Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L11: Earth and Planetary Materials II |
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Sponsoring Units: DMP DCMP GSCCM Chair: Jay Bass, University of Illinois at Urbana, Chicago Room: LACC 153C |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L11.00001: Thermochemical State of the Lower Mantle: New Insights from Mineral Physics Invited Speaker: We report recent findings in the field of high-pressure mineral physics with important implications for Earth's lower mantle. We find that the two main constituents of the lower mantle, namely (Mg,Fe)SiO$_{3}$ -- magnesium silicate perovskite -- and (Mg,Fe)O -- magnesiow\"{u}stite --, undergo electronic transitions at lower mantle pressures, in which iron is transforms from the high-spin state to the low-spin state. The transformations should profoundly alter the thermochemical state of Earth's lower mantle. Minerals bearing high-spin iron have characteristic absorption lines in the near-infrared, hindering radiative conductivity at lower-mantle temperatures. These absorption lines shift in low-spin iron-bearing minerals to the visible range (green to violet), and their intrinsic intensities decrease; the minerals thus become transparent in the near-infrared and their radiative conductivity (and therefore total thermal conductivity) increases. Other issues at stake are that of iron partitioning between mineral phases in the bottom third lower mantle. The two transition pressures correspond to the bottom third of the lower mantle (70 GPa, 1700 km depth), and to the last 300 km above the core-mantle boundary (120 GPa, 2600 km depth); these regions have very special geophysical signatures, since chemical heterogeneities have been reported in the bottom third of the lower mantle, and that the bottom 300 km of Earth's mantle is constituted by the D'' layer. Our observations could provide a mineral physics basis for these two regions of Earth's lower mantle. The implications of these transitions on the dynamics of the lower mantle will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L11.00002: Elasticity of Deep-Earth Materials at High P and T: Implication for Earths Lower Mantle Jay Bass, S.V. Sinogeikin, Estelle Mattern, J.M. Jackson, D. Lakshtanov, J. Matas, J. Wang, Y. Ricard Brillouin spectroscopy allows measurements of sound velocities and elasticity on phases of geophysical interest at high Pressures and Temperatures. This technique was used to measure the properties of numerous important phases of Earths deep interior. Emphasis is now on measurements at elevated P-T conditions, and measurements on dense polycrystals. Measurements to 60 GPa were made using diamond anvil cells. High temperature is achieved by electrical resistance and laser heating. Excellent results are obtained for polycrystalline samples of dense oxides such as silicate spinels, and (Mg,Al)(Si,Al)O3 --perovskites. A wide range of materials can now be characterized. These and other results were used to infer Earths average lower mantle composition and thermal structure by comparing mineral properties at lower mantle P-T conditions to global Earth models. A formal inversion procedure was used. Inversions of density and bulk sound velocity do not provide robust compositional and thermal models. Including shear properties in the inversions is important to obtain unique solutions. We discuss the range of models consistent with present lab results, and data needed to further refine lower mantle models. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L11.00003: Probing lattice dynamics at high pressure with inelastic x-ray scattering Daniel Farber, Michael Krisch, James Badro, Daniele Antonangeli, Florent Occelli, Chantel Aracne Inelastic X-ray scattering provides a direct measure of acoustic excitations within the crystalline lattice at pressures to 100 GPa and accuracies to better than a few percent. Inelastic X-ray scattering thus provides an extremely powerful method of determining the elastic moduli and lattice dynamics in earth materials at high-pressure. Furthermore, inelastic X-ray scattering can be performed on oriented single crystals as well as poly-crystals to provide another internal check of the method. We present measurements the single crystal elastic moduli and phonon dispersion curves of molybdenum in oriented single crystal samples to 40 GPa. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L11.00004: High-to-low spin transition in magnesiumwustite under pressure Taku Tsuchiya, Renata Wentzcovitch, Stefano de Gironcoli Mg-Fe substitution is most commonly seen in natural solid solution minerals. High resolution X-ray spectroscopy has recently demonstrated that the major lower mantle (LM) minerals undergo a high-to-low spin transition at LM pressures (23-135 GPa). Previous failures of standard DFT and ``LDA+U'' approaches to describe this phenomenon have hindered its investigation and consequences of fundamental importance to geophysics, such as heat transport in Earth?fs mantle. Here, using a rotationally invariant first principles formulation of LDA+U, where the Hubbard U parameter is computed in a internally consistent way, we report the first successful study of this transition in low solute concentration (Mg$_{1-x}$Fe$_x$)O, magnesiumwustite. This is believed to be the second most abundant phase of Earth's LM. This encouraging result appears to open for exploration a new class of problems of enormous significance to deep Earth geophysics. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L11.00005: Post-perovskite transition in NaMgF$_3$ John Parise, Koichiro Umemoto, Renata Wentzcovitch, Donald Weidner We have investigated through first principles computations the pressure-induced behavior of NaMgF$_3$. It has the same Pbnm perovskite structure as MgSiO$_3$, the major lower mantle phase. Likewise MgSiO$_3$ it displays the same post-perovskite transition. Static LDA calculations indicate this transition should occur shortly after 18 GPa and then decompose into NaF and MgF$_2$ above 40 GPa. Phonon dispersions and elastic moduli of the post-perovskite phase confirm its vibrational/mechanical stability. The existence of a post-perovskite transition at low pressures in this material makes possible experimental studies of this uncommon structure in a more easily accessible pressure range. Research supported by NSF/EAR 013533 (COMPRES), 0230319, and NSF/ITR 0428774 (VLab). [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L11.00006: Measurements of Thermal diffusivity anisotropy in the laser-heated diamond anvil cell Abby Kavner, Nathalie Conil Heat transport within the Earth and planets is limited by the diffusive heat flow at thermal boundaries. Diffusive heat transport is controlled by a material property—thermal conductivity—that is currently not very well constrained for materials at the high pressures and temperatures relevant to planetary interiors. We have measured thermal diffusivity anisotropy of graphite in the laser heated diamond anvil cell, by examining the hotspot ellipticity generated by laser heating a highly oriented graphite crystals. At ambient pressures, the thermal diffusivity ratio inferred from hotspot ellipticity measurements is in good agreement with independent measurements. In addition, we provide the first measurements of pressure dependence of thermal diffusivity anisotropy of graphite. We compare the observed hotspot ellipticity with models of the heat flow behavior in the diamond anvil cell to examine the temperature dependence of thermal conductivity of graphite. These experiments provide a proof-of-concept for high pressure/ high temperature relative thermal diffusivity measurements in the laser heated diamond anvil cell, with applications for a wide variety of Earth and Planetary materials. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L11.00007: First-principles calculation of lattice thermal conductivity of MgO Jianjun Dong, Bin Xu, Chris Brown, Jeffrey Nickerson Lattice thermal conductivity of MgO has been calculated based on the first-principles total energy theory and the Boltzmann transport theory. In this study we consider the the anharmonic interaction up to the third-order. An efficient Brillouin zone integration technique is adopted to reduce the computational loads of calculating the phonon- phonon interaction terms in the linearlized Boltzmann equation. Our first-principle calculated results will be discussed with comparison to experimental data and some previous theoretical results. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L11.00008: Thermal equation of state of bcc and hcp Fe: linear response quasi-harmonic lattice dynamics Xianwei Sha, R. E. Cohen Linear-response Linear-Muffin-Tin-Orbital calculations have been performed to understand and predict the thermal equation of state, elasticity, and phase stability of bcc and hcp Fe, for input into dynamic shock finite-element simulations. The phonon dispersion and phonon density of states have been calculated at different volumes and various c/a axial ratios for hcp structures, which show good agreements with available experimental data. The thermal conductivity and electrical resistivity at different pressure have been calculated. Free energy functional for bcc and hcp Fe has been derived, and has been further applied to establish the thermal equation of state, bulk modulus K$_{0}$, dK$_{0}$/dT, and thermal expansion coefficients under high pressures and temperatures. A detailed comparison with experiment has been made. For hcp Fe, the variations of $c/a$ ratios with temperatures and pressures have been predicted. The influence of anharmonic effects has been examined using tight-binding calculations. This work was supported by US Department of Energy ASCI/ASAP subcontract to Caltech , Grant DOE W-7405-ENG-48 (to REC). [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L11.00009: Single crystalline and aggregate elasticity of hcp cobalt at high pressure Daniele Antonangeli, Michael Krisch, Guillaume Fiquet, Daniel Farber, James Badro, Chantel Aracne, Florent Occelli The determination of elasticity at high pressure is singularly important for geophysics. In particular a comparison of single-crystalline and polycrystalline results is essential, since the various elements or minerals in Earth are present as textured aggregates.We report the first experimental determination of the complete elastic tensor of hcp cobalt under hydrostatic compression to 39 GPa by Inelastic X-ray Scattering (IXS). These results are complemented by an IXS study on polycrystalline cobalt up to 99 GPa, over the whole stability range of the hcp phase. Moreover the orientational averaging schemes and the micro-mechanical models describing the stress and strain relations of the interacting grains, currently employed to link the single crystal elastic moduli with the aggregate sound velocities in textured polycrystalline samples, will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L11.00010: Electronic topological transitions in high-pressure bcc metals John E. Klepeis, Alexander I. Landa, Per A. S\"oderlind In 1960 Lifshitz$^{\dagger}$ pointed out that variations in the topology of the Fermi surface as a function of deformation could lead to so-called electronic topological transitions (ETT). We have carried out first-principles electronic structure calculations for the shear elastic constants of bcc vanadium, niobium, and tantalum as a function of pressure and find evidence of an ETT in each case. The transition is manifested in the form of a pronounced softening in the shear elastic constants over a particular pressure range that in turn coincides with a pressure-induced topological change in the Fermi surface. Specifically, the 3-fold degenerate $\Gamma_{25}^{\prime}$ pure $d$-state is unoccupied at low pressures but shifts to lower energy and passes through the Fermi energy as pressure is increased. We have explored the detailed nature of these transitions and suggest that they should appear rather generally in most metals. \\ \\ $^{\dagger}$ I. M. Lifshitz, Sov. Phys. JETP {\bf 11}, 1130 (1960). [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L11.00011: Investigation of the Phase Diagram of Carbon from first principles Alfredo Correa, Stanimir Bonev, Roger Falcone, Giulia Galli The investigation of the phase diagram of carbon has been the subject of experimental research for several decades. Unfortunately progress has been slow due to the extreme temperature and/or pressure required to melt diamond. From the point of view of simulations, it is rather challenging to simulate melting lines under pressure completely from first principles, and only very recently elaborate tools to perform such simulations have been developed [1,2]. We present results for the high pressure portion of the carbon phase diagram, where we have determined new melting lines and predicted metallization pressure and temperature. Our results were obtained using ab-initio molecular dynamics, together with two phase simulation techniques and free energy calculations. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [1] S. Bonev et al., Nature 431, 669 (2004) [2] T. Ogitsu et al., Phys. Rev. Lett. 91, 175502 (2003) [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L11.00012: Electronic properties of carbon in the fcc phase. Cesar Cab, Alejandro Tapia, Romeo de Coss, Gerko Oskam, Gabriel Canto The observation of a new carbon phase in nanoparticles obtained from Mexican crude oil having the face-centered-cubic structure (fcc) has been reported. However, more recently has been suggested that hydrogen is present in the samples forming CH with the zincblende structure. The structural and electronic properties of C(fcc) and CH(zincblende) are unknown. In the present work we have studied the electronic structure of C(fcc) and CH(zincblende) by means of first-principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the band structure, the local density of states (LDOS), and orbital population. We find that in contrast to graphite and diamond, both fcc carbon and CH with the zincblende structure exhibit metallic behavior. This research was supported by Consejo Nacional de Ciencia y Tecnolog{\'\i}a (Conacyt-M{\'e}xico) under Grants No. 43830-F, No. 44831-F, and No. 43828-Y. [Preview Abstract] |
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