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Professor Paul F. McMillan

(UCL Chemistry and the DFRL, Royal Institution)

Paul F. McMillan is Professor of Solid State Chemistry within the UCL Chemistry department; he is also jointly Professor of Chemistry at the Royal Institution (Davy-Faraday Research Laboratory). He initiated formation of the Materials Chemistry Centre, and serves as its founding Director. He was recruited from the USA in 2000 (Arizona State University), where he spent >20 years as PhD student and post-doctoral research in geochemistry, rising through the ranks of Assistant, Associate and Full Professor of Chemistry, to become Director of the Centre for Solid State Science in 1997. He coordinated formation of the ASU Materials Research Group in High-Pressure Synthesis in 1992, and became founding Director of its Materials Research Science and Engineering Centre in 1996, supported by the U.S. National Foundation. At UCL and the RI, he has developed new experimental programmes in solid state science, including high pressure materials chemistry, studies of liquids and amorphous materials, and low-density semiconductor frameworks and nanoparticles. He also established the Materials Chemistry Centre, via SRIF-2 funding through UCL and the RI to refurbish space and establish joint-use instrumentation laboratories mainly within the Christopher Ingold building at UCL, and to develop interdisciplinary programmes at the interface between materials chemistry and chemical engineering.

Research in McMillan's group is mainly focused on high pressure research, developing the synthesis and structure-property relations in dense materials during synthesis and under extreme stress. Our research is currently directed at nitrides and oxynitride materials (Si3N4, Ge3N4, Ga3O3N), as well as transition metal nitrides (MoN, TiN, TaN), and icosahedral borides (B6O). In addition to high hardness, these compounds have potentially useful electronic or magnetic properties. For example, the metal nitrides are usually superconductors, with values of Tc up to approximately 17 K. The other nitrides and oxynitrides are wide bandgap semiconductors.

We use in situ laboratory-based spectroscopy techniques combined with synchrotron X-ray diffraction measurements at the SRS or ESRF to study the synthesis and structural properties of materials under extreme high-pressure conditions, and during their recovery to ambient pressure and temperature. The high-pressure chemistry experiments are carried out using a combination of high-pressure, high-temperature synthesis experiments, and in situ studies in the diamond anvil cell at extreme P and T. Our research is not only directed at new solid state materials and compounds, but at amorphous materials subjected to high pressure. We are investigating the new phenomenology of pressure induced polyamorphism, and liquid-liquid phase transitions that occur in response to changes in the density and entropy at high-P and high-T. A new direction in high-pressure investigation within the group involves high-pressure neuroscience. We have established a collaboration with S.A. Greenfield's group at Oxford Pharmocology (also Director of the RI), to investigate pressure responses of voltage transmission waves among dynamic neuronal complexes using voltage-sensitive dye imaging under hyperbaric conditions. The new studies will result in an improved understanding of nitrogen narcosis, rare gas (Xe) anaesthesia, and pressure-reversal of anaesthetic/narcotic effects.

Another direction being developed within the group is concerned with low-density semiconductor "clathrate" frameworks, and synthesis and characterisation of Ge and Si nanoparticles using organic solution chemistry routes. The semiconductor clathrates are tetrahedral framework structures formed by Si or Ge, that are synthesised by metastable pathways from unusual "Zintl phases" (e.g., NaSi) that contain anionic structural units of the group 14 elements. Two main clathrate structure types are known (clathrate structures "I" and "II") that mimic the structures found among H2O-clathrate hydrate structures. Large cavities within the structures are usually filled with Na, K or Ba atoms; however, a "guest-free" clathrate Si136 has recently been prepared. The new compounds have tunable electronic properties : they can be metallic or semiconducting depending on the guest atom or framework site occupancy: they can become superconducting, and they have been proposed as new classes of thermoelectric materials. Within the MCC, the PFM group has established a new interaction with Professor Paul Barnes' group from IMG Birkbeck, to use synchrotron X-ray diffraction techniques to study the formation and crystallographic properties of Si-based clathrates. In another project, we are using synchrotron (X-ray absorption (XAS) and extended X-ray fine structure (EXAFS) techniques to follow the synthesis of luminescent Ge nanoparticles within organic liquid media, following solid state reactions that involve Zintl phases (e.g., Mg2Ge).

Recent publications:

Hubert H, Garvie LAJ, Devouard B, Buseck PR, Petuskey WT, McMillan PF (1998) High-pressure, high-temperature synthesis and characterization of boron suboxide (B6O). Chem Mat 10, 1530-1537

Hubert H, Devouard B, Garvie LAJ, O'Keeffe M, Buseck PR, Petuskey WT, McMillan PF (1998) Icosahedral packing of B12 icosahedra in boron suboxide (B6O). Nature 391, 376-378

Leinenweber K, O'Keeffe M, Somayazulu M, Hubert H, McMillan PF, Wolf GH (1999) Synthesis and structure refinement of the spinel, ?-Ge3N4. Chem. Eur. J., 5, 3076-3080

Deb SK, Dong JJ, Hubert H, McMillan PF, Sankey OF (2000) An experimental and theoretical study of the Raman spectra of the hexagonal and cubic forms of Ge3N4. Solid State Comm., 114, 137-142

Dong J, Sankey OF, Deb SK, Wolf GH, McMillan PF (2000) Theoretical study of ?-Ge3N4 and its high pressure spinel ?-phase. Phys. Rev. B, 61, 11979-11992

Soignard E, Somayazulu M, Mao H-k, Dong J, Sankey OF, McMillan PF (2001) High pressure-high temperature investigation of nitride spinels in the system Si3N4–Ge3N4. Solid State Comm., 120, 237-242

Zhang Z, Leinenweber K, Bauer M, Garvie LAJ, McMillan PF, Wolf GH (2001) High-pressure bulk synthesis of crystalline C6N9H3.HCl: a novel C3N4 graphitic derivative. J. Am. Chem. Soc. 123, 7788-7796

Deb SK, Wilding MC, Somayazulu M, McMillan PF (2001) Pressure-induced amorphisation and an amorphous-amorphous transition in densified porous silicon. Nature, 414, 528-530

McMillan PF (2002) New materials from high pressure experiments. Nature Materials, 1, 19-25

Wilson M, McMillan PF (2003) Crystal-liquid phase relations in silicon at negative pressure. Phys. Rev. Lett., 90, 135703-7

Soignard E, McMillan PF, Chaplin TD, Farag S, Bull CL, Somayazulu M, Leinenweber K (2003) High-pressure synthesis and study of low-compressibility molybdenum nitride (MoN and MoN1-x) phases. Phys Rev B, 68, 132101-4

McMillan PF (2003) New materials from high pressure experiments: Challenges and opportunities. High Pressure Research, 23, 7-22

McMillan PF, Deb SK, Dong J (2003) High-pressure metastable phase transitions in ?-Ge3N4 studied by Raman spectroscopy. J. Raman Spectroscopy, 34, 567-577

McMillan PF (2003) Chemistry of materials under extreme high pressure-high temperature conditions. Chem. Comm., 919-923

Bull CL, McMillan PF, Soignard E, Leinenweber K (2004) Structure determination of ?-MoN prepared at high pressure and high temperature by neutron and X-ray diffraction. J Solid State Chem, 177, 1488-1492

McMillan PF (2004) Polyamorphic transformations in liquids and glasses. J Mat Chem, 14, 1506-1512

McMillan PF, Wilson M, Daisenberger D, Machon D (2005) A density-driven phase transition between semiconducting and metallic amorphous polymorphs of silicon. Nature Materials, 4, 680-684

Soignard E, Shebanova O, McMillan PF (2006) Compressibility and phonon properties of hexagonal transition metal nitrides at high pressure: ?-TaN and ?-MoN. Phys. Rev B, in revision

Tang X, Dong J, Hutchins P, Shebanova O, Gryko J, Barnes P, Cockcroft JK, Vickers M, McMillan PF (2006) Thermal properties of the type-II clathrate polymorph of silicon (Si136): a theoretical and experimental study. Phys. Rev. B, revised

Machon D, Daisenberger D, Soignard E, Shen G, Kawashima T, McMillan PF (2006) High pressure-high temperature studies and reactivity of ?-Mo2N and ?-MoN. phys. stat sol, in press

Machon D, McMillan PF, Dong JJ, Xu, B (2006) High-pressure study of the ?-to-? transition in Ga2O3. Phys Rev B, 73, 094125



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