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Title: Simple molecular systems under pressure
Author: Frost, Mungo David
ISNI:       0000 0004 6499 7882
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2016
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Under pressure small molecular systems exhibit a remarkable degree of polymorphism and unexpected behaviours. Two previously unexplored binary systems, those of niobium{hydrogen and water{oxygen, are explored and their phase diagrams mapped. In the case of water and oxygen, two hitherto unknown clathrate structures are found. A wide study of dense nitrogen at and above room temperature is also reported. The phase diagram of nitrogen is exceptionally complex for a single element and exhibits considerable metastability of phases. Current theoretical understanding of nitrogen has many disagreements with experimental observation. High quality structural data on various previously known phases are reported as well as a novel molecular phase. This new phase, λ-N2, has been studied using Raman spectroscopy and powder x-ray diffrraction. Combining experimental and theoretical results gives a layered structure with intermolecular interactions playing an important role and an apparently weakened intramolecular bond. The dissociation of the nitrogen triple bond is also examined. The amorphous η state is studied via Raman spectroscopy and optical and IR absorbance methods before laser heating at 255 GPa. After heating the sample becomes very much more transparent and shows signs that the nature of the bandgap has changed suggesting that the nitrogen may be in a crystalline atomic phase hitherto unknown. An effort to take nitrogen to considerably higher pressures is reported. A two stage diamond anvil cell was developed using focused ion beam micromachining. Although ultimately unsuccessful, to the author's knowledge this was the first ever attempt at designing such a system to be compatible with small molecular samples and Raman spectroscopy and various conclusions are as to how best to proceed with such developments.
Supervisor: Gregoryanz, Eugene ; Loveday, John Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: high-pressure ; nitrogen ; hydrogen ; diamond anvil