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Anomalous High-Pressure Behavior of Selenium PDF Print E-mail
A collaborative research team from multiple institutions, including Harbin Institute of Technology, Argonne National Laboratory, Harvard University, and Geophysical Laboratory has discovered unexpected dynamics and volume expansion associated with pressure-induced crystallization of amorphous selenium. The development and application of the state-of-the-art high pressure research facilities at multiple beamlines at the Advanced Photon Source during the last two years ensure have made the work possible. The initial appearance of the crystal phase and the transformation between monoclinic and trigonal phases was monitored temporally and spatially, revealing the structural origin of the long-standing enigma of time-dependent electrical properties of the material on compression. The unusual volume expansion phenomenon under pressure associated with the crystallization of an "over-pressurized" metastable phase found in this study may be more common than previously believed.

The research involved the application of several cutting-edge methods: in situ high pressure synchrotron x-ray diffraction (XRD), novel diamond anvil cell microtomography techniques, as well as first-principles electronic structure calculations. These investigations have revealed the anomalous pressure-induced behavior of amorphous selenium, in particular the direct experimental observation of a volume expansion of a material under high pressure. These results demonstrate the importance of using new time- and spatially- resolved high-pressure XRD and imaging techniques to understand the kinetics of structural transformations in materials under extreme conditions. The new microtomographic technique developed in this project could find widespread use in determinations of the equations of state of glasses and melts up to megabar pressures, which is crucial to many problems in earth, planetary, and materials sciences [Liu, et al., Proc. Nat. Acad. Sci., 105, 13229-13234 (2008)].
The Surprising Stability of Molecular Nitrogen at High P and T PDF Print E-mail
The N-N triple bond in molecular nitrogen is one of the strongest and most stable chemical bonds in nature, giving rise to the two most commonly observed states, the solid and liquid. Molecular nitrogen itself, however, has shown a surprising degree of polymorphism, leading to a complex phase diagram that is still incomplete in the high pressure and temperature regions. Particularly important is the region near the melting line at pressures higher than 50 GPa, where theoretical work and shock wave experiments have suggested the transition to a polymeric form, which occurs at much higher pressures at low temperatures, according to previous static compression experiments.

Recent work by Alexander Goncharov and colleagues at Carnegie and Lawrence Livermore National Laboratory have obtained Raman spectra up to 120 GPa and 2500 K for both solid and fluid nitrogen in an effort to clarify the behavior of this system at high pressures and temperatures. To probe the material directly at high P and T, a pulsed laser to the sample was combined with a second pulsed laser that excited the vibrational spectrum characteristic of the molecular state of the solid. This pulsed Raman spectroscopy technique allows the acquisition of spectra at precisely the peak temperature of the sample, and also reduces the background thermal radiation from a metallic laser light absorber. The spectroscopic data show that nitrogen in the fluid state remains in the molecular form throughout the pressure and temperature ranges investigated, and that there is no evidence to support a fluid-fluid transition.

As shown in the figure, the melting line reaches a maximum in temperature with increasing pressure at around 70GPa, followed by a decrease up to a triple point at approximately 87 GPa. This behavior suggests an explanation for the onset of amorphization at still higher pressures and lower temperatures as has been observed previously in SiO2 and H2O [Goncharov, et al., Phys. Rev. Lett., 101, 095502 (2008)].
Symposium in Honor of Two NSLS Beamline Scientists PDF Print E-mail





A workshop/symposium honoring the service of Jingzhu Hu and Quanzhong Guo at the NSLS X17 beamlines, Brookhaven National Laboratory, will be held on October 4, 2008 at Brookhaven. For 18 years, the two have been instrumental in pioneering x-ray diffraction techniques for high pressure research. More...
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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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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.