Structure–Property Correlation Studies of Potassium 4,4′-Bis(dinitromethyl)-3,3′-azofurazanate: A Noncentrosymmetric Primary Explosive

Abstract

The knowledge of mechanical, vibrational, electronic properties, and origin of polarizability of a noncentrosymmetric explosive materials plays a crucial role in understanding the phase transition mechanism and stand-off detection of high energy material (HEM) residues on the surfaces and interfaces. In the present study, we have explored the role of crystal structure and chemical composition in predicting the structural, dynamical, electronic, and optical properties of two newly synthesized noncentrosymmetric green primary explosives, potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K2BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K2DNABT), using density functional theory simulations. The calculated structural and mechanical properties suggest that K2BDAF and K2DNABT are mechanically stable and possess lower bulk modulus values [K2BDAF (18.91 GPa) < K2DNABT (22.4 GPa)] than toxic lead azide Pb(N3)2 (26 GPa). The Born effective charge (BEC) and vibrational, thermodynamic, and phonon dispersion studies reveals that both crystals are dynamically stable and that K2BDAF is found to be more stable, more polarizable, and easily detectable than K2DNABT. The TB-mBJ electronic band structure study reveals that K2BDAF is an indirect band gap (along R → V high symmetry points) material with a gap of 2.37 eV < K2DNABT (3.57 eV, direct). The density of states and charge density plots show that both crystals possess mixed covalent and ionic bonding nature, and these results are consistent with BEC analysis. The nitrogen, oxygen and carbon p states are found to be dominant near the Fermi level in the valence band. From the optical properties study, it is found that the studied explosive materials have the lowest dielectric constant compared with the well-known 4-N,N-dimethylamino-4-N-methylstilbazolium tosylate (DAST) crystal, i.e., K2DNABT (2.68) < K2BDAF (2.91) < DAST (5.2), and show birefringence similar to (K2BDAF static value, 0.32; at 532 nm, 0.58), (K2DNABT static value, 0.25; at 532 nm, 0.27) that of DAST (0.39, 0.55, 0.64) and 4-N,N-dimethylamino-4′-N′-methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (0.45, 0.63) NLO crystals. For both the studied explosives, the absorption is found to be strong in the [010] direction and follows the relation α(K2BDAF) < α(K2DNABT). The analysis of differences in BEC reveals that from K2DNABT to K2BDAF, the positive contribution of atoms to total birefringence increases as follows: K1 (0.09559), O1 (0.69236), O2 (0.77962), N1 (1.41018), N2 (0.92394), N3 (0.27415), N4 (0.74264), and N5 (0.84502). The negative contribution from C1 is increased by −1.29582. Over all, our results are in agreement with the experimentally reported sensitivity trend, and the reasons behind the sensitivity differences of K2DNABT and K2BDAF are explored.