Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison. Despite attracting considerable attention, the structures of many high-pressure nitrogen phases still require unambiguous determination. Here, we report the structure of the elusive high-pressure high-temperature polymorph ι–N₂ at 56 GPa and ambient temperature, determined by single crystal X-ray diffraction, and investigate its properties using ab initio simulations. We find that ι–N₂ is characterised by an extraordinarily large unit cell containing 48 N₂ molecules. Geometry optimisation favours the experimentally determined structure and density functional theory calculations find ι–N₂ to have the lowest enthalpy of the molecular nitrogen polymorphs that exist between 30 and 60 GPa. The results demonstrate that very complex structures, similar to those previously only observed in metallic elements, can become energetically favourable in molecular systems at extreme pressures and temperatures.

Molecular nitrogen exhibits one of the strongest known interatomic bonds, while xenon possesses a closed-shell electronic structure: a direct consequence of which renders both chemically unreactive. Through a series of optical spectroscopy and x-ray diffraction experiments, we demonstrate the formation of a novel van der Waals compound formed from binary Xe-N₂ mixtures at pressures as low as 5 GPa. At 300 K and 5 GPa Xe(N₂)₂-I is synthesised, and if further compressed, undergoes a transition to a tetragonal Xe(N₂)₂-II phase at 14 GPa; this phase appears to be unexpectedly stable at least up to 180 GPa even after heating to above 2000 K. Raman spectroscopy measurements indicate a distinct weakening of the intramolecular bond of the nitrogen molecule above 60 GPa, while transmission measurements in the visible and mid-infrared regime suggest the metallisation of the compound at ~100 GPa.

Diatomic elemental solids are highly compressible due to the weak interactions between molecules. However, as the density increases the intra- and intermolecular distances become comparable, leading to a range of phenomena, such as structural transformation, molecular dissociation, amorphization, and metallisation. Here we report, following the crystallization of chlorine at 1.15(30) GPa into an ordered orthorhombic structure (oC8), the existence of a mixed-molecular structure (mC8, 130(10)–241(10) GPa) and the concomitant observation of a continuous band gap closure, indicative of a transformation into a metallic molecular form around 200(10) GPa. The onset of dissociation of chlorine is identified by the observation of the incommensurate structure (i-oF4) above 200(10) GPa, before finally adopting a monatomic form (oI2) above 256(10) GPa.

We have used synchrotron x-ray and Raman spectroscopic studies combined with molecular dynamics to investigate the ${\mathrm{I}}_2-{\mathrm{H}}_2$ system at high pressure. By laser heating the mixture above 25 GPa we synthesized the molecular compound $\mathrm{HI}{\left({\mathrm{H}}_2\right)}_{13}$, with the ${AB}_{13}$ structure type and unusually high volumetric hydrogen content. The isolation of HI molecules by ${\left({\mathrm{H}}_2\right)}_{13}$ supramolecular clusters stabilizes the compound over a remarkable pressure range from 9 to at least 130 GPa. At lower pressures and 300 K another compound, hydrogen diiodane ${\mathrm{H}}_2{\left(\mathrm{HI}\right)}_2$, spontaneously forms, being stable up to 12.5 GPa.