Vibrational spectroscopy has been used to investigate the room-temperature high-pressure phases of the energetic material hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The pressure-induced alterations in the spectral profiles were studied in a compression sequence to 30.2 GPa using Raman spectroscopy and to 26.6 GPa using far-infrared spectroscopy. At pressures near 4.0 GPa, several changes become immediately apparent in the Raman spectrum, such as large frequency shifts, mode splittings, and intensity changes, which are associated with a phase transition from alpha-RDX to gamma-RDX. Our study extends the kinetic stability of gamma-RDX to pressures near 18.0 GPa. Evidence for a new phase was found at pressures between 17.8 and 18.8 GPa and is based on the appearance of new vibrational bands and associated changes in intensity patterns. The new phase has vibrational characteristics that are similar to those of beta-RDX, suggesting the two polymorphs share a related crystal structure.

The high‐pressure response of ε‐2,4,6,8,10,12‐hexanitrohexaazaisowurtizane (CL‐20) has been examined to 27 GPa in diamond anvil cells using vibrational spectroscopy. The results reveal evidence of an ε→γ pressure‐induced phase transition between 4.1 and 6.4 GPa and suggest the existence of a γ→ζ transition near 18.7 GPa. Several Raman and infrared frequencies were found to decrease in intensity as the phase boundaries are approached. An anomalous intensity increase was noted in the CNC infrared mode that is believed to result from an increase in the Raman cross‐section due to a stronger interlayer coupling under pressure.