Equation of state, phase transition, decomposition of β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) at high pressures

Pressure-volume relations and vibrational Raman spectra of unreacted HMX (octahydro-1, 3,5,7-tetranitro-1,3,5,7-tetrazocine) have been obtained in both quasihydrostatic conditions to 45 GPa and nonhydrostatic conditions to 10 GPa by using diamond-anvil cell, angle-resolved synchrotron x-ray diffraction, and micro-Raman spectroscopy. The results show that the high-pressure behavior of HMX strongly depends on the stress conditions. HMX is more compressible in hydrostatic conditions (⁠$B0=12.4 GPa$ and $B′=10.4$⁠) than in nonhydrostatic conditions (⁠$B0=14.4 GPa,$ $B′=13.3$⁠). This discrepancy in HMX compressibility can be explained in terms of chemical reactions occurring in nonhydrostatic conditions. The static isotherm is in good agreement with the shock Hugoniot, suggesting little temperature effect on the pressure–volume relation. The hydrostatic data suggest that β(monoclinic)-HMX undergoes two phase transitions: (i) a conformational transition at 12 GPa with no apparent abrupt volume change and (ii) a discontinuous one at 27 GPa with a 4% volume change. At 40 GPa, the b and c axes become nearly identical with the $c/a$ ratio 1.62 and β=123°, approaching a nearly close-packed structure.