Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P01: Materials at Extreme Conditions: Theory and Simulations |
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Sponsoring Units: GSCCM DMP DCOMP Chair: Nir Goldman, Lawrence Livermore National Laboratory Room: LACC 150A |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P01.00001: Predicting the melt lines of compressed materials using computations Josiah Bjorgaard , Daniel Sheppard Computing the melt line of materials under compression is important, since experimental values are difficult to obtain. A number of different methods have been developed with this aim, including the coexistence and Z-methods, but suffer from serious drawbacks including high computational cost and low accuracy. Here, we present our current efforts to design an improved melt line prediction using the diffusion kinetics determined from simulations and experimental literature, with the overall goal of producing improved accuracy at a fraction of the computational cost. We compare our results with the well known Z-method and experimental melt lines. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P01.00002: First-principles study of the melting curve of iron Felipe González-Cataldo , Burkhard Militzer The phase diagram of iron at high pressure is of considerable interest for geophysics and planetary science. The interior structure of rocky planets, such as the size and state of the core and related core-formation processes, depend on the detailed behavior of planetary materials, such as iron, at high pressure and temperature. Using density functional theory molecular dynamics, we investigate the melting curve of iron at megabar pressures. We derived the melting temperature by equating the Gibbs free energies of solid and liquid phases that we derived through the thermodynamic integration method. The melting curve allows us to study the crystallization behavior of iron under high compression. Our calculations show that the melting temperatures of iron are higher than extrapolations from previously calculated melting curves at lower pressures. Finally we compare its slope with the adiabatic gradient in order to determine how the cores of super-Earth planets crystallize. |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P01.00003: Rotational and Translational Disorder in High-Pressure Water Ice Jean-Alexis Hernandez , Razvan Caracas Above 2 GPa water ice presents a single sub-lattice of oxygen atoms and all the different phases differ by the behavior of the hydrogen atoms. Behind this apparent simplicity, the sub-lattice of hydrogen atoms undergoes several structural and dynamical transitions: while increasing the pressure leads to the dissociation of the water molecules (VII – X transition), increasing the temperature activates the rotations of the water molecules, which is predicted to form a plastic phase (dynamical orientational disorder of H_{2}O molecules) and superionic phases (fast diffusion of H atoms in the oxygen sub-lattice). The relations between the superionic domain, the hypothetical plastic phase, the H-bond symmetrization and the melting of the bcc sub-lattice remain poorly constrained. Here, we provide first-principles analysis of the H-bonding regimes encountered in whole stability domain of bcc ices. We report the first ab initio evidence for a plastic phase of water and we propose a coherent phase diagram for bcc water ices compatible with the two groups of melting curves and with the multiple anomalies reported in ice VII around 10 – 20 GPa. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P01.00004: Computationally tractable approaches to ultra-high temperature anharmonicity Thomas Mellan , Andrew Duff , Blazej Grabowski , Jörg Neugebauer , Michael Finnis Advances in high-performance computing and first principles methodologies herald greater opportunities than ever to make high-temperature predictions at the ab initio level of theory. This has led to recent predictions up to the melting point, for metals such as aluminium,[1] and for ceramics such as ZrC.[2] Despite major advances in beyond-quasiharmonic approaches, for real materials, full anharmonic treatments are still prohibitively expensive computationally. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P01.00005: Examination of the High Pressure Phase Diagram of Solid Aluminum Luke Shulenburger , Joshua Townsend , Raymond Clay Recent investigations have provided a detailed but somewhat contradictory picture of the equilibrium phase diagram of solid aluminum at high pressures. Laser based ramp compression coupled with x-ray diffraction is in qualitative agreement with density functional theory calculations, but the transition pressures between FCC, HCP and BCC phases are higher than expected. In this work, we present a new theoretical examination of the solid phases of aluminum under pressure where the density response is treated by quantum Monte Carlo and the lattice vibrational degrees of freedom are investigated for anharmonic character. In this way we aim to shed light on whether the disagreement stems from the equation of state or possible kinetic effects. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P01.00006: Pre-melting hcp to bcc Transition in Beryllium Dong-Bo Zhang , Peihong Zhang , Renata Wentzcovitch Beryllium (Be) is an important material with wide applications ranging from aerospace components to x-ray ray equipment. Yet a precise understanding of its phase diagram remains elusive. We have investigated the phase stability of Be using a recently developed hybrid free-energy computation method that accounts for anharmonic effects by invoking phonon quasiparticles. We find that the hcp → bcc transition occurs near the melting curve at 0 < P < 11 GPa with a positive Clapeyron slope of 41(4) K/GPa, a result that is more consistent with recent experimental measurements. This work also demonstrates the validity of this theoretical framework based on the phonon gas model and phonon quasiparticle to study phase transitions in strongly anharmonic materials. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P01.00007: Thermal conductivity of MgO:Fe at high pressure and the spin transition via ab initio calculations. Aleksandr Chernatynskiy In this work we elucidate the effect of the spin transition on the thermal transport in ferropericlase (Mg_{x}Fe_{1-x}O) from ab-initio calculations. Ferropericlase is expected to be one the components of the Earth’s mantle and at the high-pressure conditions of the lower mantle, iron atoms substituted into the MgO rocksalt lattice experience transition from a high spin state to a low spin state. Recent experimental study has demonstrated a substantial decrease of the thermal conductivity in MgO:Fe at pressures that correspond to the onset of the spin transition, and the latter has been implicated as the cause of the former. Here, we consider possible mechanisms that affect the thermal conductivity as a function of pressure across the spin transition. Those include phonon scattering on the electronic states of iron atoms as well as volume collapse which accompanies spin transition. We report our results for the lattice thermal conductivity in ferropericlase from the accurate solution of the Boltzmann Transport Equation for phonons using DFT calculations as an input. We account for impurity scattering via Virtual Crystal Approximations and take into account the effect of the electron-phonon interactions across the spin transition. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P01.00008: Finding the Right Density from the Wrong Temperature with Thermal Density Functional Theory Justin Smith , Kieron Burke Using the Mermin theorem, we define a correction to the one-body potential that ensures that a calculation performed at some reference temperature generates the exact density at some other desired temperature. We list various properties of this potential and derive a simple approximation for the free energy at the desired temperature. We calculate this effective thermal potential exactly for the two-site Hubbard model, and find that our free-energy approximation is highly accurate under many conditions. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P01.00009: Topology in the Quantum Limit Ross McDonald Extreme magnetic fields play a fundamental role in both identifying and inducing novel states of matter. I will discuss the experimental challenges and rewards of subjecting low carrier density metals to high fields. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P01.00010: Strain Functionals: A Symmetry-adapted Set of Descriptors for Characterizing Atomic Geometries and Potential Energy Functions Nithin Mathew , Sven Rudin , Edward Kober We demonstrate the use of a complete and symmetry-adapted approach to characterize atomic geometries. A Gaussian kernel is used to map discrete atomic quantities to continuous fields and the local geometry is characterized using n^{th} order central-moments of the atomic neighborhood. The Gaussian kernel guarantees that the mapping is continuous and smooth. Rotationally invariant metrics, referred to as strain functional descriptors, are derived from these n^{th} order central-moments using Clebsch-Gordan coupling. Descriptors derived from a 6^{th} order moment expansion can distinguish between different crystal structures and identify defects such as dislocations, stacking faults, and twins at finite temperature. Combined with dimensionality reduction techniques and clustering algorithms, these descriptors are used to analyze molecular dynamics simulations of high strain-rate compression of Cu, Ta, and Ti. We will also demonstrate that the strain functional descriptors form a complete, orthogonal, and minimal basis for characterizing inter-atomic potentials. Formulation of inter-atomic potentials for Cu and Ta in the strain functional basis will be presented. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P01.00011: Simulation Effect of Twinning Evolution on Metal Strength under Dynamic Loading Hao Pan , Xiomian Hu The dynamic response materials under shock wave loading and complex deformation conditions are significantly different from those of low pressure and low strain rate loading. The experimental results show that the plastic deformation mechanism has changed. For lower fault energy materials (such as Ta, Be, etc.), twinning are another important mechanism of plastic deformation. In this paper, the twinning process is modeled and added into the thermo-viscoplastic crystal plasticity. The evolution of twin in Ta and Be materials during impact loading and unloading is studied by numerical simulation. The change in the strength of the material with the twins was analyzed. The results show that the effect of twinning on the mechanical strength of Ta and Be is not the same under medium and high strain rate loading. Twins are more sensitive to the strength and texture evolution of Be. Twin is an important physical mechanism of plastic deformation to Be under the shock wave loading. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P01.00012: The stability and the structures of Fe-I and Fe-Br compounds under Earth core condition Xiaoli Wang , Jianfu Li , Frank Spera , Matthew Jackson , Maosheng Miao The reactivity of Iron with non-metals is important for the steel industry as well as for understanding the Earth core compositions and properties. For latter case, it is essential to understand the change of chemical propensity of Fe under high pressure. Using first principles electronic structure calculations and the automatic crystal structure search method based on particle swarm optimization algorithm, we studied the stability and structures of Fe-I and Fe-Br compounds under high pressure up to 350 GPa. Our calculations show that the compounds with higher Fe composition become more stable with increasing pressure. Similar stability trend is also shown for Fe-Br compounds. We also found that Fe forms more stable compounds with I than Br, which is opposite to the chemical propensity at ambient pressure. These results suggest that the I/Br distribution ratio the Earth core might be much higher than we usually believe. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P01.00013: Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides Yunwei Zhang , Weikang Wu , Yanchao Wang , Shengyuan Yang , Yanming Ma High pressure is able to modify profoundly the chemical bonding and generate new phase structures of materials with properties not accessible at ambient conditions. We here report an unprecedented phenomenon on the pressure-induced formation of semi-conducting electrides via compression of layered alkaline-earth-metal subnitrides that are conducting electrides with loosely confined electrons in the interlayer voids at ambient pressure. Our structure searches identified the high-pressure semiconducting electride phases of a tetragonal I-42d structure for Ca_{2}N and a monoclinic Cc structure shared by Sr_{2}N and Ba_{2}N, both of which contain atomic-size cavities with paring electrons distributed within. These electride structures are validated by the excellent agreement between the simulated X-ray diffraction patterns and the experimental data available. We attribute the emergence of the semiconducting electride phases to the p−d hybridization on alkaline-earth-metal atoms under compression and the filling of the p−d hybridized band due to the interaction between Ca and N. Our work provides a unique example of pressure-induced metal-to-semiconductor transition in compound materials and reveals unambiguously the electron-confinement topology change between different types of electrides. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P01.00014: Chemistry without Chemical Bonds: Reactivity of He with Ionic Compounds under High Pressure Zhen Liu , Jorge Botana , Andreas Hermann , Steven Valdez , Eva Zurek , Dadong Yan , Hai-Qing Lin , Maosheng Miao Since its discovery 150 years ago and until very recently, Helium has remained the last naturally occurring element that does not form a stable solid compound. We propose and demonstrate, using structure prediction coupled with first principles calculations, that there is a general driving force for Helium to react with ionic compounds that contain an unequal number of cations and anions. The corresponding reaction products contain cations, anions, and helium, and are stabilized not by any local chemical bonds but by the long-range Coulomb interactions that are significantly modified by the insertion of helium atoms, especially under high pressure. We study the stability of the mixtures of He and MgF2, MgO, Li2O, LiF, CaF2, Na with density functional theory. As is presumed, MgF2, Li2O, CaF2 and Na, which have different numbers of anion and cation, can form stable structures with He while their counterparts, MgO and LiF, show a positive formation enthalpy when mixed with Helium. We calculate the internal and Madelung energy difference of formation of the compounds and they show similar behavior at high pressure. It is concluded that the Madelung energy decrease when He mixes with A2B or AB2 compounds resulting in the stability of AB2He or A2BHe compounds at high pressure. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P01.00015: A first-principles study of the anharmonic lattice dynamics of sodium under pressures Shasha Li , Yue Chen The anharmonic lattice dynamics of sodium under pressures are studied from first-principles calculations to elucidate its intriguing thermal expansion and melting behaviors. It is predicted based on quasi-harmonic approximation and phonon calculations that the face centered cubic and cI16 (a distorted body centered cubic structure) phases of sodium exhibit negative thermal expansions above 90 GPa. By calculating the Grüneisen dispersions of the different phases of sodium, it is found that the large negative Grüneisen parameters of the transverse acoustic modes are mainly responsible for the predicted negative thermal expansion. Moreover, the phonon spectra of sodium at elevated temperatures are calculated using the anharmonic third-order force constants and the perturbation theory. Phonon-phonon interactions related to the low frequency vibrational modes are found to be greatly affected by external hydrostatic pressures. Based on this theoretical study, we achieve deeper insight into the intriguing melting behavior of sodium under high pressures. |
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