Our work with Group I elements brought two remarkable discoveries. For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. PWe calculated that the actual lowest-energy structure for lithium is the simple face-centred cubic (fcc) form. Previous workers had made their samples by cooling, which trapped the materials in a defective structure at temperatures too cold to rearrange: we made our samples under pressure, then cooled, and decompressing, producing the predicted fcc lithium.
In potassium, we predict a new phase of matter, in which approximately 70% of the atoms are solid and 30% liquid at the same time, all intermingled and able to interconvert.
This material has only been produced in tiny quantities within diamond anvil cells, so these properties have yet to be observed. Subsequently, we showed that rubiudium, sodium and cesium have similar phases.
The early stages of the project involved mapping out the pure hydrogen phase diagram, and interpreting the experimental signatures coming from experiments on hydrogen (H), deuterium (D) and mixtures of H-D. Standard methods struggle to simulate these light elements, while our calculations and experiments showed a spectacular agreement. The quantum rotation of molecules is crucial, and the twofold difference if mass between hydrogen and deuterium means that the normal theory of long-ranged correlated motion - "phonons" - breaks down.
Our work touched on two classic mathematical challenges:
The "Kepler conjecture" : we showed that the classic "Lennard-Jones potential" - no fewer than 50 different stability regimes were identified depending on the truncation range of the potential.
The "Kelvin problem" : We found that hydrogen is most stable when surrounded by four metal ions: so much so that metal atoms rearrange to maximise the number of tetrahedral interstices, we showed that Ba and La hydrides adopt structures where all interstitial sites are tetrahedra occupied by hydrogens.
We carried out a number of experimental studies on other diatomic molecules under pressure,. The overall picture which emerges is that with increasing pressure oxygen, nitrogen, chlorine, iodine etc. initially solidify in arrangements that optimise quadrupole interactions, then structures wherein the molecules are increasingly efficiently packed, then undergo a polymerization. We believe they ultimately metallize, but await experimental verification.
We discovered a new "form" of hydrogen, a supermolecule containing 13 rotating molecules, which in turn form a near-spherical cluster with one central molecule surrounded by 12 others. This supermolecule was created in an iodine hydride which, with a ratio of 27:1, has the largest number-fraction of hydrogens of any known compound.
In a shockwave, the high pressure region behind the shockfront occurs fast and only lasts for nanoseconds. Our new interatomic potentials were used to simulate the shock directly using molecular dynamics: it showed different phase behaviour under rapid compression. Only structures which can be accessed quickly are observed, and structures never seen under normal conditions can appear if the transformation is fast enough.
