Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J40: Focus Session: Earth and Planetary Materials II |
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Sponsoring Units: DMP DCOMP Chair: Boris Kiefer, New Mexico State University Room: Morial Convention Center 232 |
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J40.00001: The melting curve of MgSiO3 perovskite from ab initio molecular dynamics using the coexistience method Invited Speaker: Despite its importance in understanding such things as the crystallisation of the Earth's mantle from a magma ocean or the existence of melt in the current mantle, the melting temperature of the lower mantle phase MgSiO3 perovskite is poorly know. Estimates of its melting temperature at the core-mantle-boundary range from 5400 K to over 8000 K. We have used, therefore, ab initio molecular dynamics simulations to predict its melting temperature throughout the Earth's mantle using the coexistence method. We used 900 atoms (a 3x3x5 super-cell) with atoms in one half of the super-cell melted and the other half solid. Both halves are thermalised to the desired temperature individually. We then turned off the thermalisation and allowed the system to evolve in an NVE simulation, using DFT forces calculated within the GGA. Those systems which were too hot melted within 10 ps. Those which didn't remained with both solid and melt coexisting in the super-cell for over 25 ps. These where assumed to be either on the melting curve of just below it. Our results agree well with the higher temperature melting curves found experimentally, and we predict a melting temperature of about 6500 K at the core-mantle boundary. We will also present results on simulating the melting temperature of the MgO-MgSiO3 binary. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J40.00002: First-Principles Molecular Dynamics of Melts in the MgO-SiO$_{2}$ System Bijaya Karki, Nico de Koker, D. Bhattarai, Lars Stixrude We have recently completed simulations of five melt compositions in the MgO-SiO$_{2}$ system within density functional theory. These results allow us to investigate the structural and thermodynamical, transport properties of melts along the MgO-SiO$_{2}$ join as a function of pressure. In particular, we have found that the mixing in MgO-SiO$_{2}$ system is significantly non-ideal at low pressures with negative excess volume and enthalpy of mixing. With increasing pressure, the volume of mixing decreases rapidly to a value close to zero at pressures above 50 GPa whereas the enthalpy of mixing remains negative. The radial distribution functions and coordination environments are found to show interesting changes with varying composition. Also, the effects of composition on diffusivity are shown to be substantial at low pressures whereas the effects are increasingly suppressed with increasing pressure. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J40.00003: A New look into the spin transition in Fe$_2$O$_3$ Dipta Bhanu Ghosh, Stefano de Gironcoli The wide range of intriguing characteristics exhibited by Fe$_2$O$_3$ with pressure and temperature has renewed the attention of the scientific community in the last decade. Experimental and theoretical efforts are on to address and unravel the complexity of the system. The ambient pressure phase, hematite ($\alpha$-Fe$_2$O$_3$) transforms to a new structural phase (HP1). That the HP1 phase is orthorhombic perovskite (Pbnm) or Rh$_2$O$_3$-II type (Pbcn) is still a debate and yet to be explored theoretically. On top of this ambiguous assignment of HP1, there has been a long-standing issue of an isostructural high spin (HS) to low spin (LS) transition. Experimental data till date are divided into two horizons--one assigning the spin transition in the hematite phase and the other in the HP1 phase. In this work, motivated by these exotic unresolved controversies of the system, we have tried to gain an insight of the system from first principles density functional calculations. Our results favor the Rh$_2$O$_3$-II type as the HP1 phase, in agreement with recent experiments. Also a (new) mechanism governing the HS to LS transition is proposed. This mechanism, we believe, might help in removing the boundary between the two horizons as mentioned above. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J40.00004: Prediction of an ultrahigh-pressure form of Al$_2$O$_3$ Koichiro Umemoto, Renata Wentzcovitch We predict by first principles a pressure induced phase transition in alumina at $\sim$3.7 Mbar, relevant for interiors of the giant planets and terrestrial exoplanets, at room temperature from the CaIrO$_3$-type polymorph to another with the U$_2$S$_3$-type structure. This transformation should be important for the analysis of shock data in this pressure range, since alumina is used as window material. Our calculated compression curves agree with shock data excellently, indicating that the presence of two phase transitions (corundum--Rh$_2$O$_3$(II)-type and Rh$_2$O$_3$(II)-type--CaIrO$_3$-type) had gone unnoticed in shock data. Our prediction suggests that the multi-Mbar crystal chemistry of planet-forming minerals might be related to that of the rare-earth sulfides. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J40.00005: Models of Giant Planet Interiors Derived from First-Principles Simulation Burkhard Militzer, Jan Vorberger, William Hubbard Our understanding of the interior of giant planets is based on the accurate characterization of hydrogen and helium at megabar pressures and temperatures of several thousands of Kelvin. Theoretical method including first-principles computer simulations have been the preferred tool to study these dense fluids because laboratory experiments cannot yet probe deep into Jupiter's interior despite great progress in shock wave measurements with precompressed samples. Results from an extensive set of density-functional molecular dynamics simulations will be presented [J. Vorberger \textit{et al.,} ``Hydrogen-Helium Mixtures in the Interiors of Giant Planets,'' \textit{Phys. Rev. B} \textbf{75} (2006) 024206]. A new and more accurate equation of state (EOS) will be derived that spans the interior of giant planets. Differences from the widely used Saumon-Chabrier-Van Horn (SCVH) EOS will be analyzed. An updated model for the interior of Jupiter will be introduced. Estimates for the heavy element enrichment as well as for the size of Jupiter's core will be discussed and compared with previous models based on the SCVH EOS. This work is supported by NASA grants PGG04-0000-0116 and NAG5-13775 as well as NSF grant 0507321. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J40.00006: Titan's Interior Chemical Composition: Possible Important Phase Transitions Michael Howard, Joseph Zaug, Bishun Khare, Christopher McKay We study the interior composition of Titan using thermal chemical equilibrium calculations that are valid to high pressures and temperatures. The equations of state are based on exponential-6 fluid theory and have been validated against experimental data up to a few Mbars in pressure and approximately 20000K in temperature. In addition to CHNO molecules, we account for multi-phases of carbon, water and a variety of metals such as Al and Fe, and their oxides. With these fluid equations of state, chemical equilibrium is calculated for a set of product species. As the temperature and pressure evolves for increasing depth in the interior, the chemical equilibrium shifts. We assume that Titan is initially composed of comet material, which we assume to be solar, except for hydrogen, which we take to be depleted by a factor 1/1000. We find that a significant amount of nitrogen is in the form of N$_{2}$, rather than NH$_{3}$. Moreover, above 12 kbars, as is the interior pressure of Titan, a significant amount of the carbon is in the form of graphite, rather than CO$_{2}$ and CH$_{4}$. We discuss the implications of these results for understanding the atmospheric and surface composition of Titan. . [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J40.00007: Extended-Solid Phases of Carbon Dioxide at High Pressures Valentin Iota, Zsolt Jenei, Jae-Hyun Klepeis, Choong-Shik Yoo, William Evans At high pressures and temperatures, CO$_{2}$ transforms to a series of solid polymorphs with differing crystal structures, intermolecular interactions and chemical bonding. Among them are a number of covalent (extended) solid phases, with crystal structures analogous to SiO$_{2}$ polymorphs. Above 40GPa and 1500K CO$_{2}$ transforms to phase V, a network of corner sharing CO$_{4}$ tetrahedra -- structurally similar to SiO$_{2}$ tridymite. At room temperatures, CO$_{2}$ forms \textit{a-carbonia, }an amorphous extended-solid phase similar to silica glass. Recently, we reported another phase, with a structure resembling that of SiO$_{2}$ stishovite, formed by compressing associated phase II above 50GPa. Here, we present a systematic picture of the structural and bonding diagram of carbon dioxide, focusing on the relationship between its molecular and extended phases at high pressures and temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J40.00008: High-pressure Neutron Powder Diffraction and Inelastic Neutron Scattering Studies on the Mineral Jarosite KFe$_{3}$(SO$_{4})_{2}$(OH)$_{6}$ Monika Hartl, Alice Acatrinei, Luke Daemen, Hongwu Xu, Kim Tait, Yuejian Wang, Sven Vogel, Jianzhong Zhang, Yusheng Zhao The mineral jarosite KFe$_{3}$(SO$_{4})_{2}$(OH)$_{6}$ has been detected in rocks at the Meridiani Planum region of Mars [1 and cited therein]. Jarosite is typically formed in aqueous environments at acidic pH. It decomposes to ferric oxohyroxides in humid climate. This gives rise to the question under which conditions jarosite was formed on Mars and what it can tell us about the climatic cycles and the former presence of water on Mars. We are looking at the phases of jarosite at elevated temperature and pressure and were able to show the stability of jarosite up to 6 GPa at room temperature and up to 3 GPa at 300 $^{o}$C using neutron powder diffraction. Furthermore, we used inelastic incoherent neutron scattering to look at the vibrational modes of the hydroxyl groups in jarosite at various temperatures between 10K and 200K. [1] A. Banin, Science 309 (2005) 888 [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J40.00009: Lattice Dynamics and Thermal Equation of State of Platinum Tao Sun, Koichiro Umemoto, Zhongqing Wu, Jincheng Zheng, Renata Wentzcovitch Platinum is widely used as a pressure calibration standard. However, the established thermal EOS has uncertainties, especially in the high $P$-$T$ range. We use density functional theory to calculate the thermal equation of state of platinum, up to $550$~GPa and $5000$~K. The static lattice energy is computed by using the LAPW method, with LDA, PBE, and the recently proposed WC functional. The electronic thermal free energy is evaluated using the Mermin functional. The vibrational part is computed within the quasi-harmonic approximation using density functional perturbation theory and pseudopotentials. Special attention is paid to the influence of the electronic temperature to the phonon frequencies. We find that in overall LDA results agree best with the experimental ones. To provide accurate thermal EOS for pressure calibration, we combine the computed temperature dependence of the Gibbs energy with the room temperature Gibbs free energy corrected by experiments. The resulting thermal EOS seems reasonably accurate and can be used as a reference for pressure calibration. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J40.00010: Ultrafast shock wave propagation at high ambient pressure in a diamond anvil cell Michael Armstrong, Jonathan Crowhurst, Joseph Zaug, William Howard The measurement and characterization of acoustic phenomena at high pressure is critical to the modeling of planetary dynamics, seismic events, and chemistry in extreme environments. Here we present the results of experiments using ultrafast laser excitation and detection of shock waves starting from high precompression (10s GPa) in a standard diamond anvil cell (DAC) with transient single shot shock pressures $>$ 10 GPa. Using ultrafast interferometry, we directly detect surface motion with $\sim $nm spatial resolution and $\sim $ps time resolution. Such experiments enable examination of shock waves with significant strain starting at high ambient pressure using a convenient and relatively inexpensive apparatus. Ultrafast time resolution enables the observation of shock-induced chemistry on the scale of a picosecond shock rise. Furthermore, standard DACs can reach 100s GPa precompression, enabling the examination of phase transitions and chemical reactions starting from a wide range of thermodynamic initial conditions. [Preview Abstract] |
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