Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M19: Matter in Extreme Environments: Planetary and Geological MaterialsFocus Live
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Sponsoring Units: DCOMP DMP Chair: Tiange Bi |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M19.00001: Understanding Matter at Superdense and Warm Conditions* Invited Speaker: Suxing Hu Warm and extremely-dense matter, having mass densities ranging from tens to millions of gram per cubic centimeter and temperatures of 10,000 to 1,000,000 K, widely exist in the universe such as giant planet cores and brown/white dwarfs. Thanks to advances in technology, such extreme conditions can now be created in laboratories by powerful lasers or pulsed-power machines. The experimental advances have certainly helped us to unravel how matter behaves under warm and superdense conditions. On the theory and computation side, first-principles tools such as thermal density-functional theory (DFT) are often used to reveal novel properties of warm and extremely-dense matter. Many new phenomena, for example, unusual K-edge shifting,[1] interspecies radiative transition,[2] and dynamical Kα/β–emission/absorption line movements,[3] have recently been predicted by ab initio DFT calculations. Some of them are recently confirmed by experimental measurements. In this talk, we will cover the recent progress in understanding the physics of matter in such extreme environments through both computational and experimental studies. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M19.00002: Prediction of a temperature-induced phase transition in Mg2GeO4 by first principles Koichiro Umemoto, Renata M Wentzcovitch Here we present a first principles prediction of a temperature-induced phase transition in I-42d-type Mg2GeO4, a low-pressure analog [1] of ultrahigh pressure phase of Mg2SiO4 [2]. The latter was predicted to occur as a product of dissociation/recombination transitions in MgSiO3 post-perovskite at multi-Mbar pressures (post-PPV transitions) [3]. In the Mg-Ge-O system, this post-PPV phase occurs beyond ~150 GPa [1]. This new phase transition predicted here alters the sequence of post-PPV phases in Mg2GeO4 at high temperatures (>~2,000 K) and likely also in Mg2SiO4. I-42d-type Mg2SiO4 is predicted to occur in the deep interiors of super-Earths [2-5]. Therefore, this newly found phase transition should be relevant for modeling the internal dynamics and structure of super-Earth-type planets. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M19.00003: Dense and metallic nitric sulfur hydrides Xiaofeng Li, Lewis Conway, Maosheng Miao, Andreas Hermann Hydrogen sulfide (H2S) and ammonia (NH3) form hydrogen-bonded molecular mixtures at ambient conditions, but their phase behavior and propensity towards mixing under pressure is not well understood. Such mixtures dominate the interiors of icy planets and open up new routes towards hydrogen-rich superconductors. Here we report on stable phases in the H2S-NH3 system under extreme pressure conditions to 4 Mbar from first-principles crystal structure prediction. We identify four stable compositions, two of which, (H2S)(NH3) and (H2S)(NH3)4, are stable in a sequence of structures to the Mbar regime. A re-entrant stabilization of (H2S)(NH3)4 above 300GPa is driven by a marked reversal of sulfur-hydrogen chemistry. Several stable phases exhibit metallic character, which enables superconductivity. Electron-phonon coupling calculations predict superconducting temperatures up to 50K, in the Cmma phase of (H2S)(NH3) at 150GPa. The present findings suggest a reservoir for hydrogen sulfide in the upper mantle regions of icy planets in a potentially metallic mixture, which has implications for their magnetic field formation. They also shed light on potential routes towards superconducting H2S-containing molecular mixtures. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M19.00004: Prediction of crystal structures and motifs in the Fe-Mg-O system under Earth’s core pressures Renhai Wang, Yang Sun, Feng Zheng, Yimei Fang, Feng Zhang, Shunqing Wu, Zijing Lin, Cai-Zhuang Wang, Renata M Wentzcovitch, Kai-Ming Ho While the crystal structures of Fe-O and Fe-Mg binary systems under pressure have been extensively studied recently, there are still very few studies of the Fe-Mg-O ternary phases at relevant Earth’s core and super-Earth’s mantle pressures. Here, we use the adaptive genetic algorithms (AGA) to search for and study the crystal structure of FexMgyOz phases in a wide range of stoichiometries at 200 GPa and 350 GPa. Our results revealed new phases with lower enthalpy than the known compounds in the existing Fe-Mg-O high-pressure structure database. By constructing the convex hull at both pressures, we determine the ground-state phases. We also carry out structure motif analyses for AGA-found stable and metastable structures. We find a wide variety of composition- and pressure-dependent motifs. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M19.00005: Comparison of DFT-based methods for electronic stopping in warm dense aluminum Alina Kononov, Thomas Hentschel, Attila Cangi, Andrew D Baczewski, Stephanie B Hansen The transport properties of warm dense matter (WDM), which has temperatures and densities between condensed matter and classical plasmas, inform models of diverse objects ranging from planetary cores to inertial confinement fusion plasmas. WDM is challenging to model theoretically: classical plasma models falter at high densities, and condensed-matter models such as time-dependent density functional theory (TDDFT) become exceedingly computationally expensive at temperatures beyond the Fermi temperature. The average atom (AA) method represents a cheaper DFT-based alternative whose accuracy is conversely expected to improve as temperature approaches the plasma regime. In this talk, we benchmark AA against TDDFT for the case of electronic stopping of protons in aluminum at solid density and temperatures ranging from 100 K to 10 eV/kB. We report good agreement for fast protons and analyze potential sources of discrepancies near and below the Bragg peak, including differing predictions for the density of states and ionization state of aluminum. Insights from this work will help improve the accuracy and speed of WDM simulations, with important implications for astrophysics and nuclear engineering. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M19.00006: Phase relations in iron monoxides from LDA + Usc calculations Yang Sun, Matteo Cococcioni, Renata M Wentzcovitch Understanding the pressure-dependent evolution of Earth forming phases containing iron is a challenging problem given the strongly correlated nature of iron oxides. Using the LDA+Usc method, we present calculation phase relations of iron monoxides involving five polytypes in multiple spin-state configurations. The Hubbard parameter U is determined self-consistently simultaneously with the occupation matrix and structures at arbitrary pressures. The Hubbard parameter strongly depends on pressure, structure, and spin state. Comparison with experimental structural data indicates the LDA+Usc can predict structure, compression curves, phase relations, and transition pressures very well for the insulating B1 and iB8 states. However, it requires additional calculations using the Mermin functional that includes the electronic entropic contribution to the free energy to obtain an nB8 metallic state and a consistent iB8 to nB8 insulator to metal transition pressure. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M19.00007: On High Pressure Ammonium Fluoride, Its Ice Analogues, Hydrates and Hydrides. Lewis Conway, Katherine L Brown, Andreas Hermann, John Loveday Ammonium fluoride, NH4F, is a hydrogen bond-ordered analogue of ice and forms structures equivalent to ice phases at low pressures. A mixed NH4F-H2O system forms a proton ordering network akin to a spin ice system with magnetic monopoles, in a H2O-rich system NH4F defects act as proton disordering agents [1]. The H2O ice phase diagram shows numerous phase transitions and bond symmetrisation under pressure. Only ice structures with bipartite networks may have a defect free NH4F analogue, and bond symmetrisation will not occur. We therefore explore NH4F up to 300 GPa by structure searching to compare how far the analogy holds, and report new close packed high-pressure phases of NH4F that become stable above 80 GPa. We explore NH4F-rich systems, where non-bipartite water networks may have NH4F analogues with a small number of H2O ‘defects’. This may lead to the formation of a wide range of 'quasi-ammonium fluoride' phases as well as clathrate hydrate analogues and we test this on the NH4F-H2O-H2 ternary phase diagram. Finally, we expand into the binary NH3-HF system (NH4F represents 1:1) as an analogue of the H-O phase diagram and discuss new stable higher fluorides of ammonium. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M19.00008: Spin state and structural stability of ferropericlase up to 3 Mbar Tianqi Wan, Yang Sun, Renata M Wentzcovitch Ferropericlase (Fp), (Mg1-xFex)O, the second most abundant mineral in the Earth mantle, is expected to be an essential component of the deep mantle of terrestrial exoplanets. Thus, an understanding of Fp across a wide range of pressures, temperatures, and iron concentrations is crucial for modeling these planets' internal structure and dynamics. Understanding pressure-induced electronic spin transitions in iron in Fp is challenging, given the strongly correlated nature of iron. Here, we present an LDA+Usc study of Fp's structure and spin state from 200 GPa to 3 TPa and iron concentrations, xFe, varying from ~3% to ~12%. The vibrational stability of various spin states is addressed, and the quasiharmonic approximation (QHA) is used to compute the thermodynamic properties and stability field of B1 and B2 phases. Such properties will be useful for modeling the mantle of super-Earth-type planets with up to ~15 Mearth. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M19.00009: Iron oxide motifs and structures from 0.1 to 3 TPa Feng Zheng, Yang Sun, Renhai Wang, Yimei Fang, Feng Zhang, Shunqing Wu, Cai-Zhuang Wang, Renata M Wentzcovitch, Kai-Ming Ho Iron oxides are fundamental components of planet-forming materials. Understanding the Fe-O system's behavior and properties under pressure can help us identify a plethora of new phases and states possible in exoplanetary interiors, especially terrestrial ones. In this study, we use the adaptive genetic algorithm (AGA) to investigate the structure of iron oxides throughout a wide range of stoichiometries (0.25 < xO < 0.8) and pressures (P = 0.1, 1.0, and 3.0 TPa). At 0.1 TPa, we successfully identify the experimentally known iron oxides phases. However, conventional DFT incorrectly describes the relative energy ordering; it requires self-consistent DFT+U calculations to correct these phases' energetics. At ultra-high pressures, we find some unreported low-enthalpy FexOy phases. Combining the previously predicted structures at 0.35 and 0.5 TPa [Weerasinghe et al., JPCM 27, 455501 (2015)], we show that Fe-O compounds form intricate structural motifs at lower pressures. However, at 1 and 3 TPa, most iron oxides adopt simple BCC or BCT motifs. This finding provides a glimpse of iron coordination and oxidation states in more complex phases with additional elements such as Mg, Si, H, etc. at exoplanetary interior pressures, of which we know very little. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M19.00010: Anharmonic thermodynamic properties of cubic CaSiO3 perovskite from phonon quasiparticles Zhen Zhang, Renata M Wentzcovitch Cubic CaSiO3 perovskite (cCaPv) is the third most abundant phase in the Earth’s lower mantle (7 vol%). The widely used quasiharmonic approximation combined with harmonic phonon spectra cannot be used to calculate the free energy of cCaPv. The latter is strongly anharmonic and presents imaginary frequencies in harmonic phonon calculation. Here we present an ab initio study of the thermodynamic properties of cCaPv over the pressure and temperature range of the lower mantle. We compute the anharmonic phonon dispersion throughout the Brillouin zone by using the phonon quasiparticle approach. This method characterizes the intrinsic temperature-dependence of phonon frequencies and, in principle, captures full anharmonicity. Such temperature-dependent phonon dispersion is used to calculate ab initio free energy in the thermodynamic limit (N → ∞) within the framework of the phonon gas model. Accurate free energy calculations enable us to investigate the thermodynamic properties of cCaPv, e.g., thermal expansivity, Grüneisen parameter, bulk modulus, and heat capacity, where anharmonic effects are demonstrated. The present methodology is important for exploring thermodynamic and thermoelastic properties and phase boundaries in strongly anharmonic systems at high pressures and temperatures. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M19.00011: Prediction of Fe-Mg and Fe-Mg-O compounds at exoplanetary interior pressures Yimei Fang, Yang Sun, Renhai Wang, Feng Zheng, Feng Zhang, Shunqing Wu, Cai-Zhuang Wang, Renata M Wentzcovitch, Kai-Ming Ho Terrestrial exoplanets are of great interest for being geochemically and geophysically similar to Earth. Their major elements abundances are expected to be similar to Earth’s but their internal pressure and temperature conditions can vary significantly in first order with their masses/sizes. Despite much recent progress in identifying the nature of magnesium silicates up to and beyond ~3 TPa, a most important element, iron, has not been systematically included in these studies so far. Here, we use the adaptive genetic algorithm (AGA) to predict several unreported stable crystalline phases in the binary Fe-Mg and ternary Fe-Mg-O systems up to pressures at 1 TPa and 3 TPa. Analyses of local packing motifs of the low-enthalpy Fe-Mg and Fe-Mg-O phases reveal that both Fe-Mg and Fe-Mg-O systems favor BCC-like motifs at ultra-high pressures regardless of chemical composition. Our results extend the current knowledge of structural information of the Fe-Mg and Fe-Mg-O systems to exoplanetary pressures. |
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