Anisotropic compressibility and structural stability of LLM-105 under high pressure

Abstract

The structural stability of the energetic material 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105, C4H4N6O5) under high pressure is pivotal for optimizing its detonation performance and safety. However, its microscopic structural response to external compression remains insufficiently understood. In this study, high-pressure single-crystal X-ray diffraction measurements demonstrate that LLM-105 exhibits pronounced anisotropic compressibility along the b axis while retaining monoclinic P21/n symmetry up to 10.40 GPa. This anisotropic response is ascribed to the pressure-induced reduction of the folding angle within the V-shaped molecular framework. Furthermore, compression of intermolecular hydrogen bonds drives the torsional deformation of the amino groups relative to the pyrazine ring. This mechanism is strongly corroborated by in situ Raman spectroscopy, which reveals distinct splitting of multiple amino vibrational modes at ∼3 GPa; notably, the divergent blue-shift rates of the split peaks serve as robust evidence for the continuous nature of this torsional evolution. In addition, the optical band gap of the sample narrows substantially (by ∼50%) in the pressure range of 0.10 MPa–28.02 GPa, a change that most likely is attributed to the enhanced intermolecular π–π orbital overlap and interlayer coupling. Our results indicate that the coupling between framework folding and hydrogen-bonding patterns governs the structural and electronic stability of layered energetic crystals under extreme high-pressure environments.