Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most important energetic materials. Investigations on its phase transitions and chemical reactions under extreme conditions are very important to understand the explosion process and design new energetic materials. By using a diamond anvil cell combined with in situ Raman, IR and X-ray diffraction techniques up to a pressure of ∼40 GPa, we found that β-HMX undergoes four reversible phase transitions without any chemical reaction under external pressure at room temperature. Isostructural phase transitions emerge around 5 GPa (ζ-HMX) and 10-13 GPa (ε-HMX), and another two phases emerge at 16 GPa (η-HMX) and 27 GPa (ϕ-HMX). The unit cells of ζ-HMX and ε-HMX were determined as a = 6.215 Å, b = 10.417 Å, c = 8.272 Å, β = 124.88°, P21/c at 6.2 GPa and a = 6.130 Å, b = 9.846 Å, c = 8.258 Å, β = 125.06°, P21/c at 12.6 GPa, respectively. The crystal structures of β, ζ, and ε-HMX were obtained by Rietveld refinement, based on which the rotations of NO2 groups were found to be related to the phase transition at 5 GPa. Additionally, HMX decomposes at 8.7 GPa and 300 °C. Carbon dioxide, hydroxyl, imino and hydroxyimino groups were detected in the IR spectrum, which indicates that the reaction contains a hydrogen transfer process. Our investigation uncovers the structural variation of β-HMX under external pressure and identifies the decomposition products under extreme conditions, which provides new insight to understand the detonation process of energetic materials.
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