MS47 - Novel materials under high pressure

This proved to be a very popular session for which there was standing room only in the venue. All of the speakers successfully conveyed their enthusiasm and passion for high-pressure research, and the topics spanned pressure regimes from over 10 GPa down to 0.02 GPa, thus illustrating very convincingly the versatility and usefulness of pressure as a variable both for synthesis and for probing the properties of new materials.

The first speaker, Tetsuo Irifune from Ehime University in Japan, described the synthesis of polycrystalline diamond by direct conversion of graphite using a multi-anvil apparatus. TEM investigations showed the product to be sintered nano-grains that possess an unusually high hardness. Two distinct forms were observed - granular 10-20 nm particles and larger (100-200 nm) tabular crystallites. Various growth mechanisms were proposed to explain these observations and the hardness properties of the materials.

The second speaker, Olga Degtyareva from the Carnegie Institution of Washington in the USA, described the host-guest structures of arsenic and antimony formed at high pressure. These unusual structures were analysed by a combination of X-ray diffraction and Raman spectroscopy. In particular, an incommensurate-to-incommensurate phase transition was observed with a change in symmetry from monoclinic to tetragonal in both host and guest components.

Diego Gatta from the University of Milan in Italy then described the elastic and structural behaviour of the naturally occurring feldspathoid, analcite, up to 8.5 GPa. A first-order phase transition was observed near 1.0 GPa to give a triclinic phase and the mechanism for this process was interpreted in terms of tetrahedral tilting. The coordination geometry of the sodium ions increases from six to seven. The elastic behaviour of the high-pressure phase appears to be strongly anisotropic.

David Allan from the University of Edinburgh in Scotland then described methods for the growth of simple salts from solution at high pressure. The technique is particularly well suited for the preparation and in situ characterisation of new hydrates and this was illustrated with sodium formate and sodium acetate. A more complex sodium salt containing both sulfate and hydrogensulfate ions was prepared by high-pressure decomposition of sodium persulfate.

The session finished with a talk by Wendy Mao from the University of Chicago in the USA, who described the preparation and structural characterisation of hydrogen clathrates based on both water and methane. The stabilities of these clathrates as a function of pressure and temperature have been explored and for (H2)4CH4 the material was found to be stable at pressures as low as 0.2 GPa at 77 K. These clathrates contain high weight percentages of molecular hydrogen and so offer the potential as low-temperature energy-storage materials.

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