I investigate how the structural and physical properties of matter change under the application of extremes of pressure and temperature, and why. The study of materials under such conditions, which otherwise exist only deep within the Earth and other planets, provides new insight into bonding and electronic structure, and thereby provides a fruitful route to new phenomena and materials.
Of particular interest is the behaviour of the simple elemental metals at high density. Until recently, it was assumed that the valence electrons of such metals, which control their behaviour, would form a near-free electron gas around a lattice close-packed ions under high compression. Today, owing much to Edinburgh’s work on pressure-induced structural complexity in the elements, a very different picture has emerged. Rather than having simple crystal structures at high pressure, even the simplest metals – Li and Na – have a wide range of extremely complex structures: one comprises more than 500 atoms/cell, while anothers adopt complex incommensurate forms.
The Figure above shows those elements now known to have incommensurate forms at high pressures. Those highlighted in yellow were the result of Edinburgh work, while those highlighted in red were found by researchers in Japan.
Computational studies explain this complexity as arising from localisation of the gas-like valence electrons in interstitial sites in the structures due to most of the volume of the compressed solid being occupied by core orbitals.
Extreme conditions research unavoidably requires the use of very small samples in order to generate high pressures. The sample size then requires very intense x-ray beams for structural studies, such as those available available at synchrotron radiation sources in the UK (Diamond), Europe (the ESRF) and the US (the APS).
The highest pressures available using static compresison techniques, where the sample is compressed in a diamond anvil cell, is some 400 GPa (4 Megabars or 4 million times atmospheric pressure). With the aim of achieving structural information at pressures 10-times higher than this, we are starting to utilise dynamic compression techniques, where very powerful lasers are utilised to compress samples over timescales of 10’s of nanoseconds. Such research will be conducted on the Janus, Omega and NIF laser sources in the US, and on the Orion facility in the UK. We are also starting to investigate the use of ultra-intense pulses of x-rays available from x-ray lasers such as the LCLS in the US, and XFEL in Hamburg, coupled with pulsed optical lasers, to achieve extreme P-T states in materials.