Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A11: Earth and Planetary Materials I |
Hide Abstracts |
Sponsoring Units: DMP DCMP GSCCM Chair: Taku Tsuchiya, University of Minnesota Room: LACC 153C |
Monday, March 21, 2005 8:00AM - 8:36AM |
A11.00001: Theoretical and experimental evidence for a post-perovskite phase of MgSiO$_3$ in Earth's D$''$ layer. Invited Speaker: The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed to contain $\sim $75 vol.{\%} of (Mg,Fe)SiO$_{3}$ perovskite, $\sim $20{\%} (Mg,Fe)O, and $\sim $5{\%} CaSiO$_{3}$. This mineralogy was unable to explain many unusual properties of the D'' layer, the lowermost $\sim $150 km of the mantle. Using \textit{ab initio} simulations and high-pressure experiments we have demonstrated [1] that at pressures and temperatures of the D$''$ layer, MgSiO$_{3}$ transforms from perovskite into a layered CaIrO$_{3}$--type structure (space group \textit{Cmcm}); this structure was also independently found in [2]. The elastic properties of the new phase and its stability field explain most of the previously puzzling properties of the D$''$ layer: its seismic anisotropy [3], strongly undulating shear-wave discontinuity at its top$^{ }$[4], and the anticorrelation between shear and bulk sound velocities [5]. This new phase is therefore likely to be a major Earth-forming mineral, and its discovery will change our understanding of the deep Earth's interior. Latest studies of the effects of impurities [6,7] on the stability of this phase, and similar phases of other compounds will be discussed. \\ \\ REFERENCES: \\1. Oganov A.R., Ono S. (2004). \textit{Nature} \textbf{430}, 445-448. \\2. Murakami M., et al. (2004). \textit{Science} \textbf{304}, 855-858. \\3. Panning M., Romanowicz B. (2004). \textit{Science }\textbf{303}, 351-353. \\4. Sidorin I., et al. D.V. (1999). \textit{Science} \textbf{286}, 1326-1331. \\5. Su W.J., Dziewonski A.M. (1997). \textit{Phys. Earth Planet. Inter.} \textbf{100}, 135-156. \\6. Mao W.L., et al. (2004). \textit{Proc. Natl. Acad. Sci.} \textbf{101}, 15867-15869. \\7. Ono S., Oganov A.R., Ohishi Y. (2004).\textit{ Submitted.} [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A11.00002: Vibrational and thermodynamic properties of MgSiO$_3$ post-perovskite Jun Tsuchiya, Taku Tsuchiya, Renata Wentzcovitch Recently a high pressure transformation in MgSiO$_3$ perovskite, the primary constituent of Earth's lower mantle, was identified near 125 GPa and 2500 K. In order to understand the importance of the new phase on mantle properties and dynamics it is necessary to determine its thermodynamic properties and contrast with those of perovskite. Here we perform first principles quasi-harmonic approximation calculations of the free energy of MgSiO$_3$ post-perovskite and derive several thermodynamic properties of interest up to and beyond core-mantle-boundary (CMB) pressures. It is found that in general its thermal properties are very similar to those of perovskite at relevant conditions, despite the great structural difference between these phases. The post-perovskite phase is found to be mechanically stable at ambient pressure and might be retrieved metastably at cryogenic temperatures. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A11.00003: MgSiO3 post-perovskite at D'' conditions Renata Wentzcovitch, Taku Tsuchiya, Jun Tsuchiya The thermoelastic properties of the newly found post-perovskite polymorph of MgSiO$_3$, more stable than the Pbnm-perovskite phase at conditions close to those expected in Earth's D'' region, has been investigated by first-principles and contrasted with those of the perovskite phase. We predict the major seismic trends such as velocity discontinuities, ratios of velocities and density anomalies, and anisotropy in aggregates with preferred orientation that should occur in the presence of this phase change. Consequences of this model mineralogy for the D'' region will be discussed. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A11.00004: On the stability of perovskite and post-perovskite polymorphs of MgSiO$_3$ Jose Martins, Yaowen Liu, Renata Wentzcovitch The relative stability of 10 distorted perovskite structures of MgSiO$_3$ have been investigated by first principles and contrasted with that of the newly found {\it Cmcm} post- perovskite. The electronic structure of these polymorphs was analyzed and simple relationships between magnitudes of polyhedral distortions, bond-lengths, and band-gaps were found. Up to approximately 95 GPa, the {\it Pnma} phase is the most stable, has the largest band gap, and can accommodate the largest volume reduction with the smallest distortion of the SiO$_6$ octahedra. At higher pressures the post-perovskite polymorph is the most stable one. This phase transition is accompanied by an increase of octahedral volume and band gap. These results do not support a transition from {\it Pnma} to another distorted perovskite structure prior to the transition to the {\it Cmcm} polymorph as proposed by some experiments. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A11.00005: First principles study of liquid MgSiO$_3$ at conditions of the Earth's deep mantle Jones Tsz-Kai Wan, Roberto Car, Sandro Scandolo, Thomas S. Duffy Constant-pressure {\it ab initio} molecular dynamic simulations at high temperatures have been used to study MgSiO$_3$, the major constituent of the Earth's lower Mantle. In this work, we focus the properties of molten MgSiO$_3$, where its existence in the core-mantle boundary is still in debate. By using liquid configuration, we have performed variable-cell {\it ab initio} molecular dynamic simulations at relevant thermodynamic conditions across one of the measured melting curve. The calculated equilibrium volumes and densities are compared with the simulations using orthorhombic perovskite configuration under the same conditions. For molten MgSiO$_3$, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide the evidences of the existence of molten MgSiO$_3$ near the core-mantle boundary. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A11.00006: {\it Cmcm} post-perovskite: a new alumina polymorph Jun Tsuchiya, Taku Tsuchiya, Renata Wentzcovitch Alumina, Al$_2$O$_3$, is a model ceramic material with important applications in high pressure science, particularly as the ruby pressure scale. It is isoelectronic with MgSiO$_3$, the major Earth forming mineral. Here we show by first principles that the newly found post-perovskite polymorph of MgSiO$_3$, CaIrO$_3$ type structure with {\it Cmcm} symmetry, is also a stable high pressure phase of Al$_2$O$_3$ and should be stabilized in the pressure range in which the ruby scale has been calibrated. The sequence of polymorphs under pressure in these minerals is therefore analog: corundum/ilmenite $\rightarrow$ {\it Pbnm}-perovskite for MgSiO3 and Rh$_2$O$_3$(II) type for Al$_2 $O$_3$ $\rightarrow$ {\it Cmcm} post-perovskite. The reason for the greater stability of {\it Pbnm}-perovskite in MgSiO$_3$ versus Rh$_2$O$_3$(II)-type in Al$_2$O$_3$ is the difference in cation polyhedral types and volumes in the former, that favors for ABX$_3$-type composition. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A11.00007: Total energy and linear response computations for perovskite and post-perovskite phases in the MgSiO$_3$-FeSiO$_3$-Al$_2$O$_3$ system Razvan Caracas, Ronald Cohen We perform first-principles calculations for the perovskite (pv) and post-perovskite (ppv) phases in the (Mg,Fe,Al)(Si,Al) O$_3$ system, the dominant chemical system of the Earth’s lower mantle. We consider different chemical compositions in this system for which we analyze the structural, electronic, elastic and lattice dynamical properties. We use total energy and linear response techniques within LDA and GGA, as implemented in the code ABINIT. We perform calculations in the 0-180 GPa pressure range, in 30GPa increments, to characterize the behavior of these materials over the whole Earth’s mantle pressure range (up to 137 GPa). We find that the addition of Al in MgSiO$_3$ increases the pv-ppv transition pressure, while the addition of Fe largely decreases this pressure. The pv phase of FeSiO$_3$ is unstable with respect to ppv at all pressures. Fe reduces the electronic gap in both pv and ppv, the Fe-end-member being high-spin and metallic. The pv phase of FeSiO$_3$ is ferromagnetic while the ppv phase is ferromagnetic at low pressures and antiferromagnetic at high pressures. This research is supported by the NSF grant EAR-0310139 and the Carnegie Institution of Washington. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A11.00008: Sub-lattice melting in hydrogen-rich alloys Simon Gravel, Neil Ashcroft Hydrogen at low temperatures has recently been predicted to undergo a solid to quantum liquid transition at sufficiently high pressure. The resulting quantum liquid is believed to be a metal and exhibit both superfluidity and superconductivity. Pseudopotential methods combined with nonlinear response theory have been able to give a simple, qualitative account for this transition. We generalize these methods and apply them to hydrogen-rich compounds in order to determine effective pair and triplet interactions in such alloys. By comparing the binding energies obtained to the proton or deuterium zero-point energy we can determine whether sub-lattice melting can be expected, and obtain an estimate of the required pressures. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:36AM |
A11.00009: X-ray spectroscopy studies of liquid water Invited Speaker: We have investigated the electronic structure of water and ice using a combination of experimental and theoretical techniques [1]. Measurements have been performed on the liquid using both X-ray Absorption (XAS) and X-ray Raman Spectroscopy. The spectrum of the liquid is distinctly different from that of the bulk ice, where the liquid shows a distinct pre-edge feature and a strong enhancement of the intensity at the edge. Through spectrum simulations and model experiments (bulk and surface of ice) we show that the specific features in the liquid spectrum are due exclusively to asymmetric configurations with only two strong hydrogen bonds: one donating and one accepting, indicating that the liquid consists of rings or chains embedded in a disordered H-bond network [1]. Current molecular dynamics techniques fail to predict these new experimental data. Recent results on disparities in supercritical water will also be discussed. \newline \newline [1] Wernet et al, Science 304, 995-999 (2004) [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A11.00010: Pressure-induced Phase Transition of Confined Water from ab initio Molecular Dynamics Simulation Sheng Meng, E.G. Wang, Shiwu Gao We present an ab initio molecular dynamics study of pressure induced melting of an ice thin film confined between two parallel metal surfaces. The ice-to-water phase transition has been observed at a pressure of roughly 0.5~GPa, when the film is compressed by 6.6 percent. The latter is in agreement with the volume change in the melting of bulk ice. The effects of non-adiabatic compression on the layer-dependent momentum distribution and the electronic redistribution at the interfaces are presented and discussed. [Preview Abstract] |
Monday, March 21, 2005 10:48AM - 11:00AM |
A11.00011: First principles investigation of the ice VII-VIII (order-disorder) phase boundary Koichiro Umemoto, Renata Wentzcovitch, Stefano de Gironcoli, Stefano Baroni Phase boundaries among the various forms of ice are difficult to determine experimentally because of the large hystereses involved. Theoretically there are also great challenges, including order-disorder (OD). The ice VII-VIII boundary, a typical OD boundary, has been reasonably well constrained experimentally. We present a first principles study consisting in the complete statistical sampling of molecular orientations within a 16 molecules supercell. This supercell size accounts well for several aspects of this transition, including the transition temperature and its pressure dependence in the high P range. The differences at lower Ps are likely to be related with the insufficiencies of DFT, within LDA or GGA, to describe the hydrogen bond. Research supported by NSF/EAR 013533 (COMPRES), 0230319, and NSF/ITR 0428774 (VLab). [Preview Abstract] |
Monday, March 21, 2005 11:00AM - 11:12AM |
A11.00012: Dissociation of planetary ices at high P,T Jae-Hyun Klepeis, Magnus Lipp, Bruce Baer, Choong-Shik Yoo The major components of the ice layer of the Jovian planets are methane and ammonia [1]. By adopting laser-heating techniques, we examined methane and ammonia under high pressure and high temperature. XRD and Raman studies showed that ammonia generates nitrogen and that methane dissociates into various hydrogen end products and carbon end products under extreme conditions. From this study, we suggest that the inner layer of Jovian planets gradually loses methane and ammonia by their dissociation process. [1] W.B. Hubbard, Science, 214, 145 (1981). [Preview Abstract] |
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