Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L17: Matter in Extreme Environments: Planetary MaterialsFocus
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Sponsoring Units: DCOMP Chair: Thomas Duffy, Princeton University Room: BCEC 156A |
Wednesday, March 6, 2019 11:15AM - 11:51AM |
L17.00001: Material Properties of Super-Earths and mini-Neptunes Invited Speaker: Diana Valencia The most common planets in the galaxy are now known to be low-mass exoplanets that either are rocky, known as super-Earths or may have an incipient envelope, the so-called mini-Neptunes. As the data grows for measured masses and radius for these planets the types of questions that we aim to answer have moved from discovery to characterization. I will talk about synergies between the study of material properties in the PT regime of these planets and the inferences we are able to make now in terms of composition and structure. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L17.00002: Ab initio study of water speciation in forsterite Koichiro Umemoto, Tian Qin, Renata Wentzcovitch The Earth’s interior contains 0.5 to 100 times of water on the Earth’s surface based on different studies. Water or hydrogen (in hydroxyl form) can be stored as hydrous defects in nominally anhydrous minerals (NAMs) in the Earth’s mantle. Although in modest concentrations, these defects change the physical properties of their hosts, including electrical conductivity and viscosity, properties that affect mantle processes such as convection. The most likely incorporation mechanism of hydrogen in mantle minerals is the substitution of Mg and Si cations by hydrogens. However, a long-standing debate remains concerning the relative thermodynamic stability of these defects. Using ab initio calculations we investigate the energetics of these defects, (4H)x Si and (2H)x Mg, in forsterite, the Mg end-member of olivine, the most abundant upper mantle phase. We address the role of vibrational free energy and lattice and internal defect configurational entropy in the relative stability of these defects. We conclude that entropic effects are key to the stabilization of the hydrous Mg defect, which should predominate over the hydrous Si defect at typical upper mantle conditions. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L17.00003: First-principles studies of oxidized carbon in water under extreme conditions Nore Stolte, Ding Pan The properties of oxidized carbon and water mixtures at high pressure (HP) and high temperature (HT) are of great importance to the deep carbon cycle, which involves more than 90% of Earth's carbon and substantially impacts the carbon budget near Earth's surface. We studied CO2 in supercritical water up to ~13 GPa and 1400 K using ab initio molecular dynamics, and found that while the major form of dissolved carbon is CO2(aq) at ambient conditions, carbonic acid (H2CO3(aq)) can be more abundant than CO2(aq) at HP-HT when the total mole fraction of carbon is below 40%. We investigated the aqueous reaction mechanisms of H2CO3(aq) formation and dissociation at HP-HT at the molecular scale. We will discuss the possible P-T range for the stability of H2CO3(aq). Our study suggests that H2CO3(aq) may be an important carbon transport host in the deep carbon cycle. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L17.00004: First-principles prediction of new gas hydrates Lewis Conway, Andreas Hermann Gas hydrates can form under low temperature and high pressure (~kbars) conditions and as a result occur naturally on Earth and within the solar system. They have applications in industry in storage and transport of gases. We explore computationally the stability of new gas hydrates, with a focus on the chiral water network Sχ, the metastable ice structure ice XVII, associated with the C0 hydrogen hydrate and the CO2 HP-hydrate [1]. Sχ has been shown experimentally to be readily emptied [2]. Computationally, Sχ has been shown to form at least metastable hydrates with He, Ne and Ar [3]. Here we present a density functional theory (DFT) study of molecular N2 and O2 gas hydrates based on filled Sχ, and analyse the phase evolution in the ground state, which shows a strong dependance on pressure and filling ratio. O2 gas hydrate has the curious potential to form a magnetically ordered structure. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L17.00005: " Predicted novel helium compounds under high pressure via CALYPSO " Hanyu Liu, Jurong Zhang, Yinwei Li, Jian Lv, Yansun Yao, Xiaolei Feng, Simon Redfern, Changfeng Chen, Yanming Ma The knowledge of the structures that can exist in compounds containing helium is of interest for understanding the conditions where and if this inert element can form structures where closed shell electrons of helium can participate in bonding that is not describable exclusively by van der Waals interactions alone. In this work, we examine some mixtures of He and H2O, N2or some minerals at high pressures using a first-principles structure searching method (CALYPSO). We find some thermodynamically stable structures under pressure. These mechanically and dynamically stable structures are found at pressures that are now becoming accessible to high-pressure technique. The present results offer insights for the understanding of the plausible reaction between helium with minerals in the Earth’s or other exoplanetary interiors. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L17.00006: Helium insertion reactions with ammonia and water under pressure Yihong Bai, Zhen Liu, Jorge Botana, Dadong Yan, Hai-Qing Lin, Jian Sun, Chris Pickard, Richard Needs, Maosheng Miao The noble gas elements are usually quite inert to chemical reactions due to their closed shell configurations. He is the most stable noble gas since its ionization energy is almost twice as large as that of Xe. It was recently demonstrated that He could react with a large number of ionic compounds with unequal number of cations and anions due to the substantial change of the electrostatic energy under high pressure. In this work, we explore and show the reactivity of He with two molecular crystals, ammonia (NH3) and ice (H2O) by density functional theory calculations. We found that NH3 and He can form stable structure above 50 GPa with a NH3:He ratio of 1:1, while H2O and He can form stable structure above 300 GPa with a H2O:He ratio of 2:1. Although change of the electrostatic interaction is the driving force for the He insertion under high pressure, the mechanism is very different between ammonia and ice. This work extends the reactivity of He into a new area of molecular crystals, showing the richness of the chemistry of this most inert element in the periodic table. Since He, NH3 and H2O are the major components of giant gas planets, the new chemistry revealed in our work is important for the understanding of the structure and the evolution of these planets. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L17.00007: Helium reaction with sodium halides under pressure Zhen Liu, Yihong Bai, Ortega Fernando, Chris Pickard, Dadong Yan, Hai-Qing Lin, Maosheng Miao The noble gas elements are usually quite inert to chemical reactions due to the closed shell configurations. He is the most stable noble gas since its ionization energy is almost twice as large as that of Xe. It was recently demonstrated that He could react with a large number of ionic compounds with unequal number of cations and anions due to the substantial change of the electrostatic energy under high pressure. Unexpectedly, we found in the current work that He can react with NaX (X=Cl, Br, I) under high pressure also with considerable formation energy. The pressure needs to form stable NaXHe compounds decreases with increasing size of the halogen atoms. By analyzing the enthalpy components, the geometry change, and the electronic structures, we find the driving force of the He insertion reaction with AB type compound is the transformation of the structures that disproportionate the interstitial sites, making them ready to accommodate He atoms under high pressure. With the previous work, we greatly extend the reactivity of He with compounds under high pressure. |
Wednesday, March 6, 2019 1:03PM - 1:15PM |
L17.00008: Phase relations in δ-AlOOH investigated with ab-initio calculations Tianqi Wan, Ziyu Cai, Chenxing Luo, Tian Qin, Renata Wentzcovitch As a high-pressure phase of diaspore and boehmite, δ-AlOOH is a crucial hydrous phase that can transport water in subducted slabs to the Earth's interior. Knowledge of phase relations in AlOOH is vital to understanding the state of water and the mechanisms of water transportation to the deep mantle. Most previous theoretical studies focused on the low-pressure static behavior of δ-AlOOH, leaving not only issues such as hydrogen-bond symmetrization under pressure unresolved but also high-temperature high-pressure phase boundaries not fully discussed. Recent experiments [1] show that the stability field of δ-AlOOH covers the entire pressure range of the Earth's lower mantle and this phase could remain stable in colder slabs down to the core-mantle boundary. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L17.00009: Proton dynamics in high-pressure ice-VII from density functional theory Florian Trybel, Gerd Steinle-Neumann, Thomas Meier Recent Nuclear Magnetic Resonance (NMR) data on high-pressure ice-VII (Meier et al., Nature Comm., 2018) have revealed significant mobility of protons in the pressure range of 20-95 GPa at room temperature. Using a density-functional-theory-based approach, we explore the compression-dependent proton dynamics in ice-VII by directly sampling the proton potential along the diagonal O-O direction in the disordered body centered cubic configuration. We describe a configuration showing a double-well potential with a barrier that permits tunneling at compressions corresponding to 20 GPa. The double-well character disappears near 45 GPa, but a broad minimum indicates significant proton mobility to persist to 95 GPa. Tunneling frequencies are computed using the approximation of Wentzel, Kramers and Brillouin, with frequencies in the THz range. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L17.00010: Role of hydrogen bonding in the phase transformation of bulk liquid water to ice VII under shock loading Nilanjan Mitra, Dipak Prasad It has been widely reported both through experimental and theoretical investigations that bulk liquid water under shock loading partially transforms into Ice VII – one of the densest forms of ice. It is also known from literature that hydrogen bonding plays the main role in conversion of random arrangement of water molecules in bulk liquid water to that of periodic crystalline arrangement of ice in the event of conventional freezing. Thereby, it can be quite anticipated that the hydrogen bond plays the main role in phase transformation of bulk liquid water to that of ice VII on shock compression. The current manuscript describes the changes in the spectra of water in the 0-1000 cm-1 wavenumber regimes as bulk water is shock compressed to form ice VII. The molecular dynamic simulations carried out for this study also compares between different commonly used force potentials that are used to define water. |
Wednesday, March 6, 2019 1:39PM - 2:15PM |
L17.00011: First-principles modelling of hydrogen-rich planetary materials Invited Speaker: Andreas Hermann Accurate models of the interior structure of planetary bodies, in our or other solar systems, are key to understanding their formation, their evolution to the present day, and many of their properties. The stratification (or lack thereof) of molecular mixtures inside icy planets’ mantles influences their luminosity and cooling rates; and the presence (or not) of water stored inside rocky planets’ mantles influences their convection rates, the magnitude of plate tectonics and presence of surface water. In my talk I will give an overview of our ongoing research aimed at a better understanding of planetary materials using electronic structure calculations, focussing on hydrogen-containing systems on several different pressure scales. For molecular gas hydrates stabilized in the kbar range, crucial in the formation of icy moons and planets, I will show how wave-function based calculations can overcome density functionals’ ambiguities in capturing weak host-guest interactions [1]. For hydrous minerals, which store water inside Earth’s mantle in the GPa range, I will illustrate how crystal structure prediction methodology can help develop a more complete picture on their formation and internal phase transformations [2]. Lastly, I will discuss the properties of mixtures of planetary ices at conditions found in the mantle of Neptune-like bodies, at pressures beyond 1 Mbar and high temperatures [3,4]. |
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