First-principle Studies Of Phonon Spectrum, Structure And Properties Of TATB Crystal

1,3,5-triamine-2,4,6-trinitrobenzene (TATB) is a high-performance insensitive explosive. TATB has excellent stability to thermal, impact and shock stimuli. Although numerous both experimental and theoretical efforts have been devoted to understand the structure and properties of TATB, There are still lacks of conclusive evidence of structural stability (i.e. displacive structural phase transition), performance under extreme conditions, thermal transport properties, etc., especially the detonation process in energetic materials, including the mechanism of shock initiation and molecular bond breaking, In view of a close correction between the above mentioned issue and the safety, detonation behavior and invariability of energetic materials, it is necessary to gain a deeper insight into the microscopic physical origin of their properties. Phonon spectrum is a fundamental tool to determine different phenomena, which is also crucial to understand the structure and properties of TATB crystal.First-principles with van der Waals corrections have been used to calculated phonon spectrum of TATB crystal. Based on our computed results, structure and properties of TATB crystal were investigated. The main points are summarized briefly as follows:1. Three widely used non-local van der Waal correction schemes were utilized to characterize the structures and properties of TATB crystal. Lattice parameters, molecular structure, crystal density, lattice energy, equation of state, mechanical properties of TATB crystal were calculated, respectively. Vibration frequencies at ? in the range of 0-8.5 GPa, phonon spectrum, specific heat, Helmholtz free energy and entropy were further calculated by the method of vdW-DF2. This work could provide essential data.2. The Raman-active vibrational modes at r of TATB crystal were identified. Based on the deduced equation of state result, the coupling between vibrational modes under pressure was analyzed. From the results, we found that with increasing pressure, TATB molecules in neighboring layers tends to bend towards each other, causing a coupling of NH2 plane twist vibration or wag with NO2 shear vibration, which indicates a stronger intermolecular hydrogen bonding. This may provide new evidence for the strengthening of the intermolecular interactions of TATB crystal during the pressurization progress.3. Helmholtz free energy, specific heat and entropy of TATB crystal in the temperature range of 0-600 K were calculated. Restricted by the experimental conditions considerations, thermodynamic parameters of TATB crystal under extreme environments are still difficult to obtain, this work would provide the necessary parameters for further study.4. Base on the phonon spectrum of TATB crystal, the contributions of different vibrational modes to the specific heat in the temperature range of 300-600 K were derived, By combining with the Mulliken population analysis, the possible trigger bond of TATB crystal during thermolysis was predicted, and the following results were obtained:(1) Phonon density of states of TATB crystal reaches its maximum at the vibration frequency of 2.3 THz, which is in good agreement with the strong absorption peak at 2.22 THz observed by THz spectroscopy. (2) The number of "doorway" modes (i.e. the low frequency molecular vibrations that critical to detonation initiation) of TATB in the regions of 6.0-21.0 THz was estimated based on the phonon density of states. We found that, because of the interactions of intermolecular hydrogen bonding, many new "doorway" modes appear. (3) Phonon modes in the range of 0-27.5 THz would contribute 93.7% of the total specific heat at room temperature, which implies that the energy for impact or non-impact energy firstly flow into the low frequency molecular vibrations or lattice vibration region. (4) By combining of a Mulliken population analysis of TATB with the relative contribution of 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.In conclusion, first-principles with van der Waals correction have been used to study the phonon spectrum, structure and performance of TATB crystal. The known experimental and unknown information have been illustrated by this research. It may provide a basis for the further lattice dynamics study of TATB crystal.