Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session T5: Physics & Chemistry at High Static Pressure |
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Chair: Malcolm Nicol, University of Nevada Las Vegas Room: Hyatt Regency Constellation F |
Thursday, August 4, 2005 1:00PM - 1:15PM |
T5.00001: Pressure-Induced Phase Transitions in Pentaerythritol (PE) Crystals Zbigniew A. Dreger, Yogendra M. Gupta, Choong-Shik Yoo, Hyunchae Cynn Pentaerythritol [C(CH$_{2}$OH)$_{4}$] is a simple solid polyalcohol with a crystal structure determined by balance between weak van der Waals interactions and relatively strong hydrogen bonds. To determine the stability of this structure under compression we have examined the response of PE at high pressures using Raman spectroscopy and synchrotron x-ray diffraction. It is shown that PE can exist in three different phases at pressures up to 10 GPa. Analyses of Raman spectra and x-ray diffraction data indicate onset of phase transitions at $\sim $ 4.8 GPa and $\sim $7.2 GPa. The phase I$\to $II transition at $\sim $ 4.8 GPa, transforms the ambient tetragonal structure into an orthorhombic structure (\textit{Pnn2 }space group) with PE molecules occupying C$_{2}$ symmetry sites. Furthermore, we propose that the phase I$\to $II transition involves changes in the hydrogen bonding network (from quadrilateral to quasi one-dimensional). The proposed structure in phase II is consistent with available experimental data. Finally, we suggest that the crystal structure of phase III is a conformational variant of phase II, based on the similarity in their x-ray diffraction patterns. Work supported by DOE. [Preview Abstract] |
Thursday, August 4, 2005 1:15PM - 1:30PM |
T5.00002: Static X-Ray Diffraction Study of Cerium to 330 Kilobars Gary Chesnut, William Anderson, Joanna Casson Cerium is a fascinating f-electron metal with a complex phase diagram. Using the diamond anvil cell and synchrotron x-ray sources we have been able to study cerium to a pressure of 330 kilobars. Radial x-ray diffraction techniques have been used to ensure a more accurate pressure-volume curve at room temperature. In this study two different purities of cerium have been examined -- 99.99{\%} and 99.999{\%}. Even at such modest pressures differences have been observed. Regarding the structural phases, the iso-structural fcc phase transition occurs at 7.3 kilobars with a volume collapse of 14.6{\%}. The $\alpha $-fcc phase to low symmetry phase and low symmetry phase to body-centered tetragonal phase were also observed. This work produces high quality data that can be used to generate an accurate multi-phase EOS for cerium. The techniques developed in these experiments can further be utilized to obtain accurate EOS data for other materials of interest. [Preview Abstract] |
Thursday, August 4, 2005 1:30PM - 2:00PM |
T5.00003: Elastic and Electronic Properties of some Fe compounds Invited Speaker: A new stress-strain method is employed to study the elastic properties of simple elements including the recently proposed anti-ferromagnetic phase of hcp epsilon-Fe. The results are compared with the conventional energy-strain method. At low pressure, results on epsilon-Fe show a slight improvement with experimental elastic constants and phonon density of states but serious discrepancy still persist at high pressure. We also examined electron correlaton effects on the electronic spectra of Fe containing minerals with the LDA+U model. We calculated the photoelectron spectra of several Fe containing minerals and examine the recently reported spin transition in ferropericlases. It is found that local environment of the Fe impurities plays an important role on the spin transition pressure and the ground electronic state. The super-exchange interactions between Fe2+ along the Fe-O-Fe chains led to an anti-ferromagnetic ground states. The predicted spin transition pressures are found to increase with the Fe content. [Preview Abstract] |
Thursday, August 4, 2005 2:00PM - 2:15PM |
T5.00004: Study of Phase Transformations on Nanocrystalline (La,Sr)(Mn,Fe)O3 System by High-Pressure Mossbauer Spectroscopy Usha Chandra, Prerana Mudgal, Manoj Kumar We report here the pressure dependent 57Fe Mossbauer studies on the nanocrystalline La0.8Sr0.2(Mn0.8Fe0.2)O3 system up to 10 GPa using diamond anvil cell. At ambient pressure, iron is present as Fe3+ and Fe4+ in two different environments. Pressure seems to affect the higher symmetry site of Fe4+ only, while the octahedral site containing Fe3+ remains almost unaffected. Phase transformations are observed at pressures 0.52GPa and 3.7 GPa respectively. A sudden increase in the isomer shift at 0.52GPa is related to the reduction of Fe4+ ions while at 3.7GPa, a structural transition is observed with a sudden drop in isomer shift indicating that the Fe3+ ions are in identical environment. Quadrupole splitting increases continuously with pressure up to 10 GPa. [Preview Abstract] |
Thursday, August 4, 2005 2:15PM - 2:30PM |
T5.00005: X-Ray Diffraction Studies of the Structures of Dynamically Compressed Be, Al, LiF, KCl and SiO$_{2}$ Leonid Egorov, Victoria Mokhova, Alexsey Barenboym, Alexsey Samoilov The first x-ray diffraction data for restructuring process of dynamically compressed crystals have been obtained and photographs of the diffraction patterns are presented. The recorded diffraction patterns demonstrate the universal process of structural material reorganization. The diffraction patterns imply that a relaxation process of substance restructuring connects changes of the crystal's electronic structures, forming new chemical bonds. Within the framework of the interpretation one discusses the results of the measurements displaying the time rise of the quantity of an ultradispersion diamonds inside the detonating HE charge the results of the measurements of the liquid deuterium Hugoniot obtained by the group of physicists from LLNL. [Preview Abstract] |
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