HR: 0800h
AN: DI41A-0343    [Abstracts]
TI: In situ diffraction studies of magnesium silicate liquids and glasses under extremes of temperature and pressure.
AU: * Wilding, M C
EM: mbw@aber.ac.uk
AF: University of Wales, Institute of Mathematical and Physical Sciences, Aberystwyth, SY23 3BZ, United Kingdom
AU: Guthrie, M
EM: M.Guthrie@rl.ac.uk
AF: University of Edinburgh, Centre for Science and Extreme Conditions, Edinburgh, EH9 3JZ, United Kingdom
AU: Bull, C
EM: C.Bull@rl.ac.uk
AF: University of Edinburgh, Centre for Science and Extreme Conditions, Edinburgh, EH9 3JZ, United Kingdom
AU: Tucker, M
EM: M.G.Tucker@rl.ac.uk
AF: Rutherford Appleton Laboratory, ISIS, Chilton, OX11 0QX, United Kingdom
AU: Benmore, C
EM: benmore@anl.gov
AF: Argonne National Laboratory, Advanced Photon Source, Argonne, IL 60439, United States
AU: Weber, R
EM: rweber@matsdev.com
AF: Materials Development Inc, 3090 Daniels Court, Arlington Heights, IL 60004, United States
AB: Recently, there has been increasing interest in the structure of the liquid and glassy state at high pressure. Maxima in the melting temperature and corresponding negative melt slopes with pressure have now been reported in many systems which implies that both density and entropy increase with increasing pressure. In some cases, changes in amorphous structure with pressure can be abrupt and these changes are used to suggest that there may be first-order transitions between stable or metastable low and high density liquids at high pressure. Most evidence for this type of behaviour is indirect and is based on the changes in thermodynamic and other structure-related properties with pressure. Diffraction studies of liquids and amorphous materials at high pressure and temperature are therefore desirable and in this presentation we will discuss the results of X- ray and neutron scattering methods applied to silicate liquids and glasses that reveal details of structural changes that occur as a function of both pressure and temperature. Liquids in the MgO-SiO₂ system provide a good approximation for the liquids that are produced when the mantle minerals from planetary interiors melt. Their structures are however poorly understood because the liquids are refractory and do not form glasses particularly easily. Containerless synthesis techniques have been used to make glasses ranging in composition from the minerals enstatite to forsterite. Combined neutron and X- ray diffraction studies show a jump in the Mg-O coordination number between 38 and 33 Mole % SiO₂, this is interpreted as reflecting a limit to the formation of a polymerised silicate network. In-situ studies of these liquids using combined containerless and high energy X-ray diffraction techniques suggest that the limit to network formation is encountered at higher silica content in the liquids, and a change in the structure and structure-related properties such as the liquid viscosity, occurs when the liquids are cooled. The high temperature studies were carried out at ambient pressure; in situ diffraction studies of the glass structure can be made however and are used to identify related changes in structure. Neutron diffraction studies of a single composition magnesium silicate glass from ambient pressure to 9 GPa show changes in the amorphous structure. We identify changes in the Mg-O and O-O which suggest pressure-induced changes in the local environment of magnesium. Experimentally both the high temperate and high pressure experiments are challenging, although these in- situ studies reveal a wealth of structural complexity. As new facilities become commissioned and new techniques are developed, studies of these and similar liquids at elevated pressures and temperatures will soon be achievable.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting