1. Structural Determination of Solid SiH 4 at High Pressure Russell J. Hemley (Carnegie Institution of Washington) DMR 0508988 The hydrogen-rich solids are of great fundamental interest. Solid SiH 4 has been proposed to eventually undergo a transition to a metallic and then superconducting state at pressures considerably lower than may be necessary for solid hydrogen [Ashcroft, PRL 92, 187002 (2004)]. Structural information is the primary step toward understanding these electronic properties. In this study synchrotron x-ray diffraction was used to reveal a monoclinic structure of solid SiH 4 with space group P2 1 /c in phase between 10 and 25 GPa, providing the first structural information on this material under pressure O. Degtyareva et al., PRB 76, 064132 (2007). Left: Pressure dependence of the unit cell volume of the SiH 4. Solid and open symbols show data collected on pressure increase and decrease, respectively. Solid and dashed lines represent the experimental and theoretical equation of state. Inset: Integrated profile of the SiH 4 phase V at 24.8 GPa collected on pressure increase (crosses) and the Rietveld refinement fit for the P2 1 /c structure (line). Right: Crystal structure for the high-pressure SiH 4 phase at 24 GPa.

2. Pressure-Driven Gas SiH 4 to Metallic State: En Route to ``Metallic Hydrogen’’ Russell J. Hemley (Carnegie Institution of Washington) DMR 0508988 High-pressure spectroscopy was used to identify four solid phases of SiH 4, which we named phase III, IV, V, and VI. Solid SiH 4 becomes opaque at 27-30 GPa. The infrared measurements show an increase of the reflectivity starting at 60 GPa, signaling pressure- induced metallization. X. J. Chen et al., PNAS 105, 32 (2008). Infrared reflectivity spectra of SiH 4 at selected pressures up to 67.2 GPa. The red squares are the measurement results. The solid lines represent model fits to the data. Raman spectra and photos of samples of SiH 4 at room temperature.

3. Superconducting Behavior in Compressed Solid SiH 4 with a Layered Structure Russell J. Hemley (Carnegie Institution of Washington) DMR 0508988 X. J. Chen et al., Phys. Rev. Lett., 101, 077002 (2008). The electronic and dynamical properties of compressed solid SiH 4 have been calculated up to 300 GPa (3 million atmosphere) with density functional theory. We find two energetically preferred insulating phases with different symmetries at low pressures. A layered structure is the favored metallic phase over a wide pressure range above 60 GPa. The superconducting transition temperature in this layered metallic phase is found to be at 20-75 K. Evidence for superconductivity in metallic silane was recently confirmed [Eremets et al., Science 319, 1506 (2008)]. Our results indicate that layered structures could be essential for superconductivity in this and other hydrogen-rich compounds. The predicted energetically favorable structures at pressures. The Cmca structure is the most stable high-pressure superconducting phase.

4. High-Pressure Studies of Molecular SiH 4 Russell J. Hemley (Carnegie Institution of Washington) DMR 0508988 Our systematic high-pressure studies on the hydrogen-rich solid molecular SiH 4 provides insight into the metallization and superconductivity in dense hydrogen, a problem of interest in condensed matter physics and astrophysics for nearly a century. The work could lead to the development of new superconductors. Two national facilities (APS and NSLS) as well as our in-house built equipment were used in this research. Theorists and experimentalists from the USA, UK, Spain, Canada, and China collaborated in this extended series of studies of silane. Our article on the metallization of silane was selected as a science highlight on a variety of websites. National Synchrotron Light Source Advanced Photon Source