High-Pressure Equation of State of 1,3,5-triamino-2,4,6-trinitrobenzene: Insights into the Monoclinic Phase Transition, Hydrogen Bonding, and Anharmonicity.

The high-pressure equation of state (EOS) of energetic materials (EMs) is important for continuum and mesoscale models of detonation performance and initiation safety. Obtaining a high-fidelity EOS of the insensitive EM 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) has proven to be difficult because of challenges in experimental characterization at high pressures (HPs). In this work, powder X-ray diffraction patterns were fitted using the recently discovered monoclinic 2/ phase above 4 GPa, which shows that TATB is less compressible than when indexed with the triclinic 1̅ phase. First-principles calculations were performed with Perdew-Burke-Ernzerhof (PBE) and PBE0 functionals including thermal effects using the 1̅ phase. PBE0 improves the description of hydrogen bonding and thus predicts accurate planar and lattice parameters under ambient conditions. However, discrepancies in the predicted lattice parameters above 4-10 GPa compared with experimental measurements indexed with 1̅ are further evidence of a structural modification at high pressure. Layer sliding defects are formed during molecular dynamics simulations, which induces an anharmonic effect on the thermal expansion of the lattice parameter. In short, the results provide several insights into determining high-fidelity EOS parameters for TATB and other molecular crystals.