Session K21: Geophysics and Planetary Science

Ab initio Raman spectroscopy of water under extreme conditions

Water exhibits one of the most complex phase diagrams of any binary compound. Despite extensive studies, the melting lines of high-pressure ice phases remain very controversial, with reports differing by hundreds of Kelvin. The boundary between ice VII and liquid phase is particularly disputed, with recent work exploring plasticity and amorphization mediating the transition. Raman measurements are often used to fingerprint melting, yet their interpretation is difficult without atomistic modeling. Here, we report a study of high P/T water where we computed Raman spectra using a method [1] combining ab initio molecular dynamics and density functional perturbation theory, as implemented in the Qbox code [2]. Spectra were computed for the liquid at 10 and 20 GPa, both at 1000 K, and for solid ice VII (20 GPa, 500 K). Decomposing the spectra into inter and intra molecular contributions provided insight into the dynamics of the hydrogen-bonded network at extreme conditions. The relevance of our simulation results for models of water in Earth, Uranus, and Neptune will be discussed, and an interpretation of existing experiments at high pressure will be presented. [1] Wan, Q., Spanu, L., Galli, G., Gygi, F., J. Chem. Theory Comput. 9, 4124. (2013) [2] http://qboxcode.org   

Conductivity and Correlations in Fe at Earth Core Conditions

We have computed electrical conductivity in iron at Earth core conditions self-consistently within many-body theory using DFT/DMFT. We find that electron correlations are important even in the generation of Earth’s magnetic field. Earth's magnetic field was believed to arise from thermal convection of molten iron alloy in Earth's outer core, but density functional theory (DFT) calculations suggested that the conductivity of iron is too high to support thermal convection, so that new geodynamo models were being developed. The DFT computations for resistivity were based on the scattering of electrons off of atomic vibrations, or electron-phonon (e-p) scattering. We applied self-consistent density functional theory plus dynamical mean-field theory (DFT+DMFT) to iron and found that at high temperatures electron-electron (e-e) scattering is comparable to the e-p scattering, bringing theory into agreement with experiments and solving the transport problem in Earth's core, consistent with the conventional thermal geodynamo [Peng, Cohen, and Haule, Nature 517, 605, 2015]. How electron correlations change with pressure, and how this affects material properties, will be discussed. This work is supported by the US National Science Foundation and the ERC Advanced grant ToMCaT.   

A Green Function Approach to the Effects of Core-state Overlap on Interatomic Interactions at Extreme Densities

Under extreme conditions of temperature and pressure, interatomic separations in condensed matter can approach a small fraction of those under normal laboratory conditions. For example, during high-energy ($\sim 100$ keV) radiation damage cascades, interatomic separations can be as small as 0.5\AA. Under such conditions, core states between neighboring atoms could overlap and must be included as band states. Here we use Green’s function method in the framework of multiple scattering theory, also known as Korringa-Kohn-Rostoker (KKR) electronic structure methods, to seamlessly integrate these core overlap effects within an all-electron ab initio approach. To accomplish these we use multiple integration contours in the complex plane that incorporate states normally treated as bound atomic levels. We show results for Ni and NiFe alloys in extreme densities ($a/a_0 \sim 0.3$) to illustrate the convergence of the method with respect to which core states are banded as well as the angular moment cut-off required to establish absolute convergence of the total energies. Results are compared with those of plane-wave methods for different choices of the underlying pseudo potential to establish the range of validity of the various approaches.   

Dissolved carbon in extreme conditions characterized by first principles simulations

One key component to understanding carbon transport in the Earth interior is the determination of the molecular species formed when carbon bearing materials are dissolved in water at extreme conditions. We used first principles molecular dynamics to investigate oxidized carbon in water at high pressure (P) and high temperature (T), up to the conditions of the Earth's upper mantle. Contrary to popular geochemistry models assuming that CO$_2$ is the major carbon species present in water, we found that most of the dissolved carbon at 10 GPa and 1000 K is in the form of solvated CO$_3^{2-}$ and HCO$_3^-$ anions. We also found that ion pairing between alkali metal cations and CO$_3^{2-}$ or HCO$_3^-$ anions is greatly affected by P-T conditions, decreasing with pressure along an isotherm. Our study shows that it is crucial to take into account the specific molecular structure of water under extreme conditions and the changes in hydrogen bonding occurring at high P and T, in order to predict chemical reactions in dissolved carbon. Our findings also shed light on possible reduction mechanisms of CO$_2$ when it is geologically stored, depending on the availability of water.   

Novel stable compounds in the Mg-Si-O system under exoplanet pressures and their implications in planetary science

The Mg-Si-O system is the major Earth and rocky planet-forming system. Here, through quantum variable-composition evolutionary structure explorations, we have discovered several unexpected stable binary and ternary compounds in the Mg-Si-O system. Besides the well-known SiO$_{\mathrm{2}}$ phases, we have found two extraordinary silicon oxides, SiO$_{\mathrm{3}}$ and SiO, which become stable at pressures above 0.51 TPa and 1.89 TPa, respectively. In the Mg-O system, we have found one new compound, MgO$_{\mathrm{3}}$, which becomes stable at 0.89 TPa. We find that not only the (MgO)$_{\mathrm{x}}$·(SiO$_{\mathrm{2}})$y compounds, but also two (MgO$_{\mathrm{3}})_{\mathrm{x}}$·(SiO$_{\mathrm{3}})_{\mathrm{y}}$ compounds, MgSi$_{\mathrm{3}}$O$_{\mathrm{12}}$ and MgSiO$_{\mathrm{6}}$, have stability fields above 2.41 TPa and 2.95 TPa, respectively. The highly oxidized MgSi$_{\mathrm{3}}$O$_{\mathrm{12\thinspace }}$can form in deep mantles of mega-Earths with masses above 20 M$_{\mathrm{\oplus }}$ (M$_{\mathrm{\oplus }}$:Earth's mass). Furthermore, the dissociation pathways of pPv-MgSiO$_{\mathrm{3}}$ are also clarified, and found to be different at low and high temperatures. The low-temperature pathway is MgSiO$_{\mathrm{3\thinspace }}\Rightarrow $ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $SiO$_{\mathrm{2\thinspace }}+$ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}\Rightarrow $ MgO $+$ SiO$_{\mathrm{2}}$, while the high-temperature pathway is MgSiO$_{\mathrm{3\thinspace }}\Rightarrow $ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $ MgO $+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $ MgO $+$ SiO$_{\mathrm{2}}$. Present results are relevant for models of the internal structure of giant exoplanets, and for understanding the high-pressure behavior of materials.   

Spin crossover in solid and liquid (Mg,Fe)O at extreme conditions

Ferropericlase, (Mg,Fe)O, is a major constituent of the Earth's lower mantle (24-136 GPa). Understanding the properties of this component is important not only in the solid state, but also in the molten state, as the planet almost certainly hosted an extensive magma ocean initially. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to a nonmagnetic spin state. This crossover affects thermodynamic, transport, and electrical properties. Using first-principles molecular dynamics simulations, thermodynamic integration, and adiabatic switching, we present a phase diagram of the spin crossover. In both solid and liquid, we find a broad pressure range of coexisting magnetic and non-magnetic ions due to the favorable enthalpy of mixing of the two. In the solid increasing temperature favors the high spin state, while in the liquid the opposite occurs, due to the higher electronic entropy of the low spin state. Because the physics of the crossover differ in solid and liquid, melting produces a large change in spin state that may affect the buoyancy of crystals freezing from the magma ocean in the earliest Earth.   

Amorphization and nanocrystallization of silicon under laser shock compression: bridging experiment with atomic simulation.

Terawatt, nanosecond-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with \textbraceleft 111\textbraceright slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed by classical nucleation theory.   

First-principles study of the amorphization of stishovite by isotropic volume expansion

Simple synthesis of ceramics with high hardness and high toughness from Earth-abundant materials is one of the most important issues in materials science. Nishiyama et al. synthesized nano-crystalline stishovite with extremely high toughness and high hardness via compression and decompression of silica, and proposed fracture-induced amorphization mechanisms for the toughning [1]. Furthermore, it was shown that the toughening mechanisms are effective even in nanoscale order [2]. Our first-principles molecular dynamics simulations have shown rapid amorphization of stishovite within picoseconds under increasing volume, thus substantiating the proposed amorphization mechanisms. Furthermore, we have calculated the critical stress, energy difference, and energy barrier for the crystalline-to-amorphous structural transition. [1] N. Nishiyama et al., Scientific Reports 4, 6558 (2014). [2] K. Yoshida et al., Scientific Reports 5, 10993 (2015).   

Phase diagram of the itinerant helical magnet MnSi at high pressures and strong magnetic fields

We performed a series of resistivity, heat capacity and ultrasound speed measurements of a MnSi single crystal at high pressures and strong magnetic fields [1-3]. Two notable features of the phase transition in MnSi that disappear on pressure increasin are a sharp peak marking the first order phase transition and a shallow maximum, situated slightly above the critical temperature and pointing to the domain of prominent helical fluctuations. The longitudinal and transverse ultrasound speeds and attenuation were measured in a MnSi single crystal in the temperature range of 2-40 K and magnetic fields to 7 Tesla. The magnetic phase transition in MnSi in zero magnetic field is signified by a quasi-discontinuity in the c11 elastic constant, which almost vanishes at the skyrmion - paramagnetic transition at high magnetic fields. The powerful fluctuations at the minima of c11 make the mentioned crossing point of the minima and the phase transition lines similar to a critical end point, where a second order phase transition meets a first order one. \begin{enumerate} \item Alla E. Petrova and Sergei M. Stishov, Phys. Rev. B 86, 174407 (2012) \item V. A. Sidorov, et al., Phys. Rev. B 89, 100403(R) (2014) \item A. E. Petrova and S. M. Stishov, Phys. Rev. B 91, 214402 (2015) \end{enumerate}   

A DFT$+$DMFT study of magnetic properties of FeO at high pressure.

FeO is an insulator with anti-ferromagnetic (AFM) spin ordering at ambient pressure. When external pressure is increased, the N\'{e}el temperature first increases at the pressure below 40 GPa. Experiments show the AFM ordering collapses at high pressures. Using the density functional theory plus dynamical mean-field theory (DFT$+$DMFT), we examined the nature of magnetic collapse of FeO and derived its magnetic phase diagram up to 180 Gpa. We found coexistence of high spin-low spin transition and paramagnetic-AFM transition, both driven by increased pressure. The high spin-low spin transition is result of partition and pairing of 3d electrons in iron. The local moment of iron atom after high spin-low spin transition is small but finite up to 180 GPa.   

Gravitational Collapse and Shocks in Two-Phase Celestial Bodies

The phenomenon of gravitational collapse (GC) is well-known in theoretical astro- and planetary physics. It occurs when the incompressibility of substances is unable to withstand the pressure due to gravitational forces in celestial bodies of sufficiently large mass. The GC never occurs in incompressible models -- homogeneous or layered. This situation changes dramatically when different incompressible layers appear to be different phases of the same chemical substance and the mass exchange between the phases can occur due to phase transformation. The possibility of destabilization in such system becomes realistic, as it was first discovered in the Ramsey static analysis [1,2]. We will present our generalization of the Ramsey's results using dynamic approach. [1] W.H. Ramsey, "On the instability of small planetary cores", Mon. Not. R. Astron. Soc. 110 (4), 325-338 (1950). [2] H. Jeffreys, "The Earth: Its Origin, History, and Physical Constitution". Cambridge University Press (1976).