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We're all familiar with ice - water frozen into its solid state, at or below 0°C at standard atmospheric pressure (1 atm, or 101.325 kPa). But this naturally occurring crystalline solid (officially known as ice Ih or ice one h) is just one of at least nineteen phases of ice, each with a different packing geometry. The less familiar phases (polymorphs) occur at different pressures and temperatures. The ice polymorphs have differing densities, crystalline structures, and proton ordering. These strange phases of ice are just one example of what happens to matter at extremely high pressures.
The physical and chemical properties of a material depend on its structure and the distances between its atoms. Pressure has far more of an effect on interatomic distances than temperature, so varying the pressure is a powerful tool for exploring the relationship between structure and properties. Fundamental insights can be used, for example, to inform the design of new materials or to help explain phenomena such as volcanic eruptions that originate from processes deep in the Earth.
Further, the electronic structure of a material can be very different under pressure, giving rise to extraordinary effects. An insulator such as ice can become a metal or conductor (e.g. Ice XVII, or Superionic water), and metals can become insulators. E.g. Sodium, a pale grey, shiny metal transforms into a glass-like transparent insulator under pressure. Changing electron configurations at high pressure gives elements a different reactivity and chemistry, almost reinventing the periodic table.
Annette Kleppe, Principal Beamline Scientist on Diamond's I15 beamline, said;
High-pressure devices are superbly suitable for tuning structural and electronic properties of materials. In fact, pressure can change the electronic properties so dramatically that it adds a whole new dimension to the periodic table. High-pressure, when combined with different experimental analysis techniques, is a powerful tool for understanding natural phenomena or designing novel materials, for example. High-pressure research topics range from low-temperature physics to high-temperature Earth and planetary science.
It's no wonder researchers want to explore these extreme conditions, and Diamond has several facilities to accommodate them. I15 is our dedicated Extreme Conditions beamline, dedicated to X-ray powder diffraction experiments at extreme pressures and temperatures. Users can also carry out high-pressure experiments on beamline I18 (Microfocus Spectroscopy), I19 (Small Molecule Single Crystal Diffraction), and I22 (Small Angle Scattering and Diffraction).
Here's a snapshot of some of the cutting-edge high-pressure research carried out at Diamond.
Sarah Bolton, PhD student at the University of Edinburgh said;
Single crystal X-ray diffraction studies of organic molecular solids – the basic building blocks of life – have mostly been confined to pressures below 10 GPa. It is hypothesised that beyond that pressure (equivalent to 100,000 bar), the void space in these solids approaches zero, a turning point in the behaviour of molecular structures. Zero void space meaning that further compression is expected to change the intramolecular and intermolecular bonding interactions . A multidisciplinary team from the Centre for Science at Extreme Conditions at the University of Edinburgh set out to test this theory and push the boundary for high-pressure investigations on this type of molecular solid using the simple amino acid glycine.
Lewis Clough is a joint PhD student between Diamond and the University of Edinburgh. He worked with colleagues from Edinburgh, studying the behaviour of the alpha polymorph of glycine, which persists to at least 50 GPa. Using high-pressure single-crystal diffraction on I15, the team achieved the highest single-crystal pressure data set collected at Diamond on an organic material.
For the experiment, a tiny 50 μm-sized single crystal of α-glycine was loaded into a diamond anvil cell (DAC), a pocket-sized high-pressure apparatus, in which the crystal was compressed between the tips of two diamonds. Using an X-ray energy of 78 keV - significantly higher than standard for single crystal diffraction experiments - the team collected very high-quality data and solved the structure to the highest pressures of 51-52 GPa.
High-temperature superconductivity has been observed in binary hydrides such as LaH10 at high pressures close to 180 GPa, and research efforts now focus on finding hydride high-temperature superconductors at lower, more accessible pressures. A group of researchers from the University of Bristol recently used a novel approach to prepare lanthanum-hydrogen samples . Using high-pressure powder diffraction and laser-heating on the I15 Extreme Conditions Beamline, they have found evidence for high-temperature superconductivity in binary La4H23 at a pressure of 95 GPa. This halves the pressure for superconductivity in hydrides.
We have long sought an answer to the question of how life evolved on Earth. Although we still lack direct evidence of a life-forming event, we can recreate the conditions in the lab to find ways in which life might have originated. The formation of lipid material and the self-assembly into vesicles are considered key to the appearance of life, and hence the compartmentalisation of cells has been the focus of many studies.
The earliest forms of life on Earth may have appeared around deep-sea hydrothermal vents, environments with extreme temperature and pressure conditions (up to 100°C and 80 MPa). If that is the case, how did the first proto-cell membranes withstand the harsh environmental conditions?
Some present-day extremophiles that live in these conditions use apolar molecules as structural membrane components, allowing them to tune their membrane composition in response to environmental temperature and pressure changes. Researchers used early life protomembrane models to explore whether this strategy could have worked for the earliest lifeforms, using linear and branched alkanes as apolar stabilising molecules.
The protomembrane research included small-angle neutron scattering and elastic incoherent neutron scattering (EINS) experiments at the ILL (Grenoble, France) and small angle X-ray scattering (SAXS) on Diamond's I22 beamline.
At Diamond, experiments made use of a hydraulic pressure jump cell specifically designed for use at I22 that is fully integrated into the beamline’s control systems. The jump cell, which can also be used on I15, allows static and rapid pressure jump measurements in the range of 0.1-500 MPa.
Pressure cell results showed that the alkanes helped dampen membrane fluctuations fuelled by thermal energy that might otherwise lead to membrane disruption. The insertion of alkanes almost completely cancelled out the effect of pressure, and could help explain how the first living forms survived the harsh conditions on the young Earth.
Metal-organic frameworks (MOFs) lend themselves to a wide range of applications, including gas storage and separation, catalysis and selective adsorption, drug delivery and chemical sensors. Postsynthetic modification of MOFs, exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), is a powerful tool for tuning their physical properties and functionality.
A team of researchers using a DAC exposed single crystals of the MOF GUF-1 to high pressures on Diamond's I19 beamline. Their work enabled full structural characterisation across a range of pressures by single crystal X-ray diffraction and identified unexpected SBU reactivity – chemisorption of methanol – at 4.98 GPa.
These results suggest that pressure-induced postsynthetic modification has the potential to become a valuable method for tailoring the chemistry and internal pore surface of MOFs and other porous materials for specific applications.
And while researchers use high-tech tools to explore these high-pressure phenomena, their results can still be relevant for everyday applications. As Annette Kleppe concludes;
Even if materials discovered at extreme pressures and temperatures are found to be unstable at normal conditions, they give us an understanding of which properties and phenomena are possible. For applications the trick is to find other ways to reach the same results with other methods.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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