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

The pressure-induced structural phase transitions of epsilon- and gamma-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 epsilon-phase to the gamma'-phase starts at 0.9 GPa and ends at 4.4 GPa. The gamma'-phase in this work is distinctly different from the gamma-phase recognized by the energetic community in terms of the structure and properties. Subsequently, the eta-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, phi and l, 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 gamma-phase under pressure indicates that the zeta-phase appears at 1.3 GPa and sustains its stability up to 47.4 GPa. The current results prove that the newly discovered gamma'-phase is evidently distinct from the gamma-phase and they undergo different phase transition routes under loading compression.