The Characterization of the Lattice Vibrations of Ammonium Nitrate in ANFO Mixture After Authentic Detonations Using Confocal Raman Microscopy and Single Crystal X-Ray Diffraction

Abstract

This study aimed to characterize ammonium nitrate lattice vibrations in an ammonium nitrate and fuel oil mixture (ANFO) following authentic explosive events using confocal Raman microscopy and single-crystal X-ray diffraction (XRD). Two simulated IEDs were constructed and detonated, consisting of several different substrate materials, using ANFO as the main charge. Crystalline material was observed to be growing on several of the postblast substrates. Microscopical examination revealed the crystalline material to have isotropic and anisotropic characteristics. Following recrystallization from water, the material was identified as ammonium nitrate. Single-crystal X-ray diffraction enabled the identification of the unit cells and molecular structures of the crystals formed after the blast. The values corresponded to the known Phase IV structure of recrystallized ammonium nitrate; however, there were minor yet statistically significant variations in the distances in the unit cell dimensions and O–N–O angles. Ex situ analysis of isotropic crystalline fragment using confocal Raman microscopy determined that lattice vibrations within the material were different than the ANFO intact reference, with the blue shifting of several bands, the emergence of new bands, and the loss of other characteristic bands. It was determined that the isotropic crystalline material observed in the postblast residue consisted of stressed state Phase IV AN. This suggests that a thermal change of AN can be observed in the microscopical characteristics and Raman spectrum of the crystals, demonstrating the importance of low frequency Raman spectroscopy (10–250 cm⁻¹), which allows the identification of distinct spectral features of crystalline salts.