Session J39: Matter at Extreme Conditions: Theory and Simulations

Pressure-Constrained Deformation and Superior Strength: Compressed Graphite versus Diamond

The discoveries of compressed carbon phases and their ability to crack diamond anvil have generated great interest in the mechanical properties of carbon allotropes under high pressure. Significant progress has been made recently in structural identification of compressed graphite; however, its surprisingly high strength approaching or exceeding that of diamond remains unexplained. Here we explore this novel phenomenon and show by first-principles calculations that high-pressure confinement suppresses usual ambient or low-pressure deformation modes toward low-density carbon allotropes, and promotes alternative mechanisms for structural evolution leading to high-density compressed graphite phases that exhibit superior strength surpassing that of diamond. This finding explains the puzzling experimental observation and suggests new principles for structural deformation under high-pressure confinement. It also imposes stringent tests on widely used empirical hardness formulas that are unable to account for changes in pressure-constrained structural evolution and their influence on material strength.   

Atomistic Simulation of Orientation Dependence in Shock-induced Initiation of Pentaerythritol Tetranitrate (PETN)

Predicting the behavior of energetic materials requires a detailed description of how chemical reactions initiate during initial stages of detonation. In this talk, the dependence of the reaction initiation mechanism of pentaerythritol tetranitrate (PETN) on shock orientation and shock strength is investigated with molecular dynamics simulations using a reactive force field and the multi-scale shock technique. In the simulations, a single crystal of PETN is shocked along [110], [001], and [100] orientations with shock velocities in the range 3-10 km/s. Major reactions occur with shock velocities of 6 km/s or stronger, and reactions initiate through the dissociation of nitro (NO$_{2})$ and nitrate (NO$_{3})$ groups from the PETN molecules. The most sensitive orientation is [110], while [100] is the most insensitive. For the [001] orientation, PETN decomposition via nitro group dissociation is the dominant reaction initiation mechanism, while for [110] and [100] orientations the decomposition is via mixed nitro and nitrate group dissociation. For shock along the [001] orientation, we find that CO-NO$_{2}$ bonds initially acquire more kinetic energy, facilitating nitro dissociation. For the other two orientations, C-ONO$_{2}$ bonds acquire more kinetic energy, facilitating nitrate group dissociation.   

Microstructural Evolution and Grain Growth at High Speed Frictional Interfaces

We have examined the effect of evolution of grain morphology on the frictional force at polycrystalline Al-Al interfaces as a function of grain size and sliding velocity in the velocity range 40-250 m/s for grain sizes of 13.5 and 20 nm. Sample sizes for NonEquilibrium Molecular Dynamics (NEMD) simulations ranged from 10 - 140 M atoms. For velocities below a size dependent critical velocity above which a fluid layer forms, we find enhanced grain coarsening leading to a highly strained, graded final steady state microstructure that exhibits a dynamic morphhology characterized by grain growth and breakup at time scales greater than 5-10 ns. We find that the frictional force is insensitive to the initial grain size distribution that evolves to this new nonequilibrium steady state. We discuss mechanisms for grain size and shape evolution and the emergence of a dynamic length scale and compare these results to single crystal simulations in the same sliding regime.   

Shock response near the elastic to plastic transition in single crystal and porous silicon

We use molecular dynamics simulation methods to study the onset of the plastic wave transition in single crystal silicon, and characterize the altered response due to various degrees of porosity from 5 to 50 percent. Non-elastic response near onset of plasticity follows a mechanism similar to one shown previously in germanium, in which a propagating densification transition is driven by the release of shear stress in the material. This transition mechanism can be characterized as a partial transition from the ambient diamond structure to a distorted body center tetragonal ($\beta$-tin) structure. We show that this onset region is strongly influenced by porosity and large scale defects. Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Molecular dynamics study of the shock response in hydroxyl-terminated polybutadiene melts

All-atom molecular dynamics (MD) simulations using the non-reactive OPLS-AA force field were performed to study the detailed structural, mechanical, and spectroscopic response of hydroxyl-terminated polybutadiene (HTPB) melts subjected to supported shock waves. A combination of Monte Carlo and MD techniques was used to generate thoroughly equilibrated initial configurations, for monodisperse systems with chain lengths ranging from 64 to 256 backbone carbons per chain. Properties characterizing the size, shape and orientation of single chains, as well as the vibrational density of states, were evaluated prior to and following shock passage for four impact velocities between 1.0 and 2.5 km/s. The structural properties and global scaling behaviors of the unshocked systems are in excellent agreement with literature data. Results for the shocked systems, obtained using a geometric binning approach that provides spatio-temporal resolution in the reference frame centered on the shock front, indicate a transition to a glass-like state with a concomitant increase by several orders of magnitude of structural relaxation times in the shocked material.   

The volume isotope effect in ice under high pressure

The volume isotope effect (VIE) in ice has recently received considerable attention [1,2]. Ice Ih and XI, prototypical forms of low-pressure ice, have anomalous VIE, i.e., the volume of D$_{2}$O ($V_{D2O}$) is larger than that of H2O ($V_{H2O}$) [1]. In contrast, the VIE in ice VIII at 0 GPa was reported to be normal, i.e., $V_{D2O} < V_{H2O}$ [2]. Here we clarify the origin of this behavior in different forms of ice. Furthermore, we predict a reversal in the VIE in ice VIII under high pressure. [1] B. Pamuk et al., Phys. Rev. Lett. 108, 193003 (2012). [2] E. D. Murray and G. Galli, Phys. Rev. Lett. 108, 105502 (2012).   

New transitions of MgSiO3 post-perovskite under ultrahigh pressure

Understanding the behavior of MgSiO$_{3}$ post-perovskite (PPV) under extreme pressures is fundamental for modeling the interiors of super-Earth type exoplanets and the cores of solar giants. Previously, MgSiO$_{3}$ PPV was predicted to dissociate into MgO and MgSi$_{2}$O$_{5}$ and then into MgO and SiO$_{2}$ (Umemoto et al., Science 311, 983 (2006); Umemoto and Wentzcovitch, EPSL 311, 225 (2011)). Using the adaptive genetic algorithm, we predict new phase transitions in MgSiO$_{3}$. The phase diagram calculated using the quasi-harmonic approximation shows that some transitions can occur in some super-Earths type exoplanets.   

Elasticity of ferropericlase at lower mantle conditions

The discovery of spin-state changes (crossovers) in ferropericlase (Fp) and in silicate perovskite (Pv) under pressure has raised new questions about Earth's mantle properties. Despite extensive experimental work on the elasticity of Fp throughout the crossover, inconsistencies reported in the literature are still not explained. We introduce here a theoretical framework for thermoelasticity across spin-state changes, apply it to Fp by combining it with predictive first principles DFT$+$U calculations, and contrast results with available data on samples with various iron concentrations. We explain why the shear modulus of Fp should not soften throughout the spin crossover under hydrostatic conditions and show the importance of constraining well the elastic properties of minerals at lower mantle conditions and likely compositions without extrapolations.   

Dielectric properties of water and their impact on the Earth's deep carbon cycle

Knowledge of the dielectric constant of water as a function of pressure (P) and temperature (T) plays a critical role in understanding the chemistry of aqueous systems, and in particular of fluids in the Earth's mantle. By using \emph{ab initio} molecular dynamics, we computed the dielectric constant of water at T = 1000 and 2000 K, between 1 and 12 GPa, under conditions of the Earth's upper mantle. By comparing our results with available experimental data and empirical models, we discuss how the changes in the molecular dipole moments and hydrogen-bond network upon compression affect the dielectric constant of the liquid. Based on the calculated dielectric constants, the solubility products of carbonate minerals were predicted. At P $\sim$ 10 GPa and T = 1000 K, we found that MgCO$_3$ (magnesite) is slightly soluble in water at the millimolal level, which suggests that water in the Earth's mantle has the capacity to store and transport significant quantities of oxidized carbon.   

Structure change, layer sliding, and metallization in high-pressure MoS2

Based on ab initio calculations and metadynamics simulations, we predict that 2H-MoS$_2$, a layered insulator, will metallize under pressures in excess of 20-30 GPa. In the same pressure range, simulations and enthalpy optimization predict a structural transition. Reminiscent of this material's frictional properties, free mutual sliding of layers takes place at this transition, where the original 2H$_c$ stacking changes to a 2H$_a$ stacking typical of 2H-NbSe$_2$, a transformation which explains for the first time previously mysterious X-ray diffraction data. Phonon and electron phonon calculations suggest that metallic pristine MoS$_2$ will require ultrahigh pressures in order to develop superconductivity.   

Electronic structure and topological transition of SnTe at high pressure

Recent x-ray diffraction measurements and first-principles calculations have revealed intriguing structural evolution of tin telluride (SnTe) under high pressure. Here we report on a systematic study of the electronic band structure, density of states, Fermi surface and charge density of SnTe at high pressure using first-principles density functional theory calculations. Our results unveil an electronic topological transition in the cubic Fm-3m phase of SnTe with its Fermi surface changing from disconnected pockets to inter-connected quasicubic tubes near the L points of the Brillouin zone under high pressure. The pressure-induced quasicubic tubular Fermi surface is similar to that previously obtained via carrier doping. The induced change in electronic charge distribution stabilizes the Fm-3m structure and thus suppresses the transition to the rhombohedral structure, which explains experimental observations. Furthermore, our calculations show that pressure-induced electronic topological transition is also present in the orthorhombic Cmcm and Pnma phases of SnTe in the pressure range of 5 to 18 GPa, but this transition is absent in the high-pressure (above 18 GPa) Pm-3m phase.   

Achieving unusual oxidation state of matter under high pressure

Pressure has many effects to matter including the reduction of the volume, the increase of the coordination number and the broadening of the band-widths. In the past, most of the high-pressure studies focused on structural and electronic state phase transitions. Using first principles calculations and a bias-free structural search method, we will demonstrate that high pressure can lead to high oxidation state of elements that can never be achieved under ambient condition, making high pressure technique a nice tool to explore many traditional topics in solid state and molecular chemistry. As an example, we will show that Hg can transfer the electrons in its outmost d shell to F atoms and form HgF$_{4}$ molecular crystals under pressure, thereby acting as a true transition metal. Group IIB elements, including Zn, Cd, and Hg are usually defined as post-transition metals because they are commonly oxidized only to the $+$2 state. Their d shells are completely filled and do not participate in the formation of chemical bonds. Although the synthesis of HgF$_{4}$ molecules in gas phase was reported before, the molecules show strong instabilities and dissociate. Therefore, the transition metal propensity of Hg remains an open question.   

Dynamic structure of superionic protons in hydrogen fluoride crystal

Hydrogen fluoride crystal forms zig-zag chains of hydrogen fluoride molecules forming covalent bond between them. Goldman et al.(J. Chem. Phys.125,044501(2006).) have found the superionic state of the protons in the hydrogen fluoride crystal at 900 K and beyond the pressures at 33~GPa. The present study elucidates the dynamic structure of the protons in the superionic state of the crystal at the extreme conditions with the first principles molecular dynamics method. The strong covalent bond between the proton and the fluorine in the conductor has shown a different dynamic structure from that in the $\alpha $-CuI; The protons in the conductor are bonded with the nearest fluorine and the other protons are located at incommensurate sites of the bcc fluorine lattice. This is a different dynamic structure from the formation of the incommensurate dynamic copper dimers in the $\alpha $-CuI.(Tsumuraya $et \ al$. J. Phys. Soc. Jpn. 81,055603(2012).)