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

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.