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Carnegie Hosts 2010 SSAA Symposium PDF Print E-mail
For the third year, the SSAA Program Symposium was hosted by the Carnegie Institution of Washington at its historic administration building in downtown Washington, DC. The meeting was held from January 20-22 and drew participants from around the country. Individual grant recipients and center directors from the three divisions of the program (High Energy Density Physics, Low Energy Nuclear Science and Materials Properties under Extreme Conditions) gave updates on center or group activities and progress. CDAC Director Russell Hemley provided an overview of the program in his invited talk.




Xenon-Hydrogen Mixtures Yield Novel Compound at High Pressure PDF Print E-mail
Observations of van der Waals compounds in simple molecular systems at high pressure continue to bring surprises. Indeed, understanding the nature of bonding in these materials is not well understood. In the course of investigating the Xe-H2 system at high pressure, CDAC Research Scientist Maddury Somayazulu and co-workers from Carnegie and the Advanced Photon Source discovered a class of compounds with stoichiometry ranging from Xe(H2)7 to Xe(H2)8, along with the first experimental verification of bonding states in such materials.

A combination of Raman and infrared spectroscopic measurements, along with synchrotron x-ray diffraction experiments, showed that at 5 GPa, a hexagonal phase forms, which may be described as a tripled unit cell of solid hydrogen, intercalated with xenon atoms. The presence of two separate Xe-Xe distances in the structure, however, suggests the formation of Xe2 dimers, in which the Xe-Xe distance is 3.875 A, which is close to the Xe-Xe distance in solid Xe at 5 GPa. At higher pressures, additional hydrogen is taken up by the structure, which suggests a composition of Xe(H2)8 above 5.7 GPa. Infrared measurements show that the material remains an insulator up to 255 GPa.

Studies of the electron density maps show a surprising spread of electron density from the Xe2 pairs toward the surrounding array of H2 molecules. It is proposed that this spread of electron density is a signature of the depopulation of the highest occupied and fully filled Σ* molecular orbital of the Xe2 pair produced at high pressure. However, there is no indication at present of a transfer of electron density into the Σ* orbital of the H2 molecule which could lead to its dissociation. This suggests an interaction between Xe2 dimers and H2 molecules, which could lead to different bonding modes at higher pressures or different compositions in the Xe-H2 system. This work has been recently published in the online issue of Nature Chemistry, DOI:10.1038/NCHEM.445.
Future of Compression Science Workshop PDF Print E-mail
At the recent "21st Century Needs and Challenges in Compression Science" Workshop held at the Bishop's Lodge Ranch in Santa Fe, NM September 23-25, CDAC Director Russell Hemley chaired the Static Compression Panel, which included CDAC Advisory and Steering Committee members, as well as several CDAC collaborators. The workshop was convened by LANL Staff Scientists Dave Funk and Rusty Gray to help chart the course of high-pressure science for the coming decade, including opportunities at the proposed MaRIE facility at Los Alamos. The other participants on the Static Compression Panel were Christian Mailhiot (LLNL, CDAC Advisory Committee), Dana Dattlebaum and Yusheng Zhao (LANL, CDAC Steering Committee), and CDAC collaborators Reinhard Boehler (Carnegie), William Evans (LLNL), Malcolm McMahon (University of Edinburgh), Michael Pravica
(University of Nevada-Las Vegas) and Adam Schwartz (LLNL).


  • More information on the scientific issues discussed by the static compression panel can be found here.
  • The Static Compression Panel summary presented by Russell Hemley can be found here.
The Enigma of Dense Solid Hydrogen PDF Print E-mail
Hydrogen is the most abundant element in the cosmos, and its behavior at high pressure is crucial for a broad range of problems from condensed matter physics to astrophysics to stockpile stewardship.

The structure of hydrogen in the dense molecular state remains challenging both experimentally and theoretically. Working with CDAC scientists at Carnegie, Visiting Scientist Pierre Toledano of the Universite de Picardie developed a group-theory approach and combined it with available experimental data to predict the structures of the elusive high-pressure, low-temperature broken symmetry phases of solid molecular hydrogen (phases II and III). Data from spectroscopy and diffraction experiments do not provide unambiguous structural information for these orientationally ordered phases, but do suggest that the corresponding transitions are quasi-continuous and not reconstructive. This fact allows the use of the Landau theory to predict potential structures of the high pressure phases by constraining the number of possible mechanisms available for their formation.

Analysis of the behavior of order parameters with pressure suggests that H2-II has a partially ordered structure, while H2-III has an ordered isotranslational structure. In addition, the existence of another high-pressure phase, H2-I’ isostructural with H2-III, has been predicted. This new phase is defined by a boundary in the phase diagram that meets the boundaries of H2-I and H2-III at second triple point. The results have important implications for metallization predicted at higher pressures molecular systems [P. Tolédano, et al., Phys. Rev. Lett., 103, 105301 (2009)].

 

  • Carpenter, M. A., S. V. Sinogeikin, and J. D. Bass, Elastic relaxations associated with the Pm3m–R3c transition in LaAlO3: II. Mechanisms of static and dynamical softening, J. Phys.: Condens. Matter, 22, 035404 (2010)
  • Catalli, K., S. H. Shim, V. B. Prakapenka, J. Zhao, W. Sturhahn, P. Chow, Y. Xiao, H. Liu, H. Cynn, and W. J. Evans, Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties, Earth Planet. Sci. Lett., 289, 68-75 (2010).
  • Zhang, F. X., J. W. Wang, M. Lang, J. M. Zhang, R. C. Ewing, and L. A. Boatner, High-pressure phase transitions of ScPO4 and YPO4, Phys. Rev. B, 80, 184114 (2009).
  • Lucas, M. S., O. Delare, M. L. Winterrose, T. Swan-Wood, M. Kresch, I. Halevy, B. Fultz, J. Hu, M. Lerche, M. Y. Hu, and M. Somayazulu, Effects of vacancies on phonon entropy of B2 FeAl, Phys. Rev. B, 80, 214303 (2009).
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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 facilities, to perform key studies on a broad range of materials in newly-accessible P-T regimes, and to integrate and coordinate static, dynamic and theoretical results for Stewardship Science.