Thermal Expansion, Atomic Vibration, and Molecular Conformation of β-HMX at a Wide Temperature Range Prior to Phase Transformation

Crystal structure evolution of high-explosive HMX from −100 to 160 °C was studied by in situ single-crystal X-ray diffraction. Thermal expansion properties and the changes in atomic vibration and molecular conformation were investigated. Anisotropic thermal expansion was observed with a preferred direction along the b-axis. More interplanar defects may be generated between the (011) and (020) facets due to the large increasing rate of their interplanar spacings ((3.23 ± 0.09) × 10–4 and (6.98 ± 0.05) × 10–4 Å/°C), which may significantly influence the interfacial interaction between HMX crystals and polymer binders in plastic-bonded explosive. Atomic displacement parameter (ADP) investigation reveals that the farther away from the center ring, the stronger the atomic vibrations. Compared with the linear increase of the unit cell parameters, the increase of ADP is proportional to temperature, following an exponential growth model. Compression of the central ring along one direction and expansion along the opposite direction were also observed. The vibrations and rotations of the nitro groups related to the central ring were further identified and confirmed by Raman spectroscopy, demonstrating the flexibility of the nitro groups. Based on the observations, a mechanism was proposed for the molecular conformation transformation of HMX during the β–δ phase transition. All the results are not only beneficial to a comprehensive understanding of the thermal properties of HMX on the atomic and molecular levels but also provide an important reference for the manufacturing, processing, and storage of HMX and other energetic materials.