Simple molecular systems at extreme conditions

Abstract

This thesis project has focussed on the experimental study of simple molecular systems at extreme conditions. High-pressure and high-temperature techniques have been used in combination with Raman spectroscopy and X-ray diffraction diagnostics to characterise three simple molecular systems which are unified by the inclusion of nitrogen as a constituent element. The N2 molecule contains the only triple-bond amongst the elemental diatomics and is considered a model system for exploring the changes in structure and bonding induced by tuning pressure and temperature conditions. As such the nitrogen phase-diagram is a focus-point in current extreme conditions research and nitrogen has been found to exhibit a high-degree of polymorphism not observed in other simple molecular systems such as hydrogen or oxygen. Understanding molecular mixtures of nitrogen with other simple molecules at extreme conditions is significant to many scientific fields varying from chemistry to astronomy. The first system presented is the binary mixture of nitrogen and xenon which was studied as a function of pressure. The study constitutes the first comprehensive study of the xenon-nitrogen system at high-pressures. A new van der Waals compound was observed which underwent a phase transition at 14 GPa and was stable up to at least 180 GPa and 3000 K, conditions where pure nitrogen becomes amorphous. Optical measurements suggested possible metallization of the new compound around 120 GPa. The second system presented is the binary mixture of nitrogen and hydrogen which was studied both as a function of pressure and composition. Two known nitrogen-hydrogen structures were confirmed and a pressure-temperature path-dependent formation of hydrazine or ammonia was discovered. Additionally, one mixture was compressed to 242 GPa, the highest pressure investigated in the nitrogen-hydrogen system. The third system presented is the elemental nitrogen phase known as i-nitrogen, an elusive high-temperature polymorph which has hitherto eluded structure determination and proved challenging to access. i-nitrogen was successfully characterised as having an extraordinarily large unit cell containing 48 N2 molecules, making it the most complex molecular nitrogen structure to be determined unambiguously.