Materials Chemistry at High Pressure and High Temperature
Workshop on High Pressure Research Energy
Recovery
LINAC Project at CHESS
Paul F. McMillan
Department of Chemistry and Materials Chemistry Centre,
University College London, 20 Gordon Street, London WC1H 0AJ, UK
also at:
Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21
Albemarle Street, London W1X 4BS, UK
We use a combination of laser-heated diamond anvil cell (DAC) and "large volume" press (multi-anvil, piston cylinder) techniques to synthesise and study new materials formed from chemical precursors at high pressure and high temperature. Studies are being carried out in various systems: transition metal nitrides (high hardness; superconductors); light element solids (B6O-B6N: wide-gap semiconductors; superhard); layered and dense CxNy phases; (Si,Ge,Ga) nitride and oxynitride spinels. We are also exploring the phenomenon of density-driven liquid-liquid phase transitions and polyamorphism, especially among amorphous semiconductors.
Transition metal nitrides and carbides (TiN, WC etc.) are well-known high-hardness materials. Much is known about their mechanical engineering parameters, but their microscopic properties have generally not been determined. We have studied the bulk modulus for several metallic nitrides using synchrotron X-ray techniques in the diamond anvil cell. Because the materials are highly incompressible, we must obtain data to very high pressure: sample sizes are small, and intense focused synchrotron X-ray beams are necessary. Compressibility determinations for light element phases (B6O-B6N) are even more demanding: these materials are "superhard" and the X-ray scattering is very weak. Determining the phonon spectra and Gruneisen parameters of transition metal nitrides is important for understanding and predicting their superconductivity: several of these phases (cubic NbN; hexagonal d-MoN) are relatively high-Tc materials (15-19 K). We currently do this in the laboratory via Raman scattering in the DAC: new experiments using inelastic X-ray scattering will provide a more complete picture of the phonon dispersion relations. Finally, the high-P,T phase relations of transition metal nitrides and B-N-C-O systems are almost completely unknown. We have been using laser heating experiments in the DAC, combined with multi-anvil experiments at lower pressures, to explore the phase relations under high nitrogen pressures. Intense synchrotron X-ray beams are necessary to probe the small samples within the laser-heated spot, or to penetrate the capsule assemblies in the large volume experiments.
Spinel-structured nitrides in the Si3N4-Ge3N4 system were first reported in 1999, and there has been considerable effort devoted to synthesising and characterising these new solid state materials. Recently, the attention has switched to (Si,Al)- and Ga-containing oxynitride spinels. The new materials have high hardness and are wide direct-gap semiconductors. Now we are beginning to explore and understand the defect chemistry of these new phases, that involves vacancy formation on both cation and anion sites: this determines the optoelectronic properties. In situ high-P,T experiments are necessary to design the best synthesis routes and recovery strategies: synchrotron-based X-ray spectroscopy and diffraction are needed to study the optical properties and details of the crystal structure. In situ experiments in large volume devices are now being designed to study the formation kinetics of the new nitride and oxynitride spinel phases.
Ti3N4 is a transition metal nitride that has been predicted to be stabilised in the spinel structure at high pressure. We have investigated formation of Ti3N4 from "amorphous" Ti(C,N,H) chemical precursors obtained from reactions of organometallic species with ammonia, under high-P,T conditions. The resulting materials appear to be nanocomposites with nanocrystalline TiN embedded in the amorphous matrix. EXAFS experiments indicate that the matrix has a highly-defective structure based on the rocksalt TiN form, but with 40-50% vacancies on cation and anion sites. New nanocomposite materials are expected to result from various high-P,T treatments from the precursor compounds.
We are exploring the phenomenon of "polyamorphism" or liquid-liquid phase transitions occurring as a response to pressure and temperature in various systems, especially amorphous semiconductors. Amorphous Si and Ge have a negative initial melting slope indicating a likely melting curve maximum in the negative pressure regime (under tensile strain). Applying a two-state model to the liquid to understand the anomalous densification results in appearance of a critical point followed by a line of density-driven liquid-liquid phase transitions in the supercooled liquid regime. The transitions occur between low- and high-density liquid phases (LDL, HDL), that become amorphous solids (LDA, HDA) below the glass transition. We studied the compressional behaviour of a-Si by laboratory Raman spectroscopy and electrical conductivity, combined with synchrotron amorphous X-ray diffraction and MD simulations. The LDA-HDA transition is reversible and has the character of a first-order thermodynamic phase transition between semiconducting and metallic amorphous forms of the element: we can extract the Si coordination changes in the two amorphous polyamorphs from the X-ray data combined with the MD simulation results.
References:
McMillan PF; “New Materials from High Pressure Experiments”, Nature Materials, 1, 19-25 (2002)
Soignard E, McMillan PF, Chaplin TD, Farag S, Bull CL, Somayazulu M, and Leinenweber K; “High-pressure Synthesis and Study of Low-compressibility Molybdenum nitride (MoN and MoN1-x) Phases”, Phys Rev B, 68, 132101-4 (2003)
Machon D, Daisenberger D, Soignard E, Shen G, Kawashima T, and McMillan PF; “High Pressure-high Temperature Studies and Reactivity of g-Mo2N and d-MoN”, phys. stat sol, in press (2006)
Shebanova O, McMillan PF, and Soignard E; “Compressibility Measurements and in situ Raman Scattering Studies of Vibrational Excitations in High-hardness Cubic Nitrides TiNx and g-Mo2N at High Pressure”, High Press Res, in press (2006)
Soignard E, Machon D, McMillan PF, Dong J, Xu B, and Leinenweber K; “Spinel-structured Gallium Oxynitride (Ga3O3N): An Experimental and Theoretical Study”, Chem Mater, 17, 5465-5472 (2005)
Soignard E, and McMillan PF; “Defect Chemistry in g-Si3N4 and g-Ge3N4 Spinel Nitride Phases Probed by Raman Scattering in the Laser-heated Diamond Anvil Cell”, Chem Mater 16, 3533-3542 (2004)
Machon D, McMillan PF, Dong JJ, and Xu, B; “High-pressure Study of the b-to-a Transition in Ga2O3”, Phys Rev B, 73, 094125 (2006)
Wilson M, and McMillan PF; “Crystal-liquid Phase Relations in Silicon at Negative Pressure”, Phys. Rev. Lett., 90, 135703-7 (2003)
McMillan PF, Wilson M, Daisenberger D, and Machon D; “A Density-driven Phase Transition Between Semiconducting and Metallic Amorphous Polymorphs of Silicon”, Nature Materials, 4, 680-684 (2005)
Hector AL, Jackson AW, McMillan PF, and Shebanova O; “Amorphous and Nanocrystalline Titanium Nitride and Carbonitride Materials Obtained by Solution Phase Ammonolysis of Ti(NMe2)4”, J Solid State Chem, 179, 1383-1393 (2006)
McMillan PF; “Polyamorphic Transformations in Liquids and Glasses”, J Mat Chem, 14, 1506-1512 (2004)
