Pressure dependent phase transformations of energetic material 2,4−dinitroanisole using Raman spectroscopy, X-ray diffraction and first principles calculations

Abstract

The molecular structure of 2,4−dinitroanisole (DNAN) is optimized using Gaussian 09W program, two stable molecular structures with different methoxy group orientations are obtained and vibrational frequencies are calculated for the two optimized molecular structures. DNAN crystal structure is optimized using CASTEP and VASP codes and phonon frequencies are calculated and assigned. Computed frequencies compare well with experimental values. Pressure dependent phase transformation in DNAN is investigated by Raman spectroscopy and X-ray diffraction. Differences between the Raman spectra of DNAN and trinitrotoluene (TNT) point to a larger phonon energy gap in DNAN to be a possible reason for the lower impact sensitivity of DNAN. Appearance of new Raman bands and discontinuity of the bands above 1.3 GPa indicates a phase transformation. Changes in the X-ray diffraction patterns confirm the phase transformation. The transformation of the ambient monoclinic to an orthorhombic structure starts at around 2 GPa. At 6.8 GPa the monoclinic structure completely transforms into orthorhombic with a volume collapse of 10%. Bulk moduli for the monoclinic and orthorhombic phases are estimated as ∼12 GPa and 27 GPa respectively, pointing to a lower impact sensitivity of the orthorhombic phase.