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

Temperature-dependent Brillouin and Raman spectroscopic investigations are carried out on energetic material 4, 10 -Dinitro-2,6,8,12-tetr aoxa-4, 10 -diazatetracyclo [5.5.0.0(5.9).0(3.11)] -dodecane (TEX; C6H6N4O8) from -196 to 180 degrees 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 GPa and K = 17.7 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 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 degrees 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 cm(-1)), in-plane bending of NNO (552 cm(-1)), bending of NCO (610 cm(-1)), in-plane bending of NNO (635 cm(-1)), and in-plane bending of ONO and OCO (712 cm(-1)) exhibit slight hardening before and after the elastic anomaly. However, the asymmetric stretching modes at 1574 cm(-1) (asymmetric stretch of NO) and 1590 cm(-1) (asymmetric stretch of NO2) exhibit hardening below 20 degrees C but soften after 80 degrees 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 -50 degrees C and even observed at 150 degrees 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.

Automated summary

Temperature-dependent Brillouin and Raman spectroscopic investigations were conducted on energetic material TEX to study its elastic and phonon mode behaviors, revealing rapid reduction in elastic modulus and anomalous softening behavior of Raman bands between 20 to 80 degrees C. The high bulk modulus of TEX contributes to its lower sensitivity, but the decrease in elastic modulus may result in increased sensitivity just above room temperature.