Researchers Discover Unexpected Phase Transition in Highly Explosive TATB

Researchers from the Lawrence Livermore National Laboratory (LLNL), in collaboration with the University of Nevada, Las Vegas (UNLV), have discovered a phase transition induced by the unknown pressure for TATB that can help predict the performance of detonation and the safety of the explosive. The search appears in the May 13th online edition of the Applied Physics Letters and it is highlighted as a cover and feature article.

1,3,5-triamino-2,4,6-trinitrobenzene (TATB), the industry standard for insensitive explosives, is the optimal choice when safety (insensitivity) is of paramount importance. Among comparable similar materials with comparable explosive energy release, TATB is remarkably difficult to initiate by shock and has low sensitivity to friction. The causes of this unusual behavior are hidden in the high pressure structural evolution of TATB. Explosive explosive supercomputer simulations, run on the world's most powerful LLNL machines, depend on knowing the exact locations of the atoms in the crystal structure of an explosive. The precise knowledge of the atomic arrangement under pressure is the cornerstone of prediction of the detonation performance and the safety of an explosive.

The team performed experiments using a diamond anvil cell, which compressed TATB single crystals at a pressure greater than 25 GPa (250,000 times atmospheric pressure). According to all previous experimental and theoretical studies, it was thought that the atomic arrangement in the crystal structure of TATB remains the same under pressure. The project team challenged the consensus on the ground to clarify TATB's high-pressure structural behavior.

The main experimental challenge was the extremely weak crystal structure of TATB symmetry, making conventional X-ray diffraction techniques for diamond anvil cells impossible. Instead, the experimental team used X-ray diffraction on a single-crystal under pressure, for the first time in the case of a low-symmetry organic material such as TATB.

"The issue of phase transitions in the compressed TATB has been debated for decades, and we were convinced that our approach would finally solve this issue – but it was much harder to find the answer than we expected." said Oliver Tschauner, professor in the Department of Geosciences at UNLV.

Surprisingly, the experimental results revealed a hitherto unknown transition to a monoclinic phase of symmetry greater than 4 GPa. The experimental results allowed the team to determine the basic characteristics (network parameters and cell volume) of the high-pressure crystalline structure and the state equation (density versus pressure) above the phase transition. However, the team did not stop at this point

"Although the experimental results allowed us to apply important corrections to the TATB state equation, we were determined to go further and understand the nature of the TATB transition. phase and the exact structure of the high-pressure phase, "Elissaios Stavrou explained. , a staff member of LLNL's Materials Science Division.

To help unravel the high-pressure phase, LLNL theorists have used a scalable structural search algorithm (USPEX) to explore TATB's high-pressure structures. The theoretical results not only confirmed the experimental results, but also clarified the exact structure of the high pressure phase.

"Almost everything on a material can be derived from its crystalline structure," said Brad Steele, a postdoctoral researcher at the Division of Materials Science at LLNL and senior author of the research. "In this paper, we show that we can predict the crystalline structure even for a complex / complex energy material such as TATB.The methods used have many potential applications in the field of materials science."

Based on the USPEX results, the team determined that the phase transition involves a pressure-induced plane shift in graphitic layers of TATB molecules in the ambient pressure phase.

Matthew Kroonblawd, staff member of LLNL's Materials Science Division, further explained: "It is notoriously difficult to model the TATB, but we have been able to link the old and the new phases to help generalized computing tools that we have developed specifically for these complex molecular materials.This new phase solves the conjectures that have persisted since the 1970s ".

The team plans to use the same combination of advanced experimental and theoretical techniques to discover possible phase transitions in other energetic materials. However, the methodology used in this study is not limited to energetic materials and significantly increases the team's ability to reveal crystalline structures and stoichiometries under varying thermodynamic conditions.