High-pressure, high-temperature phase diagram and equations of state of RDX polymorphs

High-explosive crystals pass through a range of pressure-temperature conditions on the path to shock-induced detonation. Accurate identification of phase boundaries is essential to predicting the phases involved in initiation and at the leading edge of the detonation front. RDX (cyclotrimethylenetrinitramine) is a commonly-used explosive that is known to have a complex phase diagram to 10 GPa and 200 C. At ambient temperatures, alpha-RDX transforms to gamma-RDX around 4 GPa, further transforming to delta-RDX around 16-18 GPa. At high temperatures, both alpha-RDX and gamma-RDX transform to the epsilon phase, but the location of the alpha-gamma-epsilon triple point has been debated. Here, we explore the high-pressure, high-temperature phase diagram of RDX to 25 GPa and 150 C in a diamond anvil cell with in situ synchrotron X-ray diffraction (XRD). We confirm the alpha-to-gamma phase transition around 4 GPa at all temperatures explored, and the gamma-to-delta phase transition around 16 GPa. We did not observe the epsilon phase of RDX up to 150 C, more tightly constraining the alpha-gamma-epsilon triple point for this material. However, we did observe some time-dependent decomposition at this temperature. These data were used to inform thermal equations of state of all three phases explored.