Investigation of direct and indirect phonon-mediated bond excitation in α-RDX

Abstract

We evaluate whether lattice or internal phonons dominate the thermal excitation of the N-N bonds in α-cyclotrimethylene trinitramine (α-RDX) by computing the fractional contributions of phonon modes to the excitation of all atomic interactions. We derive a method to compute these contributions, which we call mode energy fractions, from the phonon eigenvectors and a splitting of the dynamical matrix. This enables identification of phonon modes that most strongly excite the N-N bonds that play a key role in molecular decomposition of α-RDX. Correlating these fractions with the mode populations and contributions to the specific heat and thermal conductivity, we analyze how thermal energy is distributed by phonons following a passing shock. Contrary to the common explanation that thermal energy is transferred to the N-N bonds indirectly, by internal phonons, we find that lattice phonons dominate this thermal energy transfer, implying that energy flow follows a direct route. We also comment on implications of these results for non-shock decomposition of α-RDX.

Contribution of molecular flexibility to the elastic–plastic properties of molecular crystal α-RDX

“…The force field used for all models is the Smith-Bharadwaj (SB) potential [19], which was developed using quantum chemistry calculations to reproduce structural properties for cyclotetramethylene tetranitramine (HMX) and dimethyl dinitro methyldiamine (DDMD). This potential can reproduce elastic and thermal properties of RDX [17,20], and has been used to study crystal nanomechanics [18], high pressure states [17], melting [21], vibrational properties [9], absorption spectra [22], phase transformations during shock [23], dislocations [4,[24][25][26][27], and rotational defects [5].…”

“…Phonon density of states for Models I-V, indicating the effect of limiting molecular flexibility on the phonon spectrum. The peaks are associated with specific bonds and angles, as indicated in red and blue, respectively [9]. The peaks near 2940, 2865, 2110 and 1760 cm −1 disappear for Models II and III (denoted by MII and MIII).…”

“…modes (bond and angle deformations) that contribute to the peaks in the VDOS plot are indicated in figure 8 (referred from [9]). Figure 8 shows a comparison of the VDOS computed with Models I-V.…”

Structural and Vibrational Properties of Crystalline β‐Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine at High Temperatures: Ab Initio Molecular Dynamics Studies

“…To study low temperature effect on vibrational properties, we compared the vibrational modes of HMX at 300, 534, 608, and 873 K. Figure A shows the Raman spectra of HMX at different temperatures. In the crystalline HMX at 0 K, the vibrational frequencies can be assigned as follows: cycle vibrations at 94, 213, 350, and 400 cm −1 and vibrations within the cycle (rattling mode) observed at 229, 367, 405, and 423 cm −1 . Cycle and nitro group (‐NO 2 ) deformation vibrations are at 571, 619, 713, 809, and 913 cm −1 , while the acoustic E 1g mode is at 604, 637, and 720 cm −1 .…”

Bond Strain and Rotation Behaviors of Anharmonic Thermal Carriers in ‐RDX

“…They proposed an indirect multiphonon up‐pumping mechanism to explain how long wavelength energy can scatter and allow energy to manifest in small wavelength phonons associated with N−N bond vibrations. Though this mechanism is now largely accepted, direct evidence has been difficult to show and some doubts regarding how the phonons can scatter to enable the mechanism have not been completely resolved . Questions have thus persisted about whether systematic phonon displacements along a well‐defined pathway could ultimately culminate in N−N bond scission.…”