Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X31: High Pressure V |
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Sponsoring Units: DMP Chair: Stanimir Bonev, Dalhousie University Room: E141 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X31.00001: High-pressure EXAFS investigations of Ge-bearing compounds Maria Baldini, Giuliana Aquilanti, Ho-Kwang Mao, Weng Yang, Guoying Sheng, Sakura Pascarelli, Wendy L. Mao The Ge K edge of amorphous GeO$_{2}$ was studied by Extended X-ray Absorption Fine Structure (EXAFS) upon increasing and decreasing pressure cycles up to 44 GPa [1]. Up to 13 GPa, the local structure was well described by a 4-fold coordination model. A disordered region formed by a mixture of 4- and 5-coordinated Ge polyhedra was observed in the intermediate pressure range between 13 and 30 GPa, and above 30 GPa the structural transition to the maximum coordination was complete. Preliminary EXAFS results obtained on crystalline Ge and on GeH$_{4}$ compounds will be also presented. \\[4pt] [1] M. Baldini, G. Aquilanti, H-k. Mao, W. Yang, G. Shen, S. Pascarelli, W. L. Mao, \textbf{Phys. Rev. B} \emph{in press} [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X31.00002: Stability in bcc transition metals: Madelung and band-energy effects due to alloying Alex Landa, Per Soderlind, Andrei Ruban, Oleg Peil, Levente Vitos The phase stability of Group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the body-centered-cubic phase relative to low-symmetry rhombohedral phases. We show that band-structure effects determine phase stability when a particular Group VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons, destabilize and stabilize bcc, respectively. When alloying with neighbors of higher d-transition series, electrostatic Madelung energy dominates and stabilizes the body-centered-cubic phase. This surprising prediction invalidates current understanding of simple d-electron bonding that dictates high-symmetry cubic and hexagonal phases. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X31.00003: Large scale simulations of quasi-isentropic compression in Fe and Al Stephane Mazevet, Tommaso Vinci To measure the melting point or the equation of state of iron under conditions corresponding to the earth core requires quasi-isentropic schemes. This brings conditions that are colder than regular Hugoniot measurements but significantly hotter than along the theoretical isentrope. One major uncertainty for this approach is to account for the phase transformations taking place. Using large scale molecular dynamics simulations, with sample size comparable to the experiment, we quantify the effect of the kinetics of the plastic and the solid-solid phase transformations occurring during the quasi-isentropic compression of Fe and Al samples. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X31.00004: Sequestration of noble gases in giant planet interiors Hugh Wilson, Burkhard Militzer The Galileo probe showed that Jupiter's atmosphere is severely depleted in neon compared to protosolar values. We show, via density functional theory molecular dynamics simulations of the partitioning of neon between hydrogen and helium phases, that the observed depletion can be explained by the sequestration of neon into helium-rich droplets within the postulated hydrogen-helium immiscibility layer of the planet's interior. We also demonstrate that this mechanism will not act upon argon, consistent with the observed lack of depletion of this gas. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X31.00005: Effects of temperature and pressure on the structure, transport properties and isotope fractionation in silicate melts Gaurav Goel, Jim V. Orman, Daniel J. Lacks Knowledge of the physical and chemical properties of silicate melts is essential for understanding the properties and evolution of planetary interiors. In this talk we present results from extensive classical molecular dynamics simulations of silicate melts. These simulations were motivated by and provided insights into the laboratory experiments on magmatic systems. We have explored the effect of high temperature and high pressure, conditions typical of Earth's core-mantle boundary, on the relationship between transport properties and structure of silicate melts. We have also explored the role of melt structure and polymerization resulting from changing the melt composition, i.e. MgO/SiO$_{\mathrm{2}}$ ratio. Finally, using non-equilibrium molecular dynamics simulations we investigated the effect of temperature gradient on isotopic fractionation in silicate melts. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X31.00006: Tetragonal or cotunnite, a high pressure phase of NiS$_2$? Yonggang G. Yu, Nancy L. Ross A sequence of pressure-induced phase transitions within NiS$_2$ (single crystal) has been established from static LDA calculations. A dozen of AX$_2$ type candidate structures have been studied at high pressures including cotunnite ($\alpha$-PbCl$_2$). The calculation identified that below 5 megabar, a tetragonal phase (P4$_2$/n) is more stable than the 9 bond coordinated cotunnite, which otherwise would be commonly conceived. This tetragonal structure is characterized by layers of Ni atoms in 8-fold coordination with S atoms. This phase is more stable than the vaesite phase at about 150 GPa. With further compression to about 756 GPa, the tetragonal phase transforms into a hexagonal AlB$_2$-type structure (P6/mmm) which is characterized by planar hexagonal layers of S intercalated by Ni atoms where each Ni atom is twelve fold coordinated by S atoms. Calculated band structures and valence charge density maps show S-S and Ni-S bonded interactions for NiS$_2$ under these extremely compressed conditions. The tetragonal phase may have geophysical implications if present in Earth's core. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X31.00007: Alkali and alkaline earth metal alloys at high-pressure Amanuel Teweldeberhan, Stanimir Bonev The electronic and structural properties of liquid alkali and alkaline earth metal alloys at high-pressure have been studied using first-principles molecular dynamics simulations. Pair correlation functions, static structure factors, short-range order parameters, free energies of mixing, and electronic density of states are calculated at various pressures, temperatures, and concentrations to investigate local ordering, pseudogap opening, and phase separation in the alloys. The computed electronic and structural properties are compared with pure alkali and alkaline earth liquid metals. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X31.00008: Structural Studies of Group IV Binary Hydrides at Extreme Pressures Patricia Kalita, Andrew Cornelius, Kristina Lipinska, Stanislav Sinogeikin, Olga Shebanova, Wenge Yang, Romeo De Coss, Ramiro Qijano Although binary hydrides such as TiH$_{2}$ are not ideal candidates for storing hydrogen, they can act as active species to catalyze the reversible dehydrogenation of other hydrides and of carbon nanotubes. The equation of state of TiH$_{2}$, ZrH$_{2}$ and HfH$_{2}$ was obtained using synchrotron x-ray diffraction and diamond anvil cells, with structural studies carried out \textit{in situ} on compression up to $\sim $50 GPa, under quasi-hydrostatic conditions. We discuss pressure-induced structural transformations and the experimental bulk modulus for TiH$_{2}$, ZrH$_{2}$ and HfH$_{2}$, accompanied by corresponding first principle calculations. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X31.00009: Order-disorder phase boundary in Ice VII-VIII investigated by first principles Renata Wentzcovitch, Koichiro Umemoto, Stefano de Gironcoli, Stefano Baroni Phase boundaries among the various forms of ice are difficult to determine experimentally because of large hysteresis involved, especially at the lowest temperatures. Theoretically, there are also great challenges, including the order-disorder (OD) phenomenon. The ice VII-VIII boundary, a typical OD boundary, has been reasonably well constrained experimentally and is an ideal study case. We present a first principles quasiharmonic study consisting in the complete statistical sampling of molecular orientations within a 16-molecules supercell. Our calculation accounts well for several important aspects: equation of state of ice VII, negative Clapeyron slope of the phase boundary, and the isotope effect. We will discuss also some factors to be improved, including XC functionals. Research was supported by NSF grants EAR 0810272, EAR 0635990, ATM 0428774 (VLab), EAR 0757903. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X31.00010: Stability and Graphitization of Carbon Nanotube Arrays at high Temperature and Pressure Francesco Colonna, Evert Jan Meijer, Annalisa Fasolino We investigated the effects of high temperatures and pressures on carbon nanotube arrays, focusing on structural changes like collapse, polymerization and graphitization. Building on our earlier work on graphite[1], we addressed those topics by means of Monte Carlo simulations, using a state-of-the-art bond-order potential (LCBOPII) capable to make and break bonds and to account correctly for the long-range interaction between graphitic structures. The structural changes and the graphitization mechanism appear to depend on temperature, pressure, and on the radius of the nanotubes. We discuss the possible transformation mechanism for temperatures up to 4000K and pressures up to 20GPa. [1] F. Colonna et al, Phys. Rev. B. 80, 134103 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X31.00011: Prediction of a new metastable sp$^3$-carbon allotrope at ambient conditions Takashi Miyake, Koichiro Umemoto, Renata Wentzcovitch, Susumu Saito We have investigated by first principles the viability of a new form of sp$^3$ crystalline carbon recently found in tight-binding molecular dynamics simulations of carbon nanotubes (CNT) under pressure. It consists of unique 4-membered rings and has body centered tetragonal structure (henceforth bct-C$_4$). It is also a polymerized form of (2,2) CNT, the smallest CNT. This phase is dynamically stable at zero pressure and is more stable than graphite beyond 18.6 GPa. At zero pressure it is also more stable than fcc C$_{60}$ and (7,0) and (8,0) CNTs. Inspection of this transparent polymorph shows a peculiar relationship with hexagonal diamond, which suggests that this phase might be an intermediate phase along the graphite to hexagonal diamond transformation path. This possibility appears to be supported by the good match between the simulated x-ray diffraction pattern of bct-C$_4$ and that of an intermediate transparent and hard phase of carbon produced by cold compression of graphite. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X31.00012: High pressure studies on extended phases of CO$_{2}$ Javier A. Montoya, Mal-Soon Lee, Sandro Scandolo Recent findings have shown that the chemistry of CO$_{2}$ at high pressure and temperature is richer than previously thought and that the activation of the C=O bond that can give origin to different forms of non-molecular CO$_{2}$. Such findings may have important implications for the understanding of the Earth's deep carbon cycle and CO$_{2}$ sequestration technologies. First principles simulations of CO$_{2}$'s electronic properties under different pressure and temperature conditions can constrain the thermodynamic phase diagram of CO$_{2}$ and explore P-T conditions necessary for the C=O bond activation. We have shown that at about 50 GPa molecular CO$_{2}$ can transform to a metastable amorphous form characterized by an almost equal proportion of three- and four-fold coordinated carbon atoms [1], while higher carbon coordination does not take place up to at least 900 GPa [2-3]. We have also found that doping with transition metals can reduce the activation barrier and transition pressure for the C=O bond activation in CO$_{2}$ [4]. Our results suggest that pressure can radically alter the oxidation chemistry of carbon. [1] J. A. Montoya \textit{et al.}, PRL 100, 163002 (2008) [2] J. Sun et al., PNAS 106, 6077 (2009) [3] M-S. Lee, J. A. Montoya and S. Scandolo, PRB 79, 144102 (2009) [4] J. A. Montoya, R. Rousseau, and S. Scandolo, unpublished. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X31.00013: d-Electrons' Occupancy and Crystal Structure in Transition Metal Sesquioxides Koichiro Umemoto, Renata Wentzcovitch We present a systematic density functional study of phase relations in transition metal sesquioxides: Y$_2$O$_3$, Rh$_2$O$_3$, and In$_2$O$_3$. Y$_2$O$_3$ and In$_2$O$_3$ undergo pressure-induced transitions accompanied by increase in cation coordination number from 6 to 7. However, in Rh$_2$O$_3$, such transition does not occur up to $\sim$350 GPa. This cannot be explained simply arguments based on ionic radii ratios. Inspection of the electronic structure reveals that, in Rh$_2$O$_3$, the spatial distribution of the partially-occupied 4d states plays a very important role in the extraordinary stability of the Rh$_2$O$_3$(II)-type structure against others with higher coordination numbers. This study suggests that the complexity of phases in 3d/4d-transition-metal sesquioxides is not only due to ratio ionic radii but also by filling of the d states. [Preview Abstract] |
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