Pressure-Induced Phase Transition of β-RDX Single Crystals

Previous investigations had revealed that beta-RDX facilely transforms to alpha-RDX due to the poor crystalline quality. This hindered obtaining insights into the beta-RDX pressure behavior. In this work, phase transition of beta-RDX single crystals with high quality was studied via Raman spectra and Fourier transform infrared (FTIR) spectra under high pressures up to 22 GPa. According to high-pressure Raman studies, the pressure-induced molecular deformations resulted in the lowering of the molecular symmetry. beta-RDX first transforms to epsilon'-RDX around 0.9 GPa, close to epsilon-RDX at high temperature and high pressure. The changes in N-N-C bond angles, the dihedral angles of C-C-N-N as well as the H-C-H and O-N-O angles were calculated using the density functional theory (DFT) method. The different changes in these three CH2 and nitro groups indicate the variations in hydrogen-bond interactions under high pressure, while the changes in N-N-C bond angles, the dihedral angles of C-C-N-N, and the new N-O and N-N stretching modes prove the transformation of conformation of beta-RDX at about 6 GPa. No further phase-transition behavior was observed with further loading pressure; however, the increasing pressure promoted the distortion of the ring. Except for the mutation of CH2 groups, all of the changes in beta-RDX are reversible when released back down to the ambient pressure.