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Weird Bonds of Oxygen PDF Print E-mail
Molecular oxygen (O2) changes its form dramatically with compression, transforming to a solid with spectacular colors, and eventually becoming metallic and superconducting at sufficiently high pressures. The underlying mechanism for these remarkable phenomena has been a source of debate for decades, in particular the origin of the recently discovered molecular cluster (O2)4 in the dense solid, red phase. Yue Meng (HPCAT), and researchers from the Geophysical Laboratory, University of Chicago, University of Saskatchewan, and the National Synchrotron Light Source found that under pressure the individual molecules interact through their outermost, highest energy molecular orbitals. This pairing interaction brings four oxygen molecules together to form discrete (O2)4clusters at a pressure of about 100, 000 times atmospheric pressure, or 10 gigapascals. With increasing pressure, the effective radial extent of the molecular orbitals on individual molecules increases, promoting the interaction of unpaired electrons on adjacent molecules.

Using diamond anvil cell methods along with the newly developed high pressure inelastic x-ray scattering technique, the group was able to track the behavior of the relevant molecular orbitals on the oxygen molecules with compression. The fact that the energy of the scattered x-rays increased with increasing pressure indicated that the bonding character of the highest energy molecular orbitals was changing, as illustrated in the Figure. Theoretical studies of the energetics of formation of the (O2)4 clusters provide a rationale for the formation of the tetramolecular unit as opposed to the dimer (O2)2. The work suggests that interactions of a similar nature, which are well known in organic chemistry, could occur between (O2)4 clusters at higher pressures, leading to still other, as yet undiscovered phases [Meng, et al., Proc. Nat. Acad. Sci., 105, 11640-11644 (2008)]

Superconductivity in Compressed Silane PDF Print E-mail
Xiao-Jia Chen and colleagues from the Geophysical Laboratory, in collaboration with scientists from the Chinese University of Hong Kong and South China University of Technology, have predicted from first principles the superconducting properties of the hydrogen-rich molecular compound silane (SiH4). A gas at ambient conditions, solid silane adopts a structure with the SiH4 molecules oriented so as to produce layers of hydrogen and silicon atoms. This newly-discovered superconducting phase, which forms at 60GPa, has a structure belonging to the orthorhombic space group Cmca. With this layered structure, compressed SiH4 at this pressure resembles a silicon-hydrogen alloy, and provides some insight into the long-predicted existence of metallization and superconductivity in dense hydrogen, a problem that has been an important driving force in the development of condensed matter physics and astrophysics for nearly a century.

The research, published in Physical Review Letters [X. J. Chen, et al., Phys. Rev. Lett., 101, 077002 (2008)], suggests that the layered motif observed in the Cmca phase could be essential for superconductivity in other hydrogen-rich compounds.

This theoretical study confirms the experimental finding of pressure-induced metallization in SiH4, which was reported the team in January 2008 [X. J. Chen, et al., Proc. Nat. Acad. Sci., 105, 20-23 (2008)]. At 60 GPa, silane has a superconducting transition temperature between 20 and 75 K in the layered metallic phase, demonstrating the potential to observe metallization and superconductivity in hydrogen within a potential silicon-hydrogen alloy at higher pressures, but still much lower than would be necessary for solid hydrogen, due to chemical precompression by the silicon. After submission of this work, evidence for superconductivity in metallic silane was reported in the March issue of Science by an international team led by M. I. Eremets, formerly of the Geophysical Laboratory [Eremets et al., Science, 319, 1506-1509 (2008)].

APS Workshop PDF Print E-mail
A Workshop on Understanding Condensed Matter Dynamics at the Microscopic Level was held at the Advanced Photon Source, Argonne National Laboratory on June 23-24, 2008. It was organized by Yogi Gupta (WSU), Christian Mailhiot (DOE-NNSA), and Dennis Mills (APS).

The goal of the Workshop was to see how national facilities like the APS can be utilized to examine and understand the real time response of dynamically compressed materials at the microscopic level. It was a one and a half day workshop, consisting of invited talks and posters. Participants were from universities, national laboratories, and research institutes in the U.S. and abroad. For more information on the workshop, please go to: http://www.aps.anl.gov/News/Conferences/2008/DC_Workshop/



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Citroni, M., R. Bini, P. Foggi, and V. Schettino, Role of electronic states in the high-pressure amorphization of benzene, Proc. Nat. Acad. Sci., 105, 7658-7663 (2008).

Struzhkin, V. V., A. F. Goncharov, R. Caracas, H. K. Mao, and R. J. Hemley, Synchrotron infrared spectroscopy of the pressure-induced insulator-metal transitions in glassy As2S3 and As2Se3, Phys. Rev. B. 77, 165113 (2008).
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Welcome

CDAC is an interdisciplinary center headquartered at the Geophysical Laboratory of the Carnegie Institution of Washington. Our goals are to advance and perfect an extensive set of high P-T techniques and unique facilities, to perform key studies on a broad range of important materials in newly-accessible P-T regimes, and to integrate and coordinate static, dynamic and theoretical results for Stewardship Science applications.