Elastic and phonon-mode anomalies with temperature in the energetic material C6H6N4O8

Abstract

Temperature-dependent Brillouin and Raman spectroscopic investigations are carried out on energetic material 4,10-Dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo $[{5.5.0.0}^{5,9}{.0}^{3,11}]$ -dodecane (TEX; ${\mathrm{C}}_{6}{\mathrm{H}}_{6}{\mathrm{N}}_{4}{\mathrm{O}}_{8}$) from \ensuremath{-}196 to 180 \ifmmode^\circ\else\textdegree\fi{}C, close to its sublimation point, to study its elastic and phonon mode behaviors. Ambient values of experimental shear ($G$) and bulk ($K$) moduli of TEX are obtained as $G=5.6\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and $K=17.7\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We have computed the elastic tensor and obtained the averaged bulk and shear moduli to compare with the experimentally obtained polycrystalline values. The known lower sensitivity of TEX compared with similar caged secondary explosives as CL-20, Royal Demolition eXplosive, and \ensuremath{\beta}-High Melting eXplosive is reckoned as due to its inherently high bulk modulus. We report direct experimental evidence of rapid reduction in elastic constant of TEX from 20 to 80 \ifmmode^\circ\else\textdegree\fi{}C, much before its melting point. Though there are no phase transitions in this temperature span, all Raman bands exhibit softening behavior around this temperature. We observe two distinct anomalies. Some bending modes such as bending of CNC $(356\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$, in-plane bending of NNO $(552\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$, bending of NCO $(610\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$, in-plane bending of NNO (635 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$), and in-plane bending of ONO and OCO (712 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) exhibit slight hardening before and after the elastic anomaly. However, the asymmetric stretching modes at 1574 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (asymmetric stretch of NO) and 1590 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (asymmetric stretch of ${\mathrm{NO}}_{2}$) exhibit hardening below 20 \ifmmode^\circ\else\textdegree\fi{}C but soften after 80 \ifmmode^\circ\else\textdegree\fi{}C. This hardening switching over to softening above the anomaly indicates a release of stiffness manifesting from a conformational change to exo-endo and aiding the increasing compressibility. Emergence of new diffraction peaks $\ensuremath{\sim}50{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and even observed at 150 \ifmmode^\circ\else\textdegree\fi{}C, which is well above the elastic anomaly, suggests that the material at high temperature may be isostructural to that at low temperature. In this paper, we establish that the high bulk modulus value of TEX contributes to its lower sensitivity; however, we also clearly demonstrate reduction in elastic modulus accompanied by anomalous behavior of Raman modes. This decrease in elastic modulus leading to increased compressibility may result in increased sensitivity of the explosive material just above room temperature.