Phase Confirmation and Equation of State of β‑HMX under 40 GPa

Phase transitions of the β-HMX crystal have been systematically studied via Raman, mid-infrared, and X-ray diffraction techniques under high pressures of up to 40 GPa. When using a neon pressure transmitting medium, we observed four phase transitions, including β-phase to ζ-phase at 5.4 GPa, ε-phase at 9.6 GPa, φ-phase at 21.6 GPa, and further to η-phase at 35.0 GPa. According to the high-pressure Raman spectra, the first two phase transitions of HMX are induced by the changes of the NO₂ groups and ring, whereas the ε to φ phase transition is attributed to the mutation of the CH₂ groups and ring. Under higher pressures, another newly discovered phase transition from φ to η occurred at 35.0 GPa. Based on the analysis of high-pressure X-ray diffraction results, all currently observed high-pressure phases are isostructures of monoclinic HMX, and no abrupt change in the volume has been found in the volume–pressure curve below 38 GPa. These are consistent with the isentropic compression results and the conclusion of the first-principles calculation. The present work clarifies the controversial pressure-induced phase transitions for β-HMX and the newly discovered phase transitions under high pressure.