High-pressure Silicate Liquid Structure Studies Using Synchrotron X-rays at GSECARS

A VX5 Paris-Edinburgh Press combined with a multi-channel collimator assembly has been setup at the GSECARS beamline for monochromatic X-ray scattering, with an emphasis on studying structures of low-Z liquids, especially silicate liquids at high pressure. The press is mounted at the center of a general-purpose diffractometer, with a single-photon counting area detector (Pilatus3 X 300K-W) mounted on the two-theta arm. The incident monochromatic beam, from either a Si(111) or a Si(311) monochromator, is focused both horizontally and vertically with large Kirkpatrick-Baez mirrors, to a typical size of ~50 microns. With this setup, background scatter from the surrounding pressure media is completely removed at 2q angles above 10° for samples larger than 0.5 mm in diameter by oscillating the multi-channel collimator during data collection. About 10-30 minutes is sufficient to collect scattered signals of amorphous silicate samples with 2 mm diameter, with minimal background. Atomic bond lengths acquired from sodium disilicate glass structure data at ambient conditions agree well with previous studies based on neutron scattering (Misawa et al., 1980) and theoretical calculation (Smith et al., 1995), demonstrating the reliability of our setup. The current setup allows pressure up to a maximum of 12 GPa with a cupped-toroidal Drickamer anvil. The highest temperature that we have reached so far is 2273 K via resistive heating using a TiB2 heater. Results presented here include high pressure local atomic structure data of Na2Si2O5 liquid up to 12 GPa, enstatite (MgMgSi2O6) liquid up to 9.2 GPa and diopside (CaMgSi2O6) liquid up to 5.8 GPa. The derived structure factors and atomic pair distribution functions will help us understand how bond lengths and bond angles in liquid structures respond to pressure and temperature and their possible roles in affecting transport properties in silicate melts. These studies also serve as the main driving force in large volume techniques developed for high-pressure liquid studies at the GSECARS beamlines.