Characterization of the Isothermal Compression Behavior of LLM-172

The high-pressure behavior of 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (LLM-172) has been studied to 36 GPa by Raman spectroscopy and 50 GPa by X-ray diffraction. The Raman spectra and calculated unit-cell volumes at select pressures show reasonable qualitative agreement with first-principles density functional theory calculations. Raman peaks exhibit a gradual broadening and loss of intensity upon compression to near 20 GPa. Above 20 GPa, most Raman features disappear with the exception of modes associated with the skeletal ring modes. These modes were found to persist (although with low intensity) to 36 GPa. Because these modes exhibit very low compressibility over the pressure range studied, it is speculated that the ring structure is very stable. The X-ray diffraction suggests that while the crystal maintains an orthorhombic structure to near 40 GPa, it gradually undergoes a decomposition/amorphization beginning near 10 GPa. Analysis of the Raman results suggests that decomposition proceeds through isomerization, which leads to the formation of a C-O-N-O group rather than ring cleavage.