Studies of Silicate Melt Structure at High Pressure Using Synchrotron X-rays in a Paris Edinburgh Cell at GSECARS

Structures of silicate melts at high pressures have been studied using synchrotron X-ray in a VX5 Paris-Edinburgh Press combined with a multi-channel collimator assembly. This setup at the GSECARS beamline utilizes monochromatic X-ray beam to investigate structures of non-crystalline materials with an emphasis on studying structures of low-Z liquids, especially silicate melts at high pressure. The press is mounted at the center of a general-purpose diffractometer, with a single-photon counting area detector mounted on the two-theta arm. The incident monochromatic beam, generated from a Si(111) monochromator, is focused both horizontally and vertically with large Kirkpatrick-Baez mirrors, to a typical size of ~40 microns. By oscillating the multi-channel collimator during data collection, we can effectively remove unwanted X-ray diffraction signal from the surrounding pressure media for samples that are larger than 0.5 mm in diameter at 2qangles above 10°. About 10-30 minutes is sufficient to collect scattered signals of a 2 mm diameter non-crystalline silicate sample 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 and temperature as high as 3000 K via resistive heating. Here we present high-pressure local atomic structure data of various silicate melts including sodium disilicate, enstatite, and diopside. 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.