Computational study of lattice dynamics and thermodynamic properties of energetic solid cyanuric triazide

Abstract

The structural, vibrational and thermodynamic properties of the cyanuric triazide crystal is investigated through density functional theory (DFT) simulations within the dispersion corrected generalized gradient approximation (GGA + G06) by considering the norm conserving pseudopotentials. Infrared spectra and Born effective charges are computed through linear response method using the density functional perturbation theory (DFPT). The experimentally observed vibrational modes are well reproduced in our calculations. The high intensity infrared absorption bands in the range of 1200–1500 cm−1 originate from the vibrations of the N3 group. The Born effective charges indicate that the compound to be asymmetric. The calculated phonon dispersion curve clearly show the absence of imaginary frequencies along the high symmetry directions, which confirms the dynamical stability of C3N12, and the interaction between the acoustic and low lying optical modes results in enhanced phonon scattering enabling low thermal conductivity in this compound. The phonon partial density of states reveals that the low-frequency regions (below 500 cm−1) are dominated by N atoms. In addition, the compound shows covalent nature owing to the hybridization between C and N atoms. The phonon spectra and thermodynamic properties of C3N12 were computed for the first time, which still awaits experimental confirmation. The temperature-dependent heat capacity and Debye temperature has been discussed in detail. The heat capacity CV of C3N12 increases with temperature, being 98.13 cal/cell.K for T = 306 K. Finally, the entropy, enthalpy and Helmholtz free energy, are investigated and discussed in the framework of the harmonic approximation. From the above results, it can be inferred that cyanuric triazide is an effective initiating explosive, as it possess low-thermal conductivity.