Prof Paul McMillan

Research Overview

A main part of our research explores the structure and synthesis of solid state materials under "extreme" high pressure and high temperature conditions. In situ studies to hundreds of kilobars and into the megabar range (20-100 GPa) are carried out in the diamond anvil cell. Samples are heated by passing infrared lasers (CO2, Nd3+:YAG) through the transparent diamond windows. We use Raman, FTIR and UV-visible spectroscopy to probe structural changes and compound formation at high pressure. We also carry out synchrotron X-ray diffraction and spectroscopy using national and international facilities including Diamond and ESRF. The in situ studies are complemented by "large volume" syntheses under high-P,T conditions using multi-anvil and piston cylinder devices. Recent work has focused on the synthesis of new nitride and oxynitride materials that have low compressibility and high hardness, as well as potentially useful electronic and optical properties: these include nitride and oxynitride spinels (γ-(Ge,Si)3N4, Ga3O3N), transition metal nitrides, icosahedral borides (B6O), and layered and three-dimensional CxNyHz materials. Our most recent "extreme conditions" studies have focused on melting metals and ceramics (Sn, TaC, MgO) with melting points extending up to 6000-7000 K at 50-100 GPa with applications to advanced materials science and geophysics. Synthesis of carbon nitrides to be used as precursors in the high pressure experiments has led to identification of a wide class of oligomeric "graphitic" carbon nitride materials that are shown to have tunable optoelectronic and electrochemical properties that are predicted to become important in energy science applications: as metal-free photocatalysts, battery electrodes and fuel cell catalyst supports. That work is being developed in collaboration with UCL Chemistry and Chemical Engineering colleagues. We have also studied the high pressure behaviour of molecular systems including alkali metal silicides, polyoxometallates and organometallic cluster compounds, and extended the studies to soft matter systems and biophysics. Biologically relevant molecules such as amyloid fibrils have been studied up to P ~10 GPa revealing important new details of their structure and compressibility behaviour, and studies of cellulose using Raman spectroscopy and synchrotron X-ray diffraction allowed us to compare intra- vs inter-chain compression.

High pressure bioscience is an emerging area with applications to the Earth's deep biosphere, nanotechnology, food science and biomedicine. We initiated our studies in high pressure biology with experiments on voltage wave propagation in rat hippocampal slices up to 200-300 atm using a newly-designed cell for optical imaging with voltage-sensitive dye techniques, to understand questions in hyperbaric neuroscience such as pressure-reversal of narcosis effects. This work was carried out in collaboration with Prof. S.A. Greenfield from the University of Oxford Pharmacology department. We then began high pressure biophysics experiments on amyloid fibrils and cellulose in collaboration with Dr. Filip Meersman from the Katholieke Universitat Leuven (Belgium), followed by a new study investigating the survival of E. coli bacteria to pressures in the GPa range in collaboration with Drs. Meersman and Abram Aertsen from KUL. That work has been followed by a current study of high pressure survival of Shewanella oneidensis, a member of a genus that contains several piezophile organisms and could provide clues to pressure adaptation mechanisms. Here high pressure microbiology experiments are complemented by in situ biophysics studies, including quasi-elastic neutron scattering (QENS) to investigate water dynamics within the cells and across the cell membrane.

Studying structures and properties of supercooled liquids, glasses and other amorphous solids is a long-standing interest within our group. Building on our first experimental observation of a new type of liquid-liquid phase transion in Y2O3-Al2O3 supercooled liquids, between liquid phases distinguished by their density rather than chemical composition, we continue to carry our experiments and modelling studies in collaboration with Drs. Martin Wilding and Mark Wilson from Aberystwyth and Oxford universities, respectively. The work extends to other ceramic systems where evidence for polyamorphism is found for a-TiO2, metallic glasses and perhaps amorphous transition metal nitrides, at high pressure. High pressure studies have also revealed pressure-induced amorphisation and polyamorphism in amorphous semiconductors including a-Si and a-Ge, demonstrated by Raman and X-ray scattering and measurements of electrical conductivity and superconductivity. That work is linked to studies of Si-, Ge-based luminescent nanoparticles and clathrate structures that are low-density framework structures based on the semiconducting elements. The electronic, magnetic and optical properties can be tuned by varying the framework and cage site occupancy and they might provide useful new hydrogen storage materials.

Finally, we continue to apply our long-standing expertise in optical (Raman, IR, UV-vis) spectroscopy to research areas extending from Earth and planetary science to catalysis and biomedical science. Earth science applications (D. Dobson, A. Jones) include measurements of mantle OH content by combined FTIR spectroscopy and electrical conductivity measurements in the diamond anvil cell, to studies of diamond inclusions and shocked diamond. Studies developing catalysis applications carried out in collaboration with Prof. A. Gavriilidis (Chemical Engineering) use on-line micro-Raman and FTIR techniques to develop process modelling parameters for microchannel reactor systems. Collaborations with Dr. J. Dudhia (Royal Veterinary College) have resulted in a patented protocol for early assessment of cartilage tissue disease using fibre-optic Raman spectroscopy and in vivo trials have been conducted successfully during minimally invasive arthroscopic surgery. Current research involves use of micro-Raman techniques combined with chemical imaging to study the evolution of cartilage components (collagen, proteoglycan) in response to stress and their distribution within the tissue.