Pressure and temperature are two environmental variables that are increasingly being
exploited by solid-state researchers probing structure-property relationships in the
crystalline state. Modern high-pressure apparatus is capable of generating many
billions of Pascals in the laboratory, and therefore can produce significantly greater
alterations to crystalline materials than changes in temperature, which can typically
be varied by only a few thousand Kelvin. Many systems such as single-molecule
magnets exhibit interesting properties under low-temperature regimes that can be
substantially altered with pressure. The desire by investigators to perform analogous
single-crystal X-ray diffraction studies has driven the development of new high-pressure
apparatus and techniques designed to accommodate low-temperature
environments.
[Ni(en)3][NO3]2 undergoes a displacive phase transition from P6322 at
ambient pressure to a lower symmetry P6122/P6522 structure between 0.82 and 0.87
GPa, which is characterized by a tripling of the unit cell c axis and the number of
molecules per unit cell. The same transition has been previously observed at 108 K.
The application of pressure leads to a general shortening of O···H hydrogen bonding
interactions in the structure, with the greatest contraction (24%) occurring diagonally
between stacks of Ni cation moieties and nitrate anions.
A novel Turnbuckle Diamond Anvil Cell designed for high-pressure low-temperature
single-crystal X-ray experiments on an open-flow cryostat has been
calibrated using the previously reported phase transitions of five compounds:
NH4H2PO4 (148 K), ferrocene (164 K), barbituric acid dihydrate (216 K),
ammonium bromide (235 K), and potassium nitrite (264 K). From the observed
thermal differentials between the reported and observed transition temperatures a
linear calibration curve has been constructed that is applicable between ambient-temperature
and 148 K. Low-temperature measurements using a thermocouple have
been shown to vary significantly depending on the experimental setup for the
insertion wire, whilst also adding undesirable thermal energy into the sample
chamber which was largely independent of attachment configuration.
High-pressure low-temperature single-crystal X-ray diffraction data of
[Mn12O12(O2CMe)16(H2O)4] (known as Mn12OAc) reveals a pressure-induced
expulsion of the crystallized acetic acid from the crystal structure and resolution of
the Jahn-Teller axes disorder between ambient pressure and 0.87 GPa. These
structural changes have been correlated with high-pressure magnetic data indicating
the elimination of a slow-relaxing isomer over this pressure range. Further
application of pressure to 2.02 GPa leads to the expansion of these Jahn-Teller axes,
resulting in an enhancement of the slow-relaxing magnetic anisotropy as observed in
the literature. Relaxation of pressure leads to a resolvation of the crystal structure and
re-disordering of the Jahn-Teller axes, demonstrating that this structural-magnetic
phenomenon is fully reversible with respect to pressure.
The space group of the Prussian blue analogue Mn3[Cr(CN)6].15H2O has
been re-evaluated as R-3m between ambient pressure and 2.07 GPa using high-pressure
single-crystal X-ray and high-pressure neutron powder data. Reductions in
metal-metal distances and gradual distortions of the Mn octahedral geometry have
been correlated with previously reported increases in Tc and declines in ferrimagnetic
moment in the same pressure range. Increasing the applied pressure to 2.97 GPa
leads to partial amorphization and results in a loss of long-range magnetic order as
shown by the literature.
The application of pressure (1.8 GPa) to the structure of
K2[Pt(CN)4]Br0.24.3.24H2O (KCP(Br)) causes a reduction in the Pt intra-chain and
inter-chain distances, and results in an enhancement of the overall conductivity under
these conditions as demonstrated in the literature. Almost no changes occur to the
high-pressure crystal structure upon cooling to 4 K, except in the Pt-Pt intra-chain
distances which converge and suppress the Peierls distortion known to occur at 4 K,
resulting in a comparatively greater electrical conductivity under these conditions
