The High-Pressure Equation of State of TATB: Insights on the Monoclinic Phase Transition, Hydrogen Bonding, and Anharmonicity*

The high-pressure equation of state (EOS) of energetic materials (EMs) are 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 difficult due to challenges in experimental characterization at high pressures. In this work, powder x-ray diffraction patterns were fitted using the recently discovered monoclinic I2/a phase above 4 GPa, which shows TATB is less compressible than when indexed with the triclinic P-1 phase. First principles calculations were performed with PBE and PBE0 functionals including thermal effects. PBE0 improves the description of hydrogen bonding and predicts accurate a and b lattice parameters. However, discrepancies are found in the predicted and experimental lattice parameters above 4-10 GPa for the P-1 phase. Layer sliding defects are formed during molecular dynamics simulations which produces an anharmonic effect. The results provide insights into determining high-fidelity EOS parameters of TATB.