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[Congress Report]Extreme pressures and temperatures

This session highlighted recent advances in structural studies of materials up to megabar (>100 GPa) pressures and at temperatures from cryogenic conditions to thousands of degrees. The motivation for many of these studies is the need to understand from both experiment and theory the behaviour of materials that comprise the deep interiors of the planets. R. Jeanloz (UC Berkeley, USA) provided a brief overview of geophysical problems and the importance of phase transitions for understanding the deep Earth. He then focused on recent diamond-anvil x-ray diffraction studies of whole rock samples subjected to high pressure-temperature conditions of the lower mantle, and reported evidence for a new silicate perovskite phase. H. K. Mao (Carnegie Institution, Washington) described the wide range of high-P/T techniques now available. Examples include radial diffraction for elasticity studies and phonon density of states measurements, such as those made on Fe which has been recently measured to Earth core pressures (i.e. to above 140 GPa). The new elasticity techniques were explored in more detail by A. K. Singh (Bangalore, India), who described additional applications. G.D. Price (University College London, UK) complemented these experimental talks with the results of ab initio calculations on both the solid and liquid phase of iron under core conditions (to 363 GPa). Solid phase stability, vibrational dynamics, and phase diagrams, including melting curves, were found to be in excellent agreement with the most accurate experimental data. G. Fiquet (ENS Lyon, France) presented the results of x-ray diffraction experiments on magnesite (MgCO3) and diamond samples of different isotopic composition. Magnesite was found to undergo a transition above 80 GPa to an altogether new phase, and the diamond study showed very small isotope effects, in contrast to previous claims. Finally, G. L. Chiarotti (Trieste, Italy) reported theoretical investigations of the fate of methane, water, and ammonia at extreme conditions by ab initio constant-pressure molecular dynamics simulations. The results included predictions of high pressure-temperature superionic phases of H2O and NH3, metallization of liquid water and ammonia, and the condensation of methane to form heavier hydrocarbons, with important consequences for Neptune and Uranus. The session also included four poster orals on single-crystal x-ray diffraction studies to 30 GPa that reveal significant changes in crystal chemistry of Fe under pressure (L. Zhang, Marburg U., Germany); compressibility studies of refractory materials, including evidence that Co6W6C may be even more incompressible (and possibly harder) than diamond (N.A. Dubrovinskaia, Uppsala U., Sweden); in situ x-ray diffraction of the graphite-to-diamond transition indicating that the transition can occur without melting of the catalyst and that solid MgCO3 can be an important solid catalyst (W. Utsumi, SPring8, Japan); and, finally, x-ray studies of oxygen that suggest the material remains molecular in its metallic state above 100 GPa (G. Weck, CEA, Paris, France).

R. J. Hemley, Chair