Research Highlight: In-situ crystal structure determination of a minor phase in a multiphase system at megabar pressure

Shanghai, Feburary 18, 2016-Multiple phases often coexist in a polycrystalline sample as aresult of phase equilibrium in high-pressure petrological studies. Seifertite SiO₂ likely exists as a minor phase near the Earth’s core-mantle boundary (CMB). Structure determination of a minor phase in a sample contained in a diamond anvil cell (DAC) has never been impossible using conventional powder or single-crystal diffraction techniques. Utilizing the multigrain crystallography and newly developed procedures, new work led by HPSTAR scientists, Li Zhang et al., has overcome the difficulties and obtained the first in-situ single-crystal structure of seifertite being a minor phase in a polycrystalline sample at 129GPa. By simulating the pressure and temperature conditions near the core–mantleboundary, coarse-grained seifertite SiO₂ was synthesized as a minorphase in a polycrystalline sample coexisting with the major phase (Mg,Fe)SiO₃ post-perovskite (pPv) phase at 129 GPa and 2500 K in a DAC. Data sets from six grains of seifertite having arbitrary orientation have been combined and merged for structure determination and refinement.Observed systematic absences of reflectionsfrom the six individual grains allowed only one space group: Pbcn.  Paper in pdf：Am. Mineral.  101, 231–234,2016

Group description: Dr. Li Zhang’s group mainly studies chemistry, crystal structure, and physical properties of minerals under high pressure-temperature (P-T) conditions corresponding to deep interior of planets, particularly the Earth. Laser-heated diamond anvil cell (DAC) technology coupled with synchrotron x-ray spectroscopy (XRS) and x-ray diffraction (XRD) probes have allowed us to study properties of Earth materials under P-T conditions of the mantle and core. Research focuses include but not limit to: Deep mantle mineralogy; Single-crystal structures of minerals at Megabar pressure; Melting of Earth materials at high pressure; Physics and chemistry at high pressure; Synthesis of functional materials at high P-T conditions; Development of high P-T device and probe techniques.