Novel Rubidium Poly-Nitrogen Materials at High Pressure

First-principles crystal structure search is performed to predict novel rubidium poly-nitrogen materials at high pressure by varying the stoichiometry, i. e. relative quantities of the constituent rubidium and nitrogen atoms. Three compounds of high nitrogen content, RbN_{5}, RbN_{2}, and Rb_{4}N_{6}, are discovered. Rubidium pentazolate (RbN5) becomes thermodynamically stable at pressures above \unit[30]{GPa}. The charge transfer from Rb to N atoms enables aromaticity in cyclo-N_{^{_{5}}}^{-} while increasing the ionic bonding in the crystal. Rubidium pentazolate can be synthesized by compressing rubidium azide (RbN3) and nitrogen (N2) precursors above \unit[9.42]{GPa}, and its experimental discovery is aided by calculating the Raman spectrum and identifying the features attributed to N_{^{_{5}}}^{-} modes. The two other interesting compounds, RbN2 containing infinitely-long single-bonded nitrogen chains, and Rb_{4}N_{6} consisting of single-bonded N_{6} hexazine rings, become thermodynamically stable at pressures exceeding \unit[60]{GPa}. In addition to the compounds with high nitrogen content, Rb_{3}N_{3}, a new compound with 1:1 RbN stoichiometry containing bent N_{3} azides is found to exist at high pressures

Anisotropic constitutive relationships in energetic materials: PETN and HMX

This paper presents results of first-principles density functional calculations of the equation of state (EOS) of PETN-I and beta-HMX. The isotropic EOS for hydrostatic compression has been extended to include uniaxial compressions in the [100], [010], [001], [110], [101], [011], and [111] directions up to compression ratio V/V0 = 0.70. Equilibrium properties, including lattice parameters and elastic constants, as well as hydrostatic EOS are in good agreement with available experimental data. The shear stresses of uniaxially compressed PETN-I and beta-HMX have been evaluated and their behavior as a function of compression ratio has been used to make predictions of shock sensitivity of these EMs. A comparison of predicted sensitivities with available experimental data has also been performed

First-principles anisotropic constitutive relationships in β-cyclotetramethylene tetranitramine (β-HMX)

First-principles density functional theory calculations have been performed to obtain constitutive relationships in the crystalline energetic material β-cyclotetramethylene tetranitramine (β-HMX). In addition to hydrostatic loading, uniaxial compressions in the directions normal to the {100}, {010}, {001}, {110}, {101}, {011}, and {111} planes have been performed to investigate the anisotropic equation of state (EOS). The calculated lattice parameters and hydrostatic EOS are in reasonable agreement with the available experimental data. The uniaxial compression data show a significant anisotropy in the principal stresses, change in energy, band gap, and shear stresses, which might lead to the anisotropy of the elastic-plastic shock transition and shock sensitivity of β-HMX

Nanoscale Molecular Dynamics Simulaton of Shock Compression of Silicon

We report results of molecular dynamics simulation of shock wave propagation in silicon in [100], [110], and [111] directions obtained using a classical environment-dependent interatomic potential (EDIP). Several regimes of materials response are classified as a function of shock wave intensity using the calculated shock Hugoniot. Shock wave structure in [100] and [111] directions exhibit usual evolution as a function of piston velocity. At piston velocities km/s the shock wave consists of a fast elastic precursor followed by a slower plastic front. At larger piston velocities the single overdriven plastic wave propagates through the crystal causing amorphitization of Si. However, the [110] shock wave exhibits an anomalous materials response at intermediate piston velocities around km/s which is characterized by the absence of plastic deformations

Density functional theory calculations of anisotropic constitutive relationships in alpha-cyclotrimethylenetrinitramine

Constitutive relationships in the crystalline energetic material alpha-cyclotrimethylenetrinitramine (alpha-RDX) have been investigated using first-principles density functional theory. The equilibrium properties of alpha-RDX including unit cell parameters and bulk modulus, as well as the hydrostatic equation of state (EOS), have been obtained and compared with available experimental data. The isotropic EOS has been extended to include the anisotropic response of alpha-RDX by performing uniaxial compressions normal to several low-index planes, {100}, {010}, {001}, {110}, {101}, {011}, and {111}, in the Pbca space group. The uniaxial-compression data exhibit a considerable anisotropy in the principal stresses, changes in energy, band gaps, and shear stresses, which might play a role in the anisotropic behavior of alpha-RDX under shock loading

First-principles investigation of anisotropic constitutive relationships in pentaerythritol tetranitrate

First-principles density functional theory (DFT) calculations have been used to obtain the constitutive relationships of pentaerythritol tetranitrate (PETN-I), a crystalline energetic material. The isotropic equation of state (EOS) for hydrostatic compression has been extended to include uniaxial compressions in the , , , , , , and crystallographic directions up to a compression ratio of V/V0=0.70. DFT predicts equilibrium properties such as lattice parameters and elastic constants, as well as the hydrostatic EOS, in agreement with available experimental data. Our results show a substantial anisotropy of various properties of PETN-I upon uniaxial compression. To characterize the anisotropic traits of PETN, different physical properties of the uniaxially compressed crystal such as the energy per atom, band gap, and stress tensor have been evaluated as a function of compression ratio. The maximum shear stresses were calculated and examined for a correlation with the anisotropy in shock-initiation sensitivity

Molecular dynamics simulations of an anomalous response of diamond to shock compression

We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear-deformation and overdriven rehybridization shock wave regimes

Energetic Materials at High Compression: First-Principles Density Functional Theory and Reactive Force Field Studies

We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated energetics and isothermal equation of state (EOS) upon hydrostatic compression obtained by DFT and ReaxFF are both in good agreement with available experimental data. Our calculations of the hydrostatic EOS at zero temperature are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression were also calculated by both ReaxFF and DFT