High Pressure Materials

Research in P.F. McMillan's group uses high pressure techniques to synthesise new materials in "large volume" devices (multi anvil, piston cylinder). Precursors for the high pressure syntheses are often obtained from organometallic chemistry. Materials under investigation include high-hardness transition metal nitrides and carbides (Ti(N,C), MoN, NbN) that are also superconducting, borides, Si, Ge and Ga oxides and nitrides, and CxNy phases. The high pressure laboratory within the MCC consists of (a) a home-built 500 ton press fitted with a Walker-style split-cylindrical pressurisation module; (b) a "Depths of the Earth" piston cylinder with different pressure plates permitting experiments up to 4 GPa; (c) a Rockland 1000 ton multi-anvil press with a Walker split-cylinder module. Multi-anvil syntheses are carried out routinely to above 10 GPa and T~1500oC; higher pressures (P ~25 GPa) and temperatures (~2000oC) are obtained for smaller sample sizes in selected experiments. The high pressure syntheses are complemented by in situ studies in the diamond anvil cell that permits spectroscopy and structural studies into the megabar range (P > 1,000,000 atm). Pressurised samples are heated inside the cell using infrared lasers (Nd3+:YAG; CO2) or resistively. Synchrotron X-ray studies allow structural changes to be explored during densification and material parameters such as the compressibility are obtained. The recoverability of new materials synthesised under high-P,T conditions is also established during diamond cell experiments. Experimental studies are complemented by ab initio calculations and molecular dynamics simulations (M. Wilson, UCL).

Research funding for research projects in high pressure solid state chemistry and materials science is currently enabled through grants from the EPSRC (Portfolio award to C.R.A. Catlow, P.F. McMillan and P. Barnes) and the Wolfson Foundation-Royal Society.

Selected publications:

• Machon D, McMillan PF, Dong JJ, Xu, B (2006) High-pressure behaviour of β-Ga2O3. Phys Rev B, in revision
• Machon D, Daisenberger D, Soignard E, Shen G, Kawashima T, McMillan PF (2006) High pressure-high temperature studies and reactivity of γ-Mo2N and δ-MoN. phys. stat sol, accepted<
• Hector AL, Jackson AW, McMillan PF, Shebanova O (2006) Amorphous and nanocrystalline titanium nitride and carbonitride materials obtained by solution phase ammonolysis of Ti(NMe2)4. J Solid State Chem, in press
• 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) Pressing on: The legacy of P.W. Bridgman. Nature Materials, 4, 715-718
• 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, 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
• Soignard E, McMillan PF (2004) Defect chemistry in γ-Si3N4 and γ-Ge3N4 spinel nitride phases probed by Raman scattering in the laser-heated diamond anvil cell. Chem Mat 16, 3533-3542
• McMillan PF (2003) Chemistry of materials under extreme high pressure-high temperature conditions. Chem. Comm., 919-923
• 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 (2002) New materials from high pressure experiments. Nature Materials, 1, 19-25
  

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