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

The pressure-induced\nstructural phase transitions of ε- and γ-CL-20 were studied\nby Raman and mid-infrared spectroscopy up to 60 GPa. In this work,\nthe phase transition of CL-20 from the ε-phase to the γ′-phase\nstarts at 0.9 GPa and ends at 4.4 GPa. The γ′-phase in\nthis work is distinctly different from the γ-phase recognized\nby the energetic community in terms of the structure and properties.\nSubsequently, the η phase starts at 6.9 GPa and ends at 10.6\nGPa because of the slight cage distortion. With\nfurther increase of the loading pressure, two new phases, φ\nand ι, were observed at 28 and 50 GPa, respectively. The infrared\nresults are consistent with Raman results and show that similar phase\nregions are observed for CL-20 under high pressures. The behavior\nof the γ-phase under pressure indicates that the ζ-phase\nappears at 1.3 GPa and sustains its stability up to 47.4 GPa. The\ncurrent results prove that the newly discovered γ′-phase\nis evidently distinct from the γ-phase and they undergo different\nphase transition routes under loading compression.