Chemistry at high pressure

Abstract

Chemistry involves the study of the reactions and properties of the elements and their compounds. After over two centuries of systematic chemical research, combined with the modern use of spectroscopy and diffraction methods to determine structure and valence states and detailed quantum mechanical investigations of interatomic bonding; it is now generally accepted that the fundamentals of chemistry are understood and established. New frontiers that are being developed now lie in the supramolecular chemistry of biological molecules, or in studying the synthesis and properties of polymeric macromolecules and nanoparticles. However, we must remember that most of our chemical knowledge has been gained from studies carried out at or near one atmosphere pressure at the Earth’s surface, while much of the matter in the Universe exists under much higher pressure conditions, deep inside planets and stars. Chemistry is concerned with the behaviour of the outermost electrons of atoms that determine the bonding, reactivity and structures of molecules and solids. By the time a typical solid or liquid is compressed to above a few hundred thousand atmospheres, its molar volume is reduced by approximately 50%. Once the megabar range (1,000,000 atm; P 5 100 GPa; 1 Mbar 5 1,000 kbar) is reached, average interatomic distances can be decreased by up to a factor of two. It is to be expected that large changes will occur in the outer electron shells under extreme densification conditions, and that these will lead to substantial modifications of the chemical and physical properties. It is known that such large changes in molecular and electronic structure do in fact occur, and that the very arrangement of the Periodic Table might have to be modified for high pressure conditions. As a simple example, we can consider the typical alkaline earth metals such as Ca and Sr that possess a fully closepacked fcc structure at ambient conditions. However, pressurising Ca to P . 200 kbar (20 GPa) causes it to transform to a less efficiently packed bcc structure, with a lower coordination of the metal atoms (Fig. 1). A similar transition occurs for Sr at lower pressure (3.5 GPa). This transformation is due to pressure-induced mixing occurring between 3d and 4s electronic shells, giving Ca and Sr the character of transition metals rather than alkaline earth elements at high pressure. Likewise, K begins to form compounds and solid-state solutions with Ag and Ni above P . 10 GPa, indicating a transition metal-like character for this element also. Such changes in the chemical reactivity and molecular or solid state packing in highly densified elements and their compounds can be linked to expected changes in preferred oxidation states, along with the appearance of unusual valencies and bonding patterns, in high pressure chemistry. In this special issue of Chemical Society Reviews, we present topics of current interest in high pressure research that are relevant to chemistry. The present issue is particularly timely, in that it appears just over one hundred years after P. W. Bridgman began his pioneering experiments that revolutionised high pressure research. Modern high pressure experiments are enabled by various types of high-pressure device, ranging among the large mechanical presses developed by Bridgman and subsequent generations of high-pressure researchers, through the hand-held diamond anvil cells (DACs) that now enable physics and chemistry experiments to be carried out in the laboratory into the multimegabar range (P . 100 GPa), to speciallydesigned windowed pressure cells for chemistry and biology experiments in the low-pressure range (Fig. 1). The most extreme P–T conditions currently available are developed in dynamic shock-wave environments, that allow sampling of deep planetary interiors including the gas giants and small stars, and also conditions experienced Department of Chemistry and Materials Chemistry Centre, University College London, Christopher Ingold Laboratory, 20 Gordon Street, London, WC1 H0AJ, United Kingdom. E-mail: p.f.mcmillan@ucl.ac.uk.; Fax: 020 7679 7463; Tel: 020 7679 4610 Davy–Faraday Research Laboratory, Royal Institution, 21 Albemarle Street, London, W1X 4BS, United Kingdom