Phase transformation of heat-resistant energetic material BDNAPM studied by Raman spectroscopy and X-ray diffraction

Bis(3,5-dinitro-4-aminopyrazolyl) methane (BDNAPM) is a new insensitive secondary explosive with a high thermal decomposition temperature. The higher thermal stability and insensitivity towards impact and friction make BDNAPM potentially useful in mining and fracking industry. The sensitivity of explosives is correlated to the phonon modes of the material. Pressure and temperature induced phase transformation study of BDNAPM is important for applications because different phases show different detonation properties and sensitivity towards external stimuli. We have calculated the vibrational frequencies of BDNAPM molecule using Gaussian software, phonon frequencies of the crystal using density functional perturbation theory, and assigned the modes. We report the pressure dependent phase transformations in BDNAPM using Raman spectroscopy and X-ray diffraction. Appearance of new modes, especially in the external mode region indicates a structural phase transformation at 3.3 GPa. Also, many internal and external modes show discontinuity above 2.8 GPa. The changes in the bands like splitting, discontinuity and red-shift occur mainly for bands corresponding to the N–H, C–H and N–O movements, indicating change in the hydrogen bonding that could lead to a reorientation of the molecule and a structural phase transformation beginning at 3.3 GPa. Pressure dependent X-ray diffraction results corroborate the Raman spectroscopic results. The crystal lattice exhibits anisotropic compression under pressure. We were able to fit the ambient orthorhombic structure to the XRD patterns up to 2.6 GPa and then the structure transformed into a monoclinic phase (space group P2₁/n) with a 13% volume reduction. Bulk modulus is obtained by fitting third-order Birch Murnaghan equation of state to the PV data for the ambient and high-pressure phase.