High-Pressure Geophysics, Geochemistry, and Petrology
Studies of the Earth's deep interior present us with a rich array of large-scale processes and phenomena that are not fully understood. Resolving these questions requires a focused study on iron which is the most abundant element in Earth, and plays a key role in processes from the crust to the core. For the metallic core, I have working on a series of experiments to test and explore various hypotheses and constraints on the alloy compositions, phase relations, elasticity, and rheology of iron (GRL 2002; GRL 2004; PEPI 2006). For the lower mantle, I have been studying an iron-rich, ferromagnesian silicate (PNAS 2005; PNAS 2004b;Science 2006a) at the very inhospitable conditions (125 GPa, 2500 K) existing between the liquid, iron core and the overlying solid, silicate mantle.
These studies have prepared me to launch further, in-depth investigations over an extensive P-T and to collaborate with petrologists, geochemists, seismologists, and geodynamicists to unravel some of the central questions regarding the deep Earth. They include understanding the origin of inner core seismic anisotropy, the composition and temperature profile of the inner and outer core, the chemistry of the mantle, radioactive element partitioning between the mantle and core, the source of enigmatic D” seismic features (velocity discontinuity, shear-wave splitting, ultra-low velocity zones, radial and lateral velocity heterogeneities, Vs-Vp anti-correlation, etc.), the fate of subducting plates, and the source of hot spots and plumes.
Volatiles in Planetary Systems and Applications to Energy and Environmental Issues
Gases (H2, He, Ne, etc.) and ices (H2O, CH4, CO2, NH3, etc.) are major constituents of the giant planets and their icy satellites. New studies have invariably led to astonishing and often unexpected discoveries. For instance, in a mixture of hydrogen (the most abundant element in the universe) and water (the most abundant compound), we found a new H2(H2O)2 solid with sII clathrate structure which can be synthesized at moderate pressures and temperatures (<0.2 GPa and 240 K) and is quencheable to ambient pressure at 110 K (Science 2002; PRL 2004). This finding provides a new mechanism for hydrogen retention in the outer solar system. In another study of hydrogen and methane (the most abundant organic compound), I found a vastly expanded pressure stability at low-temperature in the novel van der Waals compound (H2)4CH4 (CPL 2005). These pioneering findings for the chemistry in planetary gases and ices are certainly just the tip of an enormously rich iceberg.
Discovery of novel molecular compounds at high pressures has also opened new frontiers in hydrogen energy applications (PNAS 2004a; U.S. Patent 6,735,960). The H2(H2O)2 sII clathrate contains sufficient amount of hydrogen to meet the DOE 2005 goal for its Hydrogen Storage Initiative (Science 2002; PRL2004), and is environmentally superior since the only byproduct after fuel consumption is H2O. The (H2)4CH4 compound contains the highest amount of hydrogen among all known compounds. I plan to continue research in multi-component hydrogen-containing systems to address the long-term resolution of the energy issue.
Experimental Mineral Physics
Synchrotron radiation has enabled a wide variety of novel techniques for characterizing materials at extreme conditions. Conducting the first high-pressure x-ray Raman study of the near K-edge structure of carbon at high pressure (Science 2003), I resolved the mystery of what happens to cold-compressed graphite, a problem that had puzzled scientists for nearly four decades, because the appropriate measurements were not possible. A more recent discovery found that at high pressure, x-radiation caused ice VII to decompose into a new O2-H2 compound, opening a new area of high pressure radiation chemistry (Science, 2006b) I have been using new synchrotron techniques coupled with high pressure including nuclear resonant and nonresonant inelastic x-ray scattering, high resolution x-ray emission spectroscopy, laser heating coupled with x-ray diffraction, and radial x-ray diffraction, and formed an optimization method for the measurement of key properties of minerals (e.g. density, unit cell parameters, bulk and shear moduli, single-crystal elasticity (Cij or Sij), compressional and shear wave velocities, phonon density of state, phonon dispersion, etc.) to the accuracy necessary for constraining Earth models. We have also been using the neutron diffraction and optical Raman and infrared spectroscopy to study high-pressure hydrogen crystallography and bonding (Science 2002; PRL 2004; CPL 2005).