Thermal analysis of the phases of HMX using X-ray diffraction

Anisotropic structure deformation of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine under high pressure: vibration spectra calculation and resolution based on AIMD simulation

The phase transition of the β-HMX crystal has been widely studied under high pressure, but the microscopic transition mechanism is not sufficiently understood. In this article, we perform a series of ab initio molecular dynamics simulations focusing on structure deformation and the corresponding vibration spectra resolution of β-HMX at 0-40 GPa. Several typical pressure-induced phase transition processes are confirmed by analyzing the chemical bond, dihedral angle, charge transfer, and IR and Raman spectra. The corresponding relationship between molecular structure and spectral signal is constructed through the partial spectra calculations of special functional groups within the HMX molecule. The anisotropic effects of different groups on the initial structural phase transition are uncovered. The equatorial C-N and axial N-N bonds have the largest compression ratio as pressure increases, which is the intrinsic factor for the initiation of structure transformation. The C-N molecular ring plays an important role in the entire phase transition process. In addition, the phase transition of β → ζ is also closely related to the deformation of NO2, while that of ζ → ε is induced by the axial N-NO2 group. Regarding the higher-pressure phase transition, the synergetic effect of N-NO2, CH2 groups, and molecular rings becomes more considerable.

Tracking Thermal Decomposition Chemistry in Secondary Solid Explosives with X-ray Diffraction

We present an approach for measuring thermal decomposition kinetics in crystalline solids using X-ray diffraction to track the loss of crystallinity that accompanies condensed phase decomposition chemistry. We apply this method to systems for which extracting thermodynamic parameters has been historically difficult: organic molecular crystals that thermally decompose below their melting points, such as solid explosives. To demonstrate this method, we measured the rate of solid, thermal decomposition versus temperature in three different secondary solid explosives and the sugar fructose. In all cases, we observed an acceleration in the thermal decomposition rate with increasing temperature, which forms a vertical asymptote, a phenomenon known as melt acceleration. We show that observing the vertical asymptote in the thermal decomposition rate allows for identifying the thermodynamic melting point, which is not trivial to determine when melting and thermal decomposition happen simultaneously. We expect this method to be useful for studying thermal decomposition and for extracting thermodynamic data for secondary solid explosives, data that are needed for modeling and understanding faster phenomena, such as detonation. We also expect this method to be relevant to other organic molecular crystals in which thermal decomposition and melting overlap, such as sugars or pharmaceuticals.