Structural, electronic and optical properties of well-known primary explosive: Mercury fulminate

Mercury Fulminate (MF) is one of the well-known primary explosives since 17th century and it has rendered invaluable service over many years. However, the correct molecular and crystal structures are determined recently after 300 years of its discovery. In the present study, we report pressure dependent structural, elastic, electronic and optical properties of MF. Non-local correction methods have been employed to capture the weak van der Waals interactions in layered and molecular energetic MF. Among the non-local correction methods tested, optB88-vdW method works well for the investigated compound. The obtained equilibrium bulk modulus reveals that MF is softer than the well known primary explosives Silver Fulminate (SF), silver azide and lead azide. MF exhibits anisotropic compressibility (b>a>c) under pressure, consequently the corresponding elastic moduli decrease in the following order: C22>C11>C33. The structural and mechanical properties suggest that MF is more sensitive to detonate along c-axis (similar to RDX) due to high compressibility of Hg...O non-bonded interactions along that axis. Electronic structure and optical properties were calculated including spin-orbit (SO) interactions using full potential linearized augmented plane wave method within recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. The calculated TB-mBJ electronic structures of SF and MF show that these compounds are indirect bandgap insulators. Also, SO coupling is found to be more pronounced for 4d and 5d-states of Ag and Hg atoms of SF and MF, respectively. Partial density of states and electron charge density maps were used to describe the nature of chemical bonding. Ag-C bond is more directional than Hg-C bond which makes SF to be more unstable than MF. The effect of SO coupling on optical properties has also been studied and found to be significant for both of the compounds.Comment: 15 pages, 10 figure

Structural stability, vibrational and bonding properties of potassium 1,1′'-dinitroamino-5,5′' bistetrazolate: An emerging green primary explosive

Potassium 1,1$'$-dinitroamino-5,5$'$ bistetrazolate (K$_2$DNABT) is a nitrogen rich (50.3 $\%$ by weight, \ce{K2C2N12O4}) green primary explosive with high performance characteristics namely velocity of detonation (D = 8.33 km/s), detonation pressure (P = 31.7 GPa) and fast initiating power to replace existing toxic primaries. In the present work, we report density functional theory (DFT) calculations on structural, equation of state, vibrational spectra, electronic structure and absorption spectra of K$_2$DNABT. We have discussed the influence of weak dispersive interactions on structural and vibrational properties through the DFT-D2 method. We find anisotropic compressibility (b $<$ a $<$ c) from pressure dependent structural properties. The predicted bulk modulus reveals that the material is harder than cyanuric triazide (\ce{C3N12}) and softer than lead azide (\ce{Pb(N3)2}). A complete assignment of all vibrational modes has been made and compared with the available experimental results. The calculated zone centre IR and Raman frequencies show a blue-shift which leads to a hardening of the lattice upon compression. In addition, we have also calculated the electronic structure and absorption spectra using the recently developed Tran Blaha-modified Becke Johnson potential. It is found that K$_2$DNABT is a direct band gap insulator with a band gap of 3.87 eV and the top of the valence band mainly dominated by $2p$-states of oxygen and nitrogen atoms. K$_2$DNABT exhibits mixed ionic (between potassium and tetrazolate ions) and covalent character within tetrazolate molecule. The presence of ionic bonding suggests that the investigated compound is relatively stable and insensitive than covalent primaries. From the calculated absorption spectra, the material is found to decompose under ultra-violet light irradiation

Hydrophobic and Ionic Interactions in Nano-sized Water Droplets

We investigate the solvation of methane and methane decorated with charges in spherically confined water droplets. Free energy profiles for a single methane molecule in droplets, ranging in diameter D, from 1 to 4 nm, show that the droplet surfaces are strongly favorable as compared to the interior. From the temperature dependence of the free energy in D=3 nm, we show that this effect is entropically driven. The potentials of mean force (PMFs) between two methane molecules show that the solvent separated minimum in the bulk is completely absent in confined water, independent of the droplet size since the solute particles are primarily associated with the droplet surface. The tendency of methanes with charges (Mq+ and Mq- with q+ = q- = 0.4e, where e is the electronic charge) to be pinned at the surface depends dramatically on the size of the water droplet. When D=4 nm, the ions prefer the interior whereas for D<4 nm the ions are localized at the surface, but with much less tendency than for methanes. Increasing the ion charge to e makes the surface strongly unfavorable. Reflecting the charge asymmetry of the water molecule, negative ions have a stronger preference for the surface compared to positive ions of the same charge magnitude. With increasing droplet size, the PMFs between Mq+ and Mq- show decreasing influence of the boundary due to the reduced tendency for surface solvation. We also show that as the solute charge density decreases the surface becomes less unfavorable. The implications of our results for the folding of proteins in confined spaces are outlined.Comment: 27 pages, 9 figures plus supporting material (6 pages, 5 figures) To be published in The Journal of the American Chemical Societ

Polymorphism and thermodynamic ground state of Silver fulminate studied from van der Waals density functional calculations

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases namely orthorhombic (\emph{Cmcm}) and trigonal (\emph{R$\bar{3}$}) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semiempirical dispersion correction methods incontrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the \emph{Cmcm} structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto \emph{Cmcm} and \emph{R$\bar{3}$} phases denoted as $\alpha$ and $\beta$-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that $\alpha$, $\beta$ phases are indirect band gap insulators with a band gap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable $\beta$-form of AgCNO

Structure, elastic and dynamical properties of KN3_3 and RbN3_3: A van der Waals density functional study

We report a detailed first principles study on the structural, elastic, vibrational and thermodynamic properties of layered structure energetic alkali metal azides KN$_3$ and RbN$_3$. All the calculations were carried out by means of plane wave pseudopotential method with and without including van der Waals interactions. The calculated ground state structural properties are improved to a greater extent by the inclusion of dispersion corrections, implies that the van der Waals interactions play a major role on the physical properties of these systems. The elastic constants and the related bulk mechanical properties for the tetragonal KN$_3$ and RbN$_3$ have been calculated using both the methods and found that the compounds are mechanically stable systems. The magnitude of the calculated elastic constants increases in the order RbN$_3$ $<$ KN$_3$ implying higher elastic stiffness for KN$_3$, the fact also confirmed by the higher values of bulk, shear and Young's moduli of KN$_3$ than RbN$_3$. Moreover, the calculated elastic constants follows the inequality C$_{33}$ $<$ C$_{11}$ which indicates the presence of more number of intermolecular interactions along a-axis over c-axis of the azide lattices. A correlation has been proposed to relate the calculated elastic constants to the decomposition phenomena for the metal azides. The experimentally reported vibrational frequencies at the gamma point were exactly reproduced by the present calculations. In addition, we also present the thermodynamic properties such as heat capacity which compares well with the experiment.Comment: 8 Figure

Structural and vibrational properties of nitrogen-rich energetic material guanidinium 2-methyl-5-nitraminotetrazolate

We present density functional theory calculations on the crystal structure, equation of state, vibrational properties and electronic structure of nitrogen-rich solid energetic material guanidinium 2-methyl-5-nitraminotetrazolate (G-MNAT). The ground state structural properties calculated with dispersion corrected density functionals are in good agreement with experiment. The computed equilibrium crystal structure is further used to calculate the equation of state and zone-center vibrational frequencies of the material. The electronic band structure is calculated and found that the material is an indirect band gap semiconductor with a value of 3.04 eV

Density functional study of electronic structure, elastic and optical properties of MNH2_2 (M=Li, Na, K, Rb)

We report systematic first principles density functional study on the electronic structure, elastic and optical properties of nitrogen based solid hydrogen storage materials LiNH$_2$, NaNH$_2$, KNH$_2$, and RbNH$_2$. The ground state structural properties are calculated by using standard density functional theory and also dispersion corrected density functional theory. We find that van der Waals interactions are dominant in LiNH$_2$ whereas they are relatively weak in other alkali metal amides. The calculated elastic constants show that all the compounds are mechanically stable and LiNH$_2$ is found to be stiffer material among the alkali metal amides. The melting temperatures are calculated and which follows the order RbNH$_2$ $<$ KNH$_2$ $<$ NaNH$_2$ $<$ LiNH$_2$. The electronic band structure is calculated by using the Tran-Blaha modified Becke-Johnson potential and found that all the compounds are insulators with a considerable band gap. The [NH$_2$]$^-$ derived states are completely dominating in the entire valence band region while the metal atom states occupy the conduction band. The calculated band structure is used to analyze the different interband optical transitions occur between valence and conduction bands. Our calculations show that these materials have considerable optical anisotropy

Lattice dynamics and electronic structure of energetic solids LiN3 and NaN3: A first principles study

We report density functional theory calculations on the crystal structure, elastic, lattice dynamics and electronic properties of iso-structural layered monoclinic alkali azides, LiN3 and NaN3. The effect of van der Waals interactions on the ground- state structural properties is studied by using various dispersion corrected density functionals. Based on the equilibrium crystal structure, the elastic constants, phonon dispersion and phonon density of states of the compounds are calculated. The accurate energy band gaps are obtained by using the recently developed Tran Blaha-modified Becke Johnson (TB-mBJ) functional and found that both the azides are direct band gap insulators

High pressure structural, electronic, and optical properties of polymorphic InVO4 phases

In the present work, we report a detailed density functional theory calculation on polymorphic InVO$_4$ phases by means of projector augmented wave method. The computed first-order structural phase transformation from orthorhombic \emph{(Cmcm)} to monoclinic \emph{(P2/c)} structure is found to occur around 5.6 GPa along with a large volume collapse of 16.6$\%$, which is consistent with previously reported experimental data. This transformation also leads to an increase in the coordination number of vanadium atom from 4 to 6. The computed equilibrium and high pressure structural properties of both InVO$_4$ phases, including unit cell parameters, equation of state, and bulk moduli, are in good agreement with the available experimental data. In addition, compressibility is found to be highly anisotropic and the \emph{b}-axis being more compressible than the other for both the structures. Electronic band structures for both the phases were calculated, and the band gap for orthorhombic and monoclinic InVO$_4$ are found to be 4.02 and 1.67 eV, respectively, within the Tran-Blaha Modified Becke-Johnson potential as implemented in linearized augmented planewave method. We further examined the optical properties such as dielectric function, refractive index, and absorption spectra for both the structures. From the implications of these results, it can be proposed that the high pressure InVO$_4$ phase can be more useful than orthorhombic phase for photo catalytic applications.Comment: 20 pages, 10 figure

Structural, vibrational, and quasiparticle band structure of 1, 1 - diamino-2, 2 - dinitroethelene from ab-initio calculations

The effect of pressure on the structural and vibrational properties of the layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored by first principles calculations. We observe significant changes in the calculated structural properties with different corrections for treating van der Waals interactions to Density Functional Theory (DFT), as compared with standard DFT functionals. In particular, the calculated ground state lattice parameters, volume and bulk modulus obtained with Grimme's scheme are found to agree well with experiments. The calculated vibrational frequencies demonstrates the dependence of the intra and inter-molecular interactions in FOX-7 under pressure. In addition, we also found a significant increment in the N-H...O hydrogen bond strength under compression. This is explained by the change in bond lengths between nitrogen, hydrogen and oxygen atoms, as well as calculated IR spectra under pressure. Finally, the computed band gap is about 2.3 eV with GGA, and is enhanced to 5.1 eV with the GW approximation, which reveals the importance of performing quasiparticle calculations in high energy density materials