Phase transition and thermal expansion of molecular perovskite energetic crystal (C6N2H14)(NH4)(ClO4)3 (DAP-4)

Research on the structural responses to thermal stimuli for crystalline energetic materials is crucial to their practical applications. DAP-4 is a metal-free molecular perovskite high-energetic material attracting increasing attentions on its application potential as heat-resistant explosive. In order to reveal its structural responses to thermal stimuli, herein we investigated the structural phase transitions and thermal expansion of DAP-4 by DSC, single-crystal X-ray diffraction, and variable-temperature capillary powder X-ray diffraction. The results show that DAP-4 undergoes two-step reversible phase transitions at 300.4/298.9 K and 548.4/547.7 K, respectively, which are caused by two-step order-disorder transition of the molecular components during a heating/cooling cycle. The axial and the volumetric expansion coefficients of DAP-4 are estimated based on the temperature-dependent cell parameters obtained by Pawley refinement in a large temperature range of 173–353 K, and they are close to those estimated for β-HMX. Notably, although a volume change of 0.77% occur in the near-room-temperature phase transition, all the crystalline phases of DAP-4 possess cubic structures with isotropic expansibility, rather than the commonly-observed anisotropic one in the most of known energetic crystals, which may be propitious to reduce the adverse effect of its volume change on the formulation design.