Prof Paul McMillan
Research in our group has three main thrusts: (a) synthesis of new materials under high pressure-high temperature conditions, (b) studies of inorganic liquids and glasses, and (c) synthesis and structural characterisation of "expanded" open framework forms of group IV semiconductors. Our recent studies in high pressure research have focused on the exploratory synthesis of borides, nitrides, and oxide materials, using the pressure variable to "tune" their physical properties, or to obtain new dense polymorphs. For example, using high pressure, we could obtain nearly stoichiometric B6O, currently the third hardest known material. At pressures above 3 GPa (30,000 atm), the solid particles formed as unusual macroscopic icosahedra, as a result of a multiple twinning process or as radiating growth on the 20 triangular faces (Mackay packing). Other experiments on germanium nitride (Ge3N4) subjected to pressures above 20 GPa led to synthesis of the first nitride spinel (GeviGe2ivN4), containing Ge atoms in both octahedral and tetrahedral coordination to nitrogen. We are currently pursuing further experiments on nitrides at high pressure and high temperature.
In the area of glasses and liquids, we have been exploring the phenomenon of "polyamorphism", in which distinct glassy states exist for a given composition, within different pressure and temperature regimes. We recently discovered evidence for a new type of phase transition in glasses in the technologically important Y2O3-Al2O3 system. During cooling from the high temperature liquid, a low density form of the liquid was observed to nucleate and grow within the first liquid, and both could be quenched to coexisting glasses. Various studies have shown that the two liquids have the same composition, but different structures and thermodynamic properties.
In expanded framework forms of semiconductors, we have been investigating the synthesis and characterisation of Si- and Ge-"clathrates". These are open framework structures based upon the tetrahedral group IV atoms, with alkali or alkaline earth metal atoms trapped as "guests" within cages. The structures are up to 15% expanded relative to the normal semiconductor forms. Two clathrate structure types are known: "structure I" has formula e.g., Na8Si46, whereas "structure II" is usually non-stoichiometric (NaxSi136: 0<x<24). The clathrates have interesting semiconducting properties, and are under investigation as a potential new class of thermoelectric materials, for solid state refrigeration applications. We have recently removed the Na atoms from samples of NaxSi136, to obtain a new, low density form of elemental silicon.
- Poole PH, Grande T, Angell CA, McMillan PF (1997) Polymorphic phase transitions in liquids and glasses. Science 275 , 322-323.
- Hubert H, Devouard B, Garvie LAJ, O'Keeffe M, Buseck PR, Petuskey WT, McMillan PF (1998) Icosahedral packing of B12 icosahedra in boron suboxide (B 6 O). Nature 391 , 376-378 Grzechnik A, Chizmeshya AVG, Wolf GH, McMillan PF (1998) An experimental and theoretical investigation of phonons and lattice instabilities in metastable decompressed SrGeO 3 perovskite. J Phys Condensed Matter 10 , 221-233.
- Diefenbacher J, McMillan PF, Wolf GH (1998) Molecular dynamics simulations of Na2Si4O9 liquid. J Phys Chem B 102 , 3003-3008.
- Ramachandran G.K., Dong JJ, Diefenbacher J., Gryko J, Marzke RF, Sankey OF, McMillan PF (1999) Synthesis and X-ray characterization of silicon clathrates. J Solid State Chem 145 , 716-730.
- McMillan PF, Hubert H, Chizmeshya A, Garvie LAJ, Petuskey WT, Devouard B (1999) Nucleation and growth of icosahedral boron suboxide clusters at high pressure. J Solid State Chem , 147 , 281-290.
- Leinenweber K, O'Keeffe M, Somayazulu M, Hubert H, McMillan PF, Wolf GH (1999) Synthesis and structure refinement of the spinel, gamma-Ge3N4. Chem. Eur. J. , 5 , 3076-3080.
- Dong JJ, Sankey OF, Deb SK, Wolf GH, McMillan PF (2000) Theoretical study of beta-Ge3N4 and its high pressure spinel gamma-phase. Phys. Rev. B , 61 , 11979-11992.
- Ramachandran GK, McMillan PF, Deb SK, Somayazulu M, Gryko J, Dong JJ, Sankey OF (2000) High-pressure phase transformation of the silicon clathrate Si136. J Phys Condensed Matter , 12, 4013-4020.
- Gryko J, McMillan PF, Marzke RF, Ramachandran GK, Patton D, Deb SK, Sankey OF (2000) A low-density form of crystalline silicon with a wide optical bandgap. Phys. Rev. B, in press
- McMillan PF (2004) Polyamorphic transformations in liquids and glasses. J Mat Chem, 14, 1506-1512
- Soignard E, McMillan PF (2004) Defect chemistry in g-Si3N4 and g-Ge3N4 spinel nitride phases probed by Raman scattering in the laser-heated diamond anvil cell. Chem Mat 16, 3533-3542.
- Soignard E, McMillan PF, Leinenweber K (2004) Solid solutions and ternary compound formation among Ge3N4-Si3N4 nitride spinels synthesized at high pressure and high temperature. Chem Mat,.16, 5344-5349.
- Wilding MC, Wilson M, McMillan PF (2005) Combined neutron and X-ray
diffraction studies of rare earth aluminate glasses: structural constraints on the polyamorphic transition in Y2O3-Al2O3. Phil Trans R Soc A, 363, 589-607.
- McMillan PF, Gryko J, Arledge R, Bull CL, Cressey B, Kenyon AC, (2005) Solid state synthesis of amorphous and nanocrystalline Si and Ge by a chemical metathesis route. J. Solid State Chem, 178, 937-949.
- McMillan PF (2005) Designed for adversity. Materials World, 13, 22.
- 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.
- McMillan PF (2005) Pressing on: The legacy of P.W. Bridgman. Nature
Materials, 4, 715-718.
- Soignard E, Machon D, McMillan PF, Dong J, Xu B, Leinenweber K (2005) Spinel-structured gallium oxynitride (Ga3O3N): An experimental and theoretical study. Chem Mater, 17, 5465-5472
- McMillan PF (2005) A stranger in paradise. Science (perspective), 310, 1125-1126.