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).
Dr Dominique Laniel from 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.
Read more on Diamond website
Image: Photograph of a single crystal of α-glycine compressed to 52.76 GPa in a diamond anvil cell. A section of the crystal structure determined at this pressure is overlaid on the crystal, showing the layers that increase in proximity upon compression, revealing a network of inter-layer hydrogen bonding interactions.
