6 Materialchemie
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The ease with which an energetic material can be initiated by mechanical impact is a critical parameter directing material safety and application. While impact sensitivity metrics are traditionally derived experimentally, recent developments have highlighted that the phenomenon is amenable to first principles simulation. In this chapter, we will outline a fully ab initio approach to predict the relative impact sensitivities of energetic materials based on the mechanochemical principles that link the impact event to vibrational energy transfer.
This mechanism is key to rationalizing how a mechanical impact—which deposits energy into the low-frequency lattice vibrations—results in a molecular response. By simulating the vibrational energy levels (the so-called phonon density of states, PDOS) using first-principles computational methods (typically dispersion-corrected plane-wave density functional theory, PW-DFT) we can calculate the relative rate of energy propagation from the delocalized low-energy lattice vibrations through to the localized molecular modes. The latter traps the energy, which eventually results in bond rupture through heightened vibrational excitation.
This method, based on vibrational up-pumping, offers a route toward predicting the impact sensitivities of a broad range of energetic materials, provided the crystal structure of the compound (or salt or co-crystal) is known. While it does not offer insight into the sensitizing roles undoubtedly played by crystal defects or grain boundaries, it does provide a level of understanding at the molecular and crystal packing levels. Correspondingly, this approach offers a feedback mechanism to chemists and materials scientists to guide the design of new materials with desired impact sensitivity behavior.
The impact sensitivity (IS) of FOX-7 polymorphs is predicted by phonon up-pumping to decrease as layers of FOX-7 molecules flatten. Experimental validation proved anomalous owing to a phase transition during testing, raising questions regarding Impact sensitivity measurement and highlighting the need for models to predict IS of polymorphic energetic materials.
The reliable determination of gas-phase and solid-state heats of formation are important considerations in energetic materials research. Herein, the ability of PM7 to calculate the gas-phase heats of formation for CNHO-only and inorganic compounds has been critically evaluated, and for the former, comparisons drawn with isodesmic equations and Atom equivalence methods. Routes to obtain solid-state heats of formation for a range of singlecomponent molecular solids, salts, and co-crystals were also evaluated. Finally, local vibrational mode analysis has been used to calculate bond length/force constant curves for seven different chemical bonds occurring in CHNO-containing molecules, which allow for rapid identification of the weakest bond, opening up great potential to rationalise decomposition pathways. Both metrics are important tools in rationalising the design of new energetic materials through computational screening processes.
Single crystals which exhibit mechanical flexibility are promising materials for advanced technological applications. Before such materials can be used, a detailed understanding of the mechanisms of bending is needed. Using single crystal X-ray diffraction and microfocus Raman spectroscopy, we study in atomic detail the high-pressure response of the plastically flexible coordination polymer [Zn(μ-Cl)2(3,5-dichloropyridine)2]n (1). Contradictory to three-point bending, quasi-hydrostatic compression of (1) is completely reversible, even following compression to over 9 GPa. A structural phase transition is observed at ca. 5 GPa. DFT calculations show this transition to result from the pressure-induced softening of low-frequency vibrations. This phase transition is not observed during three-point-bending. Microfocus synchrotron X-ray diffraction revealed that bending yields significant mosaicity, as opposed to compression. Hence, our studies indicate of overall disparate mechanical responses of bulk flexibility and quasi-hydrostatic compression within the same crystal lattice. We suspect this to be a general feature of plastically bendable materials.