The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity to mesoscale structure, and has been a topic of extensive study for over 50 years.

We use molecular dynamics simulations to investigate the role of cation disorder on oxygen diffusion in Gd 2 Zr 2 O 7 (GZO) and Gd 2 Ti 2 O 7 (GTO) pyrochlores, a class of complex oxides which contain a structural vacancy relative to the basic fluorite structure. The introduction of disorder has distinct effects depending on the chemistry of the material, increasing the mobility of structural carriers by up to four orders of magnitude in GZO. In contrast, in GTO, there is no mobility at zero or low disorder on the ns timescale, but higher disorder liberates the otherwise immobile carriers, allowing diffusion with rates comparable to GZO for the fully disordered material. We show that the cation disorder enhances the diffusivity by both increasing the concentration of mobile structural carriers and their individual mobility.The disorder also influences the diffusion in materials containing intrinsic carriers, such as additional vacancies V O or oxygen interstitials O I . While in ordered GZO and GTO the contribution of the intrinsic carriers dominates the overall diffusion of oxygen, O I in GZO contributes along with structural carriers, and the total diffusion rate can be calculated by assuming simple additive contributions from the two sources. Although the disorder in the materials with intrinsic defects usually enhances the diffusivity as in the defect-free case, in low concentrations, cation antisites A B or B A , where A ¼ Gd and B ¼ Zr or Ti, can act as traps for fast intrinsic defects. The trapping results in a lowering of the diffusivity, and causes a non-monotonic behavior of the diffusivity with disorder. By contrast, in the case of slow intrinsic defects, the main effect of the disorder is to liberate the structural carriers, resulting in an increase of the diffusivity regardless of the defect trapping.

A new parameterization for density functional tight binding (DFTB) theory, lanl31, has been developed for molecules containing carbon, hydrogen, nitrogen, and oxygen. Optimal values for the Hubbard Us, on-site energies, and the radial dependences of the bond integrals and repulsive potentials were determined by numerical optimization using simulated annealing to a modest database of ab initio-calculated atomization energies and interatomic forces. The transferability of the optimized DFTB parameterization has been assessed using the CHNO subset of the QM-9 database [R. Ramakrishnan et al., Sci. Data 1, 140022 (2014)]. These analyses showed that the errors in the atomization energies and interatomic forces predicted by our model are small and in the vicinity of the differences between density functional theory calculations with different basis sets and exchange-correlation functionals. Good correlations between the molecular dipole moments and HOMO-LUMO gaps predicted by lanl31 and the QM-9 data set are also found. Furthermore, the errors in the atomization energies and forces derived from lanl31 are significantly smaller than those obtained from the ReaxFF-lg reactive force field for organic materials [L. Liu et al., J. Phys. Chem. A 115, 11016 (2011)]. The lanl31 DFTB parameterization for C, H, N, and O has been applied to the molecular dynamics simulation of the principal Hugoniot of liquid nitromethane, liquid benzene, liquid nitrogen, pentaerythritol tetranitrate, trinitrotoluene, and cyclotetramethylene tetranitramine. The computed and measured Hugoniot loci are in excellent agreement with experiment, and we discuss the sensitivity of the loci to the underestimated shock heating that is a characteristic of classical molecular dynamics simulations.

We use density functional tight binding (DFTB) molecular dynamics (MD) simulations to determine the reaction rates of nitromethane CH 3 NO 2 (NM) under high pressure (P = 14−28 GPa), and temperature (T = 1450−1850 K). DFTB-MD simulations performed with the same initial conditions (P 0 , T 0 ) reveal a stochastic behavior, both in terms of reaction times and chemical paths. By running series of MD simulations, we are able to obtain average reaction times with quantified errors and devise a simple two-step model for NM explosion: ignition/explosion. While our model bypasses the chemical complexity due to the numerous reaction paths and intermediates observed during reactions, the chemistry is accounted for via the accurate parameterization of the DFTB model, and our results suggest a single main reaction pathway for the pressure range considered here, dominated in the earlier stages by the formation of the aci-ion, CH 2 NOO − . By fitting our data to a Frank−Kamenetskii model, we extract prefactors and pressure-independent activation energies and volumes for the ignition and explosion stages. A two-step model is then built and compared to experimental observations. Single and two-step Arrhenius models are also provided for comparison with literature data. This work presents an efficient way of investigating the reactivity of high explosives by performing electronic structure-based MD simulations and provides reaction rates for simplified models that can be implemented into hydrocodes.

The multifunctional properties of complex ternary oxides such as pyrochlores are often influenced by surface structure. Optimizing the surface-driven attributes of these materials necessitates a detailed understanding of the structure and chemical composition of those surfaces. Here we report atomistic simulations elucidating the diverse atomic-scale structures of a set of low-index surfaces [(100), ( 110), (111), and ( 112)] in pyrochlore compounds as a function of both A and B cation chemistry. In pyrochlores, the low-index facets are all dipolar, requiring the introduction of surface defects to eliminate the surface dipole. We find that, due to the corresponding higher coordination of the surface cations, the (110) facet is the most energetically stable in all of the compounds considered, an interesting contrast to fluorite, in which the (111) surface is most stable. We also reveal a correlation between the surface energy and the energy to disorder the pyrochlore as a function of B cation chemistry, implying a similar physical origin for the two phenomena. Further, we find that surface rumpling is common across all pyrochlore compounds. An even more interesting feature emerging at these surfaces is the formation of extended structural defects such as steps and trenches, which are found to be stable after high-temperature annealing. As the formation of these features is a consequence of surface defects introduced to eliminate the surface dipole, we propose that the superior surface properties of materials of pyrochlores are due to these extended structural features, which are a direct consequence of the inherent dipole at the surfaces.

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We present a study of the diffusion of krypton in UO using atomic scale calculations combined with diffusion models adapted to the system studied. The migration barriers of the elementary mechanisms for interstitial or vacancy assisted migration are calculated in the DFT+U framework using the nudged elastic band method. The attempt frequencies are obtained from the phonon modes of the defect at the initial and saddle points using empirical potential methods. The diffusion coefficients of Kr in UO are then calculated by combining this data with diffusion models accounting for the concentration of vacancies and the interaction of vacancies with Kr atoms. We determined the preferred mechanism for Kr migration and the corresponding diffusion coefficient as a function of the oxygen chemical potential μ or nonstoichiometry. For very hypostoichiometric (or U-rich) conditions, the most favorable mechanism is interstitial migration. For hypostoichiometric UO, migration is assisted by the bound Schottky defect and the charged uranium vacancy, V. Around stoichiometry, migration assisted by the charged uranium-oxygen divacancy (V) and V is the favored mechanism. Finally, for hyperstoichiometric or O-rich conditions, the migration assisted by two V dominates. Kr migration is enhanced at higher μ, and in this regime, the activation energy will be between 4.09 and 0.73 eV depending on nonstoichiometry. The experimental values available are in the latter interval. Since it is very probable that these values were obtained for at least slightly hyperstoichiometric samples, our activation energies are consistent with the experimental data, even if further experiments with precisely controlled stoichiometry are needed to confirm these results. The mechanisms and trends with nonstoichiometry established for Kr are similar to those found in previous studies of Xe.