Phase Transition Routes for ε- and γ-CL-20 Crystals under High Pressures of up to 60 GPa

The pressure-induced structural phase transitions of ε- and γ-CL-20 were studied by Raman and mid-infrared spectroscopy up to 60 GPa. In this work, the phase transition of CL-20 from the ε-phase to the γ′-phase starts at 0.9 GPa and ends at 4.4 GPa. The γ′-phase in this work is distinctly different from the γ-phase recognized by the energetic community in terms of the structure and properties. Subsequently, the η phase starts at 6.9 GPa and ends at 10.6 GPa because of the slight cage distortion. With further increase of the loading pressure, two new phases, φ and ι, were observed at 28 and 50 GPa, respectively. The infrared results are consistent with Raman results and show that similar phase regions are observed for CL-20 under high pressures. The behavior of the γ-phase under pressure indicates that the ζ-phase appears at 1.3 GPa and sustains its stability up to 47.4 GPa. The current results prove that the newly discovered γ′-phase is evidently distinct from the γ-phase and they undergo different phase transition routes under loading compression.