectively. The lower pressure of transition is due to the size of acetonitrile and its
alignment in the cavity. In the methanol solvate, it is the host that changes geometry
resulting in the phase transition [52]. The bulk modulus of the low-pressure phases is
relatively similar (methanol, 8.3(12); acetonitrile, 8.5(3); formic acid, 13.6(4) GPa
[51]), and the high-pressure phases of acetonitrile and formic acid clathrates also
have a similar bulk modulus at the onset pressure (acetonitrile, 42(1), and formic
acid, 38(2) GPa). Further work in the area of host-guest complexes will be discussed
in more detail in Sect. 4 where we focus on the effects of pressure on metal-organic
framework materials.
In summary, for many materials the choice of PTM for compression may be
limited to the solubility of the compound in the medium, but these examples
highlight issues that can occur especially given the drive towards investigating
framework materials at high pressure.
2.3 Developments
In this section we will explore the developments that have been made in the area of
high pressure and those that will have a positive impact on science going forward.
2.3.1 Merrill-Bassett Diamond Anvil Cell (DAC)
The Merrill-Bassett design of DAC was a pioneering and highly impactful addition
to the area of high-pressure science [53]. The ability to perform single-crystal
diffraction experiments using a standard goniometer has enabled high-pressure
Crystallography Under High Pressures
153
phase transition which together with the stabilisation of the host interactions provides the driving force for the phase transition to occur. The nature of host-guest
effects in hydroquinone was explored further using methanol and acetonitrile as the
guest molecules with reference to the unsolvated β-form of hydroquinone [52]. In
this case, the solvents play a large role in stabilising the geometry of the host
structure much like the helium PTM did in the previous example. The unsolvated
form is unstable on compression at fairly low pressures (0.32(5) GPa) where it
collapses into the α-form over a 10-h period. Supercompression of the phase to 1.2
(5) GPa was achieved for the limited time of a single-crystal X-ray diffraction data
collection before it collapsed into a polycrystalline product. Kinetics obviously
played its part as different crystals were used to construct the compression dataset
using silicone oil albeit the transformation was also observed in pentane/isopentane
mixture. The exploration of the host-guest structure with methanol or acetonitrile in
the cavity proved that the ambient pressure structure was stable to much higher
pressures using pentane/isopentane as the PTM (greater than tenfold increase in
stability). In each of these cases, the structure undergoes a phase transition to new
high-pressure phases at 6.2(1) and 4.0(1) GPa for methanol and acetonitrile, respectively. The lower pressure of transition is due to the size of acetonitrile and its
alignment in the cavity. In the methanol solvate, it is the host that changes geometry
resulting in the phase transition [52]. The bulk modulus of the low-pressure phases is
relatively similar (methanol, 8.3(12); acetonitrile, 8.5(3); formic acid, 13.6(4) GPa
[51]), and the high-pressure phases of acetonitrile and formic acid clathrates also
have a similar bulk modulus at the onset pressure (acetonitrile, 42(1), and formic
acid, 38(2) GPa). Further work in the area of host-guest complexes will be discussed
in more detail in Sect. 4 where we focus on the effects of pressure on metal-organic
framework materials.
In summary, for many materials the choice of PTM for compression may be
limited to the solubility of the compound in the medium, but these examples
highlight issues that can occur especially given the drive towards investigating
framework materials at high pressure.
2.3 Developments
In this section we will explore the developments that have been made in the area of
high pressure and those that will have a positive impact on science going forward.
2.3.1 Merrill-Bassett Diamond Anvil Cell (DAC)
The Merrill-Bassett design of DAC was a pioneering and highly impactful addition
to the area of high-pressure science [53]. The ability to perform single-crystal
diffraction experiments using a standard goniometer has enabled high-pressure
Crystallography Under High Pressures
153