Session P13: Focus Session: Extreme Conditions and High Pressure I: Chemistry

Shock-induced Reactions in Pentaerythritol Tetranitrate Studied by Molecular Dynamics Simulation

Molecular dynamics simulations were performed using the reactive force field, ReaxFF, as implemented in the General Reactive Atomistic Simulation Program code for systems consisting of a single crystal of PETN with not fewer than 237000 atoms. The crystals were shocked along the [100] direction using two different piston velocities. The resulting chemical reactions were tracked in an attempt to elucidate short-time initiation mechanisms. Here, we present the primary, secondary, and intermediate products as a function of time and position behind the shock front.   

Molecular dynamics simulations of uniaxial shock compression of RDX crystals

Using the Hugoniostat methodology atomistic molecular dynamics simulations of uniaxial shock compressions along [001], [100], and [010] directions of RDX crystal have been conducted over a wide range of shock pressures. The Hugoniostat simulations allow modeling of shocked material without the necessity to have extremely large simulation cell required to explicitly resolve the shock wave propagation. Hugoniostat simulations on systems containing only few thousand molecules allowed us to determine Hugoniot elastic limit and to investigate shock-induced shear banding and phase transition in RDX crystal.   

A molecular dynamics study of the role of pressure on the response of reactive materials to thermal initiation

Reactive materials have the potential for implementation into a wide variety of commercial and military applications. However, the fundamental physical and chemical processes that control the energy release are not well understood. To elucidate the mechanisms of energy release, we simulated the exothermic alloying reactions of a Ni/Al bilayer with initial temperatures of 1100 K and 1500 K using both microcanonical (NVE) and isoenthalpic (NPH) molecular dynamics simulations with an embedded atom method (EAM) potential. The mechanism of the mixing is the same for all simulations: as mixing and reaction occurs at the interface, the heat generated first melts the Al layer, and subsequent mixing leads to further heat generation after which the Ni layer melts, leading to additional mixing until the alloying reactions are completed. The results indicate that pressure has a significant influence on the rates of atomic mixing and alloying reactions. In addition, two-phase coexistence simulations were used to determine the melting temperatures of pure Al and pure Ni at various pressures using this potential, and these values are discussed within the context of the Ni/Al bilayer results.   

Mbar Chemistry: Novel States of Matter in Extreme Conditions

Compression energy at 100 GPa often exceed several eV/atom, rivaling the energy of strong chemical bonds. Therefore, the application of such a high pressure significantly alters the chemical, electronic/optical, thermomechanical properties of solids and, in turn, provides a way to test condensed matter theory and to exploit novel materials with advanced properties. Furthermore, reeent advances in diamond-anvil cell high-pressure technologies coupled with advanced third-generation synchrotron x-ray offer unprecedented opportunities to discover exotic states of matter at high pressure-temperature conditions of the Earth and planetary interiors. In this paper, I will discuss several recent results of high-pressure chemistries that occur in simple low Z molecular solids to novel novel nonmolecular extended solids. Broadly speaking, these molecular-to-nonmolecular transitions occur as a result of the pressure-induced electron delocalization arising from a rapid increase in electron kinetic energy at high density. Yet, the details are substantially more complicated because of the phase metastability, large lattice strain, and governing kinetics. As a result, there are many outstanding questions regarding the exact nature of chemical bonding, phase stability, and transition mechanisms. Also, presented are several future directions of high pressure materials research in an complementary phase and time scales of dynamic and static high pressures.   

Novel Catalytic Behavior of Dense Hot Water in PETN Decomposition Reactions

Under extreme conditions, water is known to exhibit fascinating physical behaviors. Its remarkable structural and phase complexity strongly suggests that its chemical properties may be unusual as well, which have remained largely unrevealed. Using \textit{ab inito} molecular dynamics simulations, we have recently discovered that water plays an unexpected role in catalyzing complex reactions of a high explosive pentaerythritol tetranitrate (PETN). This finding is in contrary to the current view of water as a stable final product of high explosive reactions, and has possible implications in geochemistry, such as reactions in planetary interiors.   

The formation of carbon nitride clusters in shocked insensitive explosives

Many high explosives are organic molecular crystals that contain both oxidizing and reducing functional groups. These solids rapidly release their energy in supersonic detonation waves. It has been observed that explosives rich in carbon tend to have much longer reaction zones than those that do not. These explosives form graphitic or diamond-like carbon particles during detonation. The slow diffusion-limited process of forming the bulk solid from carbon clusters is believed to play a central role in determining the reaction zone length of a given explosive. In this work, we identify an altogether new mechanism for the slow reactivity of carbon rich explosives. Quantum–based multi-scale simulations of shocked 1,3,5-triamino- 2,4,6-trinitrobenzene (TATB) provide the first evidence for the formation of nitrogen-rich heterocyclic clusters that impede the formation of fluid nitrogen and solid carbon.   

Ab initio molecular dynamics of hypervelocity chemistry

Resolving chemical dynamics of decomposition of energetic molecules is crucial for understanding detonation initiation in energetic materials and predicting their sensitivity to shock and impact stimuli. We employ Born-Oppenheimer molecular dynamics driven by density-functional methods to identify possible decomposition pathways in nitric esters (including pentaerythritol tetranitrate) and to understand the effect of collision orientation and velocity. Studies of the potential energy surface in the bond-breaking region, unimolecular decomposition, and binary hypervelocity collisions of model nitric esters (methyl and ethyl nitrates) will be reported. Methodological challenges in describing extensive changes in the electronic structure that accompany decomposition will be discussed.   

Boron carbides from first principles

In this work, we focus on the understanding gained from the investigation of the physical properties of boron-carbides with theoretical methods based on density functional theory (DFT). Comparison of computed and experimental vibrational or NMR spectra has shown that the atomic structure of B$_{4}$C consists in C-B-C chains linking mostly B$_{11}$C icosahedra, and a few percent of B$_{10}$C$_{2}$ icosahedra. In particular, C-C-C chains are excluded and can not be responsible for B$_{4}$C amorphization under shockwaves. In this work we find that at lower carbon concentration all models are metastable with respect to B$_{4}$C plus $\alpha $-boron. This could explain actual difficulties in the synthesis of clean samples. Furthermore we discuss effects of temperature and/or pressure on stabilities and properties. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B$_{13}$C$_{2}$.   

Dynamical stability of the cubic metallic phase of AlH3 at ambient pressure

We have characterized the high-pressure cubic phase of AlH$_{3}$ using density functional theory to determine mechanical as well as electronic properties and lattice dynamics from the response function method [1]. Metallization in AlH$_{3}$ under pressure has been studied, which is of great interest not only from a fundamental physics point of view for the study of phenomena related to metallic hydrogen, but also, because metallic AlH$_{3}$ possesses weaker Al-H bonds than other insulating phases [2]. Our phonon calculations show the softening of a particular mode with decreasing pressure, indicating the onset of a dynamical instability that continues to persist at ambient conditions. We find from analyzing the atomic and electronic interactions using theoretical calculations that finite-temperature effects lead to the desired stabilization of metallic AlH$_{3}$ at ambient conditions.\\[0pt] [1] PRB \textbf{78}, 100102(R) (2008). \\[0pt] [2] APL \textbf{92}, 201903 (2008).   

Tight binding multi-scale simulations of detonating energetic materials

We present density-functional tight-binding (DFTB) molecular dynamics simulations of shock and detonation waves propagating through a series of explosives ranging from insensitive TATB to sensitive hydrogen azide and identify key differences in behavior. The simulations are performed using the Multi-Scale Shock Method (MSST) which we have extended to maintain thermodynamic equilibrium between electrons and ions to correctly treat electronic heat capacity.   

\textit{Ab Initio} Simulation of the Equation of State and Kinetics of Shocked Water

We report herein first principles simulations of water under shock loading and the chemical reactivity under these hot, compressed conditions. Using a novel simulation technique for shock compression, we observe that water achieves chemical equilibrium in less than 2 ps for all shock conditions studied. The decomposition occurs through the reversible reaction H$_{2}$O $\Delta $ H$^{+}$ + OH$^{-}$. We make comparison to the experimental results for the Hugoniot pressure and density final states. We develop and employ a new quantum correction method to the calculated temperatures which provides validation of both previous experiments and our simulations. Near the approximate intersection of the Hugoniot and the Neptune isentrope, we observe high concentrations of negatively charged species that contribute electronic states near the band gap. *This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Dynamical (in)stabilities of high-pressure H2O ices VII, VIII and X

We study high-pressure solid H2O ice: the lattice dynamical properties of ice X and the transition path between molecular ices VII/VIII and the ionic ice X with first-principles calculations using density functional theory in the ABINIT implementation. Our work [PRL 101, 085502] defines the dynamical stability of ice X between about 120 GPa up to about 400 GPa. Based on phonon band dispersion we show that the phase transition sequence at low temperature and high pressures in ice is ice VIII - disordered ice X - ice X - ice Pbcm. The disordered ice X is due to a phonon collapse in the whole Brillouin zone at pressures below 120 GPa, phonon that corresponds to hydrogen atoms bouncing back and forth between every two oxygen neighbors in a double well potential. Post-ice X is orthorhombic Pbcm and appears due to a phonon instability in M at pressures higher than 400 GPa that distorts the bcc cubic sublattice of oxygen atoms into a hcp-like structure. Our calculations validate earlier theoretical predictions for a phase transition to a post-ice X structure in H2O [Benoit et al. PRL 76, 2934]. We also identify and discuss the (meta)stability of several intermediate phases between ice VIII and ice X.