Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B30: High Pressure I |
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Sponsoring Units: DMP Chair: Javier Antonio Montoya, Carnegie Institution of Washington Room: D139 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B30.00001: Plasma phase transition in high pressure hydrogen from ab-initio simulations Miguel Morales, Carlo Pierleoni, Eric Schwegler, David Ceperley We performed a detailed study of molecular dissociation in liquid hydrogen using Born-Oppenheimer molecular dynamics with Density Functional Theory and Coupled Electron-Ion Monte Carlo simulations. We observe a range of densities for which $(dP/d\rho)_T = 0 $ and find sharp discontinuities in the electronic conductivity; both are clear evidence of the plasma phase transition for temperatures $600 K \leq T \leq 1500 K$. Both levels of theory exhibit the transition, although Quantum Monte Carlo predicts higher transition pressures. Based on the temperature dependence of the discontinuity in the electronic conductivity, we estimate the critical point of the transition at temperatures slightly below 2000 K. Using Path Integral Molecular Dynamics we examine the influence of zero point motion on the predicted transition, which still exhibits a first order behavior; the main effect of nuclear zero point energy is to shift the transition to smaller pressures. We calculate the melting curve of molecular hydrogen up to pressures of 200 GPa, finding a reentrant melting line in good agreement with previous calculations. The melting line crosses the metalization line at 700 K and 220 GPa with density functional theory and at 550 K and 290 GPa within Quantum Monte Carlo. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B30.00002: First-Principles Investigation on the Structure of Phase III of Hydrogen Marc Dvorak, Xiaojia Chen, Zhigang Wu First-principles computations based on the density functional theory have been performed to search for the most stable structure of phase-III solid hydrogen. Specifically the phase diagram at zero temperature is predicted by calculating the static total energy and the zero-point motion. Our results suggest that solid hydrogen under pressures in a range of $\sim 150-300$ GPa could have lower symmetry than predicted previously. We will discuss the topological considerations on phase transitions from Phase I to II to III, and the available experimental evidences supporting current finding as well. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B30.00003: On the structure of compressed liquid hydrogen Stanimir Bonev, Isaac Tamblyn We report first-principles results that predict the existence of a hitherto unknown short-range orientation order in both molecular and non-molecular liquid hydrogen. The appearance of this order provides a physical explanation for the sharpness of the dissociation transition, and has implications for the accuracy of previous equation of state calculations and the expected finite-temperature crystalline phases of hydrogen. In addition, we present results mapping the dissociation transition line. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B30.00004: Meta-stable solid hydrogen Bernard Kozioziemski, Alexander Chernov, John Lugten, Evan Mapoles, James Pipes, James Sater A meta-stable solid of the hydrogen isotopes H$_2$, D$_2$, and the deuterium-tritium mixture to be used for inertial confinement fusion (ICF) studies is observed when solidified through their respective triple-points. The meta-stable solid is distinguished from the stable hexagonal-close-packed (HCP) solid by three observations. First, the ``triple-point'' temperature of the meta-stable solid is 16 - 35 mK below that of the HCP solid, with the value isotope dependent. Second, the shape of the solid spreading over the surface is approximately isotropic in contrast to the anisotropic growth shape of the HCP solid. Finally, the meta-stable solid will rapidly transform to an often polycrystalline HCP solid. The meta-stable solid grows from the 5 - 20 $\mu$m diameter borosilicate fill-tubes recently used for ICF capsule assemblies much more often than from the 30 - 40 $\mu$m diameter silica glass tubes previously used in the hydrogen ICF layering studies. The meta-stable solid has reached sizes up to 50 $\mu$m thick and 1 mm in diameter. We will discuss our hypotheses for the nature of this meta-stable solid. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B30.00005: Repulsive interactions of deuterium and nitrogen under high pressures Minseob Kim, Choong-Shik Yoo High-pressure studies of simple diatomic mixtures are fundamental to understanding the nature of intermolecular interactions and, thereby, their physical and chemical transformations. In this paper, we present the Raman and x-ray studies of D$_{2}$:N$_{2}$ mixtures to 70 GPa. Our results indicate that the evolution of Raman spectra of D$_{2}$ under pressure is apparently coupled to the structural phase transitions in the host N$_{2}$ lattice and their crystal structures. A large blue-shift of D$_{2}$ vibron in N$_{2}$ lattice at high pressures indicates a highly repulsive nature of intermolecular interactions between the host N$_{2}$ and guest D$_{2}$ molecules. We will discuss about the origin of such repulsive interaction in terms of the crystal structure of the mixture at high pressures. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B30.00006: Xe(H$_{2})_{7}$ - A hydrogen-rich van der Waal compound stable to multimegabar pressures Maddury Somayazulu, Przemyslaw Dera, Stephen Gramsch, Russell Hemley We have recently reported the occurrence and stability of a sequence of hydrogen-rich compounds of Xe and H$_{2}$ [1]. The first solid phase that forms at 4 GPa changes stoichiometry at discrete pressures culminating in a solid whose stoichiometry is determined to be Xe(H$_{2})_{7}$. The Raman and IR spectra of this solid display remarkable complexity that can be explained in terms of a tripled hydrogen lattice. We report the details of this spectroscopy that have been measured to a maximum pressure of 255 GPa. Single crystal diffraction data of very high quality was collected at the HPCAT beamline 16-BM-D at the APS. The low pressure data was used to identify not only the structural details but also evaluate the changes in the electron density of xenon indicative of interaction between the xenon dimers and the surrounding hydrogen molecules. Acknowledgements: This work was supported by DOE-BES (DE-FG02-06ER46280), NSF-DMR (DMR-0805056) and DOE-NNSA(CDAC). A.P.S. is supported by DOE-BES under contract DE-AC02-06CH11357. \\[4pt] [1] Somayazulu et. al Nature Chemistry (in press) [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B30.00007: Electronic phase transitions in XeF$_{2}$ under extreme pressures Mathew Debessai, Minseob Kim, Choong-Shik Yoo The application of high external pressure decreases the interatomic/molecular distance of solid in a substantial way, and often gives rises to novel transitions and properties such as insulator-metal transition, superconductivity, and magnetism. Fluorine is one of few materials that have not been metalized, presumably because of the formidably high transition pressure. Yet, it has been suggested that the formation of molecular compounds may drop the transition pressure as recently found in hydrogen-rich group IV systems such as CH$_{4}$ and SiH$_{4}$. In this paper, we present the experimental evidence that XeF$_{2}$ indeed metalizes at the pressure well below the metallization pressures of Xe (124 GPa) or F$_{2}$ (predicted to be above 500 GPa) and thereby confirms the concept of a chemically precompressed state. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B30.00008: Interactions between hydrogen and silane at high pressure Shibing Wang, Wendy L. Mao, Ho-kwang Mao, Xiao-Jia Chen Understanding the behavior of hydrogen-rich systems at extreme conditions has significance to both condensed matter physics and applied research areas like hydrogen storage. We report the high-pressure study of the SiH$_{4}$-H$_{2}$ binary system at 300K in a diamond anvil cell. Raman measurements indicate significant intermolecular interactions between H$_{2}$ and SiH$_{4}$. We found that H$_{2}$ vibron frequency is significantly softened with the presence of SiH$_{4}$ for the fluid phase compared with pure H$_{2}$ fluid at the same pressures. In contrast, the Si-H stretching modes of SiH4 shift to higher frequency in the mixed fluid compared with pure SiH$_{4}$. Pressure induced solidification of the H$_{2}$-SiH$_{4}$ fluid shows a binary eutectic point at $\sim$ 72 mol\% H$_{2}$ and $\sim$ 6.1 GPa, above which the fluid crystallizes into a mixture of two nearly end-member solids. We were able to superpressurize the sample above the eutectic pressure before complete crystallization, indicating extended metastability. The properties of the two nearly end-member solids will also be presented. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B30.00009: Pressure-Induced Interactions in Silane-Hydrogen Timothy Strobel, Maddury Somayazulu, Russell Hemley We report pressure-induced formation of a novel molecular silane-hydrogen compound with intermolecular interactions unprecedented for hydrogen-rich solids. A complex H2 vibron spectrum with anticorrelated pressure-frequency dependencies and a striking H-D exchange below 10 GPa reveal strong and unusual attractive interactions between SiH4 and H2 and molecular bond destabilization at remarkably low pressure. Structural analysis from single crystal X-ray diffraction data, hydrogen rotational dynamics from low-temperature Raman measurements, H-D isotopic exchange measurements and similarities to other molecular hydrides will be discussed. The unique features of the observed SiH4(H2)2 compound suggest a new range of accessible pressure-driven intermolecular interactions for hydrogen-bearing simple molecular systems and a new approach to perturb the hydrogen covalent bond. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B30.00010: The crystal structure and SiH$_{4}$-H$_{2}$ interactions of high-pressure SiH$_{4}$(H$_{2})_{2}$ from first principles Kyle Michel, Yongduo Liu, Vidvuds Ozolins Mixtures of SiH$_{4}$ and H$_{2}$ have recently been found to crystallize at pressures above 6.8 GPa. Here, the crystal structure, bonding, and vibrational properties of SiH$_{4}$(H$_{2})_{2}$ over a range of applied pressures are studied using first-principles density functional theory (DFT) calculations. Results show a decrease in the frequencies of the intramolecular H$_{2}$ stretching modes with increasing pressure, contrary to the behavior of bulk H$_{2}$ under an applied pressure. This softening of the H$_{2}$ bond is found to be much more prominent for H$_{2}$ located in tetrahedral sites rather than in octahedral sites. DFT calculations suggest that the behavior of the H$_{2}$ bond is explained by an increased orbital overlap and electron sharing between silane and hydrogen molecules. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B30.00011: Pressure-Induced Decomposition of Hydrogen Peroxide Jing-Yin Chen, Minseob Kim, Choong-Shik Yoo, Dana Dattelbaum , Steve Sheffield We have studied the pressure-induced chemical decomposition of pure ($\sim $97.5{\%}) hydrogen peroxide to 50 GPa, using confocal micro-Raman and synchrotron X-ray diffraction. Our results indicate that pure hydrogen peroxide crystallizes into a tetragonal structure (P4$_{1}$2$_{1}$2), the same structure of 90 {\%} H$_{2}$O$_{2}$ previously reported below 8 GPa and of pure H$_{2}$O$_{2}$ at low temperatures. The tetragonal phase (H$_{2}$O$_{2}$-I) is stable to 15 GPa, above which transforms into an orthorhombic structure (H$_{2}$O$_{2}$-II) over a large pressure range between 15 and 20 GPa. The diffraction pattern of H$_{2}$O$_{2}$-II is analogous to that of $\varepsilon $-oxygen, suggesting a similar packing of oxygen atoms between H$_{2}$O$_{2}$-II and $\varepsilon $-O$_{2}$. In fact, we found that H$_{2}$O$_{2}$-II eventually decomposes to into H$_{2}$O and O$_{2}$ at 45 GPa. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B30.00012: Metallic nitrogen at high pressure and temperature Brian Boates, Stanimir Bonev A polymeric, metallic phase of solid nitrogen is predicted at high pressure and temperature from first principles. The structure is found using a novel approach that makes use of structural information from simulations of liquid nitrogen in an effort to incorporate already known finite-temperature behavior. We have determined the finite-temperature phase boundaries of several competitive phases and report results for the conductivity of both solid and liquid nitrogen. Routes for experimental detection of the new structures are proposed. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B30.00013: Ab initio investigation of the melting line of molecular nitrogen at high pressure Giulia Galli, Davide Donadio, Leonardo Spanu, Ivan Duchemin, Francois Gygi Understanding the behavior of molecular systems under pressure is a fundamental problem in condensed matter physics. In the case of Nitrogen, the determination of the phase diagram and in particular of the melting line, are largely open problems. Two independent experiments have reported the presence of a maximum in the nitrogen melting curve, below 90 GPa, however the position and the interpretation of the origin of such maximum differ.By means of ab initio molecular dynamics simulations based on density functional theory and thermodynamic integration techniques, we have determined the phase diagram of nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting line, connected to the presence of a triple point, which is related to a first order liquid-liquid phase transition, from molecular N$_2$ to polymeric nitrogen. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B30.00014: Methane under pressure: dissociation reactions from ab initio simulation Detlef Hohl, Leonardo Spanu, Davide Donadio, Eric Schwegler, Francois Gygi, Giulia Galli Using ab initio molecular dynamics [1], we have investigated the stability of methane under pressure up to $\sim 24$ GPa and in the temperature range $2-4000$ K, studied in recent diamond anvil cell experiments [2]. In particular, we have explored the possible formation of alkanes from methane dissociation. We have calculated structural and vibrational properties, and compared the relative stability of several hydrocarbons mixtures. Our results show that the temperature is the main driving force for methane dissociation, with pressure enhancing the formation of higher hydrocarbons, at temperatures where dissociation is observed. [1] Qbox code: http://eslab.ucsvais.edu [2] A. Kolesnikov, Kutcherov VG. and Goncharov AF. {\it Nature Geoscience} , (2009) {\bf 8} (566-570) [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B30.00015: Water-methane mixtures at extreme conditions Mal-Soon Lee, Sandro Scandolo The solubility of methane in water is low at ambient conditions but grows at extreme conditions. A methane-water fluid mixture makes up more than 90\% of the middle layer of Neptune and Uranus, where it is subject to pressures up to several hundred gigapascal. Separate studies on the mixture end-members, water and methane, show that extreme conditions lead pure water to form an ionic fluid, while methane dissociates forming hydrocarbons and precipitating diamond. Our first-principle molecular dynamic simulations show that the properties of a fluid water-methane mixture are qualitatively different, at extreme conditions, with respect to those of the individual components. Mixing of methane and water at extreme conditions is a consequence of the pressure-induced softening of their intermolecular interaction. Ionized water causes the progressive ionization of methane and the mixture becomes electronically conductive at milder conditions than pure water, suggesting that the planetary magnetic field of Uranus and Neptune may originate at shallower depths than currently assumed. [Preview Abstract] |
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