Negative linear compressibility and structural stability of the energetic material trans-hexanitrostilbene under pressure

The structural stability of the energetic material 2,2',4,4',6,6'-hexanitrostilbene (trans-HNS) under high pressure is critical for optimizing its detonation performance and low sensitivity. However, its structural response to external pressure has not been sufficiently investigated. In this study, high-pressure single-crystal X-ray diffraction data of trans-HNS demonstrate that the sample exhibits pronounced anisotropic strain, demonstrating an unusual negative linear compressibility (NLC) along the c axis, with a coefficient of -4.4(5) TPa-1 within the range of 0.1 MPa to ~2.90 GPa. The expansion of the c axis is attributed to significant reorientations of the HNS molecules towards the bc plane within the herringbone network. Subsequently, the NLC reverts to the PLC under further compression due to the limited space within the lattice, which restricts the reorientations of irregularly shaped HNS molecules. The enhanced CH∙∙∙O intermolecular contacts facilitate the rotations of the -NO₂ nitro groups relative to the phenyl rings, as demonstrated by the conformational changes observed in the HNS molecules through Raman spectroscopy measurements. In addition, the band gap of the sample gradually decreases, reducing by ~8.8% in the range of 0.1 MPa and 10.01 GPa. However, the sample could not be returned to its initial state once the pressure exceeded 10 GPa, which is most likely due to the occurrence of a chemical reaction or amorphization. Our results indicate that the NLC behavior may present in a wide range of unexamined herringbone-type energetic molecular crystals with different molecular structures.