Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P31: Focus Session: Materials at High Pressure IV: Geophysical Materials and Magnetic Transitions |
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Sponsoring Units: DMP GSCCM DCOMP Chair: Maddury Somayazulu, Carnegie Institution Room: C145 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P31.00001: Pressure-induced phase transitions in NaMgF3 post-perovskite Koichiro Umemoto, Renata Wentzcovitch Understanding the behavior of MgSiO$_{3}$ postperovskite(PPV) under extreme pressures is fundamental for modeling the interiors of solar giants and extrasolar planets. In 2006, MgSiO$_{3}$ post-perovskite was predicted to dissociate into MgO and SiO$_{2}$ at 1.1 TPa (Umemoto et al., Science 311, 983 (2006)). However, the predicted dissociation pressure is too high to be easily verified experimentally. Instead, a low-pressure analog, NaMgF$_{3}$ neighborite, has been studied to test for structural predictions in MgSiO$_{3}$. NaMgF$_{3}$ was predicted to dissociate at $\sim$40 GPa (Umemoto et al., Geophys. Res. Lett. 33, L15304 (2006)), but this has not been confirmed experimentally (Martin et al., Geophys. Res. Lett. 33, L11305 (2006); Grocholski et al. Geophys. Res. Lett. 37, L14204 (2010)) and the dissociation MgSiO$_{3}$ PPV is now being questioned. Here, we reexamine in detail the pressure dependence of crystal structures and phonon frequencies in NaMgF$_{3}$ and reveal the apparent reason why dissociation was not observed in this material. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P31.00002: First-principles calculation of thermal conductivity of silicate perovskite at high pressures and high temperatures Jianjun Dong, Xiaoli Tang, Abby Kavner, Moses Ntam The lattice thermal conductivity of silicate perovskite, the most abundant mineral in the Earth's lower mantle, is calculated by combining the first-principles electronic structure theory and Peierls-Boltzmann transport theory. The phonon scattering rate due to lattice anharmonicity and Mg/Fe mass disorder is evaluated for each mode at the extreme P-T conditions of the lower mantle. The predicted thermal conductivity of single crystal MgSiO3 perovskite at ambient condition, about 5.7 W/m/K, is in excellent agreement with experiment. Adding about 6{\%} Fe will lower the thermal conductivity by nearly 40{\%}. Our calculation also reveals an unique pressure dependence for the thermal conductivity of perovskite, and the calculated thermal conductivity of iron bearing perovskite is almost an order of magnitude lower than the previously estimates based on long extrapolation of single crystal data. Including a re-evaluation of radiative contribution, we discuss the implications of our results for the heat flow in deep Earth. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P31.00003: Identification of post-pyrite transition in SiO2 by a genetic algorithm Shunqing Wu, Koichiro Umemoto, Kai-Ming Ho, Min Ji, Cai-Zhuang Wang, Renata Wentzcovitch Here we propose a new phase of SiO$_{2}$ beyond the pyrite-type phase. SiO$_{2}$ is one of the most important minerals in Earth and planetary sciences. So far, the pyrite-type phase has been identified experimentally as the highest-pressure form of SiO$_{2}$. In solar giants and extrasolar planets whose interior pressures are considerably higher than that on Earth, a post-pyrite transition in SiO$_{2}$ may occur at $\sim$ 1 TPa as a result of the dissociation of MgSiO$_{3}$ post-perovskite into MgO and SiO$_{2}$ [Umemtoto et al., Science 311, 983 (2006)]. Several dioxides considered to be low-pressure analogs of SiO$_{2}$ have a phase with cotunnite-type (PbCl$_{2}$-type) structure as the post-pyrite phase. However, a first-principles structural search using a genetic algorithm shows that SiO$_{2}$ should undergo a post-pyrite transition to a hexagonal phase, not to the cotunnite phase. The hexagonal phase is energetically very competitive with the cotunnite-type one. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P31.00004: Spin crossover systems in the deep mantle Invited Speaker: In recent years there has been much interest on spin crossovers found experimentally in the most abundant minerals of Earth's lower mantle ((Mg,Fe)O and (Mg,Fe)(Si,Fe)O$_3$-perovskite) under pressure. Spin crossovers are strongly dependent on thermodynamic conditions and a full understanding of this problem requires its investigation as function of pressure and temperature. There are several controversies, especially in the perovskite systems, and surprises are revealed by electronic structure calculations. The geophysical consequences of these crossovers are yet to be fully understood but could be fascinating. I will review progress we have made in understanding spin crossovers and give an overview of this phenomenon and its potential implications for the Earth.\\[4pt] Research carried out in collaboration with H. Hsu, K. Umemoto, P. Blaha, J. F. Justo, and C. R. S. da Silva. Research supported by the MRSEC Program of NSF under Award Number DMR-0212302 and DMR-0819885, and by NSF/ATM-0428774, EAR-0810212, and EAR-1047629. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P31.00005: Insulator to Metal and Magnetic Transitions in FeO under High Pressure: DFT-DMFT Computations R.E. Cohen, Kristjan Haule, Gabi Kotliar We have applied DFT+Dynamical Mean Field Theory (DMFT) to FeO under varying pressure and strain to understand possible transitions in FeO. We use an LAPW basis set, and the lattice terms are evaluated using the WIEN2K LAPW code. The impurity model is solved using continuous time quantum Monte Carlo (CTQMC). Temperature enters explicitly, so we made special efforts to understand high temperature behavior relevant to geophysics. The computations are fully self-consistent, including the impurity levels and crystal field splitting, and the total energy is evaluated using the full potential and charge density of the lattice plus impurity models. We find with increasing pressure in paramagnetic FeO in a cubic lattice and U=8 eV a high-spin low-spin transition, with a possible intermediate spin state (characterized by intermediate occupancies of the t2g and eg states) between. We find that at 300K cubic FeO remains insulating to a factor of two compression (over 600 GPa). However, high temperatures (e.g. 2000K) and rhombohedral lattice strain promote a metal insulator transition. We are delineating the phase boundaries. This work is supported by NSF. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P31.00006: First-principles study of spin-state crossovers and hyperfine interactions of ferric iron in magnesium silicate perovskite Han Hsu, Peter Blaha, Matteo Cococcioni, Renata Wentzcovitch The spin-state crossover in iron-bearing MgSiO$_3$ perovskite, the most abundant mineral in the Earth, may significantly affect the properties of Earth's lower mantle. However, details of this phenomenon have been very unclear, owing to the complicated nature of this mineral, mainly the coexistence of ferrous and ferric iron. Using the density functional theory plus Hubbard $U$ (DFT+$U$) methods, we investigated the spin states and hyperfine interactions of ferric iron in this mineral. We show that a crossover from high-spin to low-spin state occurs within the lower-mantle pressure range, and it is accompanied by a noticeable volume reduction and an increase in iron nuclear quadrupole splitting (QS). These results are consistent with recent x-ray diffraction and M\"ossbauer spectroscopy measurements [K. Catalli \textit{et al}., Earth Planet. Sci. Lett. \textbf{289}, 68 (2010)]. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P31.00007: Ab initio melting curve of Fe and Fe-S alloys at extreme pressures: implications for Earth's and exoplanets' cores Johann Bouchet, Guillaume Morard, Stephane Mazevet, Francois Guyot Exoplanets with masses similar to that of Earth have recently been discovered in extrasolar systems [1]. A first order question for understanding their dynamics is to know whether they possess Earth like liquid metallic cores. However, the iron melting curve is unknown at conditions corresponding to planets of several times the Earth's mass (over 15 Mbar for planets with 10 times the Earth's mass [2]). In the density-temperature region of the cores of those super-Earths, we calculate the iron melting curve using first principle molecular dynamics simulations based on density functional theory. We also propose an equation of state for iron in this pressure range. Finally we show the melting curve of Fe$_{3}$S and discuss the effects of the addition of sulfur to the melting curve of pure iron. \\[4pt] [1] J. P. Beaulieu, D. P. Bennett, P. Fouque et al., Nature 439 (7075), 437 (2006).\\[0pt] [2] D. Valencia, R. J. O'Connell, and D. Sasselov, Icarus 181, 545 (2006). [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P31.00008: Elasticity of iron-bearing olivine polymorphs investigated by first principles Maribel N\'u\~nez Valdez, Yonggang Yu, Renata Wentzcovitch We calculate by first principles the effect of iron on the high pressure-temperature elasticity of olivine polymorphs: $\alpha$-phase (olivine), $\beta$-phase (wadsleyite) and $\gamma$-phase (ringwoodite), the major constituents of the Earth's upper mantle and transition zone (TZ). We combine the LDA, the quasiharmonic approximation, and a model vibrational density of states for the solid solution to calculate the full elastic tensor $C_{ij}$, bulk ($K$) and shear ($G$) moduli of (Mg$_{0.875}$Fe$_{0.125}$)$_2$SiO$_4$. Comparison with experimental data at ambient conditions validates our results. In the pressure and temperature range of the upper mantle and TZ we study single crystal wave propagation anisotropy and polarization anisotropy in aggregates with preferred orientation.\\[4pt] Research supported by NSF EAR-1019853 and EAR-0810272. Computations were performed at the Minnesota Supercomputing Institute. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P31.00009: Polymerization of methane in the deep Earth Leonardo Spanu, Davide Donadio, Detlef Hohl, Eric Schwegler, Giulia Galli Determining physical and chemical properties of carbon fluids at high pressure and temperature is a key step towards understanding carbon reservoirs and fluxes in the deep Earth. The stability of carbon-hydrogen systems at depth greater than few thousands meters is poorly understood and the abiogenic hypothesis on the synthesis of higher hydrocarbon (HCs) in the Earth mantle remains controversial. We have used ab initio molecular dynamics simulations to investigate the formation of higher HCs from dissociation of pure methane, and of methane in contact with carbon surfaces and transition metals, in a range of pressure of $2-30$ GPa and temperature T=$800-4,000$ K [1]. We present results on the range of stability of pure methane and discuss how the interaction with transition metals or carbon deposits (graphite and diamond) affects the formation of higher HCs.\\[4pt] [1] Leonardo Spanu, Davide Donadio, Detlef Hohl, Eric Schwegler, Giulia Galli (\it{submitted}) [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P31.00010: Solubility and erosion of icy cores in giant planets Hugh Wilson, Burkhard Militzer The core-mantle boundary of a giant planet consists of an interface between dense rock/ice below and fluid hydrogen-helium above. Whether this phase boundary remains stable, however, or whether the core material is dissolved and redistributed throughout the interior, remains unknown, and has major consequences for planetary interior and formation models. In this work we use density functional theory molecular dynamics calculations to compute the free energy of solubility for the icy components of the core into fluid hydrogen, to investigate whether solubility is thermodynamically preferred at the extreme temperature and pressure conditions prevalent at the core-mantle boundaries of Jupiter and Saturn. The consequences for Jupiter and Saturn, as well as for giant exoplanets, will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P31.00011: Post-stishovite transition in AlOOH-incorporated SiO2 Katsuyuki Kawamura, Koichiro Umemoto, Renata Wentzcovitch, Kei Hirose In 2007, Lakshtanov et al. [Proc. Nat. Acad. Sci. 104, 13588 (2007)] showed that the incorporation of AlOOH into SiO$_{2}$ significantly reduces the transition pressure between stishovite and CaCl$_{2}$-type phases. In the present paper, we investigate theoretically the effect of hydrogen and aluminum on this transition. First-principles calculations show that aluminum has no effect on the transition pressure. However, hydrogen bonds play a crucial role, suggesting that a cooperative redistribution of hydrogens aids the post-stishovite transition. Large-scale molecular dynamics simulations using model potentials confirm this effect and reveal the nature of the hydrogen motion. This effect produces a strong temperature dependence on the transition pressure and should make the latter sensitive to hydrogen content in the material. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P31.00012: Thermodynamic properties of MgSiO$_3$ majorite and phase transitions near 660-km depth in MgSiO$_3$ and Mg$_2$SiO$_4$: a first principles study Yonggang Yu, Renata Wentzcovitch, Victor Vinograd, Ross Angel Thermodynamic properties of MgSiO$_3$ tetragonal majorite have been calculated at high $P$-$T$ within the quasiharmonic approximation based on DFT using both LDA and GGA. The LDA results compare exceptionally well with measured thermodynamic properties. A classical Monte Carlo simulation based on a cluster expansion method demonstrates that disorder between Mg and Si in the octahedral sites in majorite does not occur below 3600 K within 30 GPa. The calculated phase boundaries between majorite (mj), perovskite (pv), and ilmenite (il) MgSiO$_3$ agree much better with experiments by using GGA than by LDA. The Clapeyron slopes (CS) predicted by GGA and LDA are close to each other: $0.9$ -- $1.7$ MPa/K for mj-pv, 6.9 -- 7.9 MPa/K for mj-il, and $-7$ -- $-3$ MPa/K for il-pv transition. The triple point predicted by GGA is at $21.8 \pm 1$ GPa and $1840 \pm 200$ K which is $\sim$400 K lower than most experiments. Our calculations also reveal that wadsleyite decomposes to an assemblage of majorite plus periclase above 2280 K with a large negative CS ($-22$ -- $-12$ MPa/K) and that ringwoodite decomposes to ilmenite plus periclase below 1400 K (1.2 MPa/K). The geophysical implications to mantle convection and the composition of the Earth's transition zone will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P31.00013: A first-principles investigation of hydrous defect and IR frequencies in forsterite: The case for Si vacancies Marc Hirschmann, Koichiro Umemoto, Renata Wentzcovitch, David Kohlstedt, Anthony Withers We investigate charge-balanced hydrous magnesium and silicon defects ((2H)$^{X}$$_{Mg}$, (4H)$^{X}$$_{Si}$) by first principles. Here we propose two new lowest-energy hydrogen configurations for (4H)$^{X}$$_{Si}$. With these new configurations, the distribution of OH-stretching phonon frequencies in Group I ($>$ 3450 cm-1) are better reproduced. Substitution of silicon with 4 hydrogens gives rise to significant elongation of distances between oxygen ions at the tetrahedron of the silicon vacancy. Our calculations indicate that the correlation between O-O distances and O-H-stretching phonon frequencies, which has been well established for hydrous minerals, does not apply directly to nominally anhydrous minerals and should not be used to determine the identity of the hydrous defects responsible for infrared absorption peaks. [Preview Abstract] |
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