Downloaded By: [McMillan, Paul F.] At: 17:18 11 October 2007 Phase Transitions, Vol. 80, Nos. 10–12, October–December 2007, 1003–1032 Metastable phase transitions and structural transformations in solid-state materials at high pressure PAUL F. MCMILLAN*yz, OLGA SHEBANOVAyz, DOMINIK DAISENBERGERy, RAUL QUESADA CABRERAy, EDWARD BAILEYyz, ANDREW HECTORx, VICTORIA LEESy, DENIS MACHON{, ANDREA SELLAy and MARK WILSONy yDepartment of Chemistry and Materials Chemistry Centre, Christopher Ingold Laboratories, University College London, 20 Gordon Street, London, WC1H 0AJ, UK zDavy-Faraday Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London, W1S 4BS, UK xSchool of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK {Laboratoire de Physique de la Matie`re Condense´e et Nanostructures, Universite´ Claude Bernard Lyon 1, F-69622 Villeurbanne, France (Received 15 December 2006; in final form 5 March 2007) We use a combination of diamond anvil cell techniques and large volume (multi-anvil press, piston cylinder) devices to study the synthesis, structure and properties of new materials under high pressure conditions. The work often involves the study of structural and phase transformations occurring in the metastable regime, as we explore the phase space determined as a function of the pressure, temperature and chemical composition. The experimental studies are combined with first principles calculations and molecular dynamics simulations, as we determine the structures and properties of new phases and the nature of the transformations between them. Problems currently under investigation include structural studies of transition metal and main group nitrides, oxides and oxynitrides at high pressure, exploration of new solid-state compounds that are formed within the C–N–O system, polyamorphic low- to high-density transitions among amorphous semiconductors such as a-Si, and transformations into metastable forms of the element that occur when its ‘‘expanded’’ clathrate polymorph is compressed. Keywords: High pressure; Solid state chemistry; Metastable transitions; Transition metal nitrides; Silicon clathrates 1. Introduction The high-pressure variable is well-studied in Earth and planetary sciences, where density-driven phase transitions within silicate minerals, metals or light element *Corresponding author. Email: p.f.mcmillan@ucl.ac.uk Phase Transitions ISSN 0141-1594 print/ISSN 1029-0338 online ß 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/01411590701473010