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Intermolecular interactions and dynamics in the solid state: An analysis of beta-HMX
A study of intermolecular interactions and lattice dynamics for the energetic material β-HMX using a series of complementary spectroscopic technique is presented. Employing Brillouin, Raman and piezomodulated Raman scattering, the fundamental properties of the lattice potential were detailed. The interpretation of the results for each method is qualitatively related to several current models of detonation initiation. ^ Brillouin scattering from a variety of acoustic phonons propagating in orthogonal planes allowed the experimental determination of all thirteen elastic constants of β-HMX. The elastic constants provide a window into the microscopic interactions within the material. Intermolecular forces associated with the observed (011) cleavage plane are qualitatively used to corroborate the relative ordering of C11, C22 and C33. This observation is further substantiated by patterns in crystal growth and molecular packing. ^ Through single-crystal, polarized, Raman scattering the Raman-active vibrational energies and symmetries for β-HMX have been characterized over the range of 3150 to 10 cm−1. By identifying combination and overtone bands, 24 Ag and 24 Bg fundamental molecular vibrations were assigned. Additionally, all six Raman-active optical phonons were characterized. From assignment of the 78.5 cm−1 Ag and 79.5 cm−1 Bg modes as optical phonons, the possibility for mediating excitation of the nitro-group deformation vibrations through phonon-phonon coupling is discussed. ^ Piezomodulated Raman spectra are presented from 200 to 25 cm−1 with stress directions along the c*- and a-crystallographic axes. Through an analysis of the PRS bands, the first determination of the anharmonic coupling constants and strain Grüneisen parameters for β-HMX are reported. The optical phonons displayed a larger mode anharmonicity relative to the intramolecular vibrations. This general trend was also reflected in the calculated strain Grüneisen parameters. For the lattice vibrations, anharmonic coupling was enhanced with strain parallel to the a-crystallographic axis. Conversely, the internal vibrations were more sensitive to strains along the c*-crystallographic axis. These results illustrate the necessity of detailing anisotropy in the molecular and lattice potentials to provide a greater understanding of detonation initiation. ^
Stevens, Lewis L, "Intermolecular interactions and dynamics in the solid state: An analysis of beta-HMX" (2005). ETD collection for University of Nebraska - Lincoln. AAI3186884.