First-principles study of the phonon spectrum and heat capacity of TATB crystal

Abstract

The widely used energetic material 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) is an extremely powerful explosive and known for its extraordinary insensitivity to external stimuli (i.e., shock, friction, impact). TATB crystal exhibits graphitic-like sheets with significant inter- and intra-molecular hydrogen bondings within each layer and weak van der Waals (vdW) interactions between layers. Although TATB has been extensively studied both theoretically and experimentally, a fully understanding of its unique detonation phenomenon at a microscopic level is still lacking. Before establishing the exact pathway through which the initial energy is transferred, a fundamental knowledge of both the lattice vibrations (phonons) and molecule internal vibrations must be gained at the first step. Recently, it has been demonstrated that density functional theory (DFT) is inadequate in treating conventional energetic materials, within which dispersion interactions appear to be major contributors to the binding forces. In the present work, phonon spectrum and specific heat of TATB crystal are calculated in the framework of DFT with vdW-DF2 correction, which has been validated in our previous studies of the equation of state, structure and vibration property of TATB crystal under pressures in a range of 0-8.5 GPa. Structure optimization is preformed at zero-pressure, followed by calculating the equation of state, crystal density and lattice energy. The computed results are found to fit well with the experimental and other theoretical values. Frozen phonon method is used to calculate the phonon spectrum and phonon density of states. We find that the phonon density of states reaches its maximum at a vibration frequency of 2.3 THz, which is in good agreement with the strong absorption peak at 2.22 THz observed by THz spectroscopy. The assignment of several Raman active vibrations of TATB above 7.5 THz is given, and a comparison with other published results is also made in this study. Furthermore, the contributions of different phonon vibration modes to the specific heat are derived from the phonon density of states. The number of doorway modes (i.e., the low frequency molecular vibrations that is critical to detonation initiation) of TATB in a range of 6.0-21.0 THz is estimated based on the phonon density of states. It is shown that the phonon modes in a range of 0-27.5 THz would contribute 93.7% of the total specific heat at room temperature. By combining a Mulliken population analysis of TATB with the relative contribution of phonon vibration modes to the specific heat at 300-600 K, we conclude that C-NO2 bond might be the trigger bond of TATB during thermolysis.