Structure, Bonding,\nand Vibrational Dynamics of a\nTriamine High Energy Density Material under Pressure

High energy density materials have complex intermolecular\ninteractions\nwhich influence their stability and performance. We used a combination\nof synchrotron X-ray diffraction, synchrotron infrared spectroscopy,\nand Raman vibrational spectroscopy, supplemented by density functional\ntheory calculations, to probe pressure-induced changes in structure\nand intermolecular interactions of 1H,4′<i>H</i>-[3,3′-bis(1,2,4-triazole)]-4′,5,5′-triamine\nas a model high energy density material up to 40 GPa. We find that\ncompression of the triamine is accompanied by increased intermolecular\ninteractions that give rise to an interesting evolution of the structure\nand bonding with pressure. Analysis of the equation of state determined\nfrom the X-ray diffraction indicates a change in compression mechanism\nnear 19 GPa consistent with changes in vibrational spectra that provide\nevidence for a structural rearrangement associated with changes in\nhydrogen bonding near that pressure. The overall compressional behavior\ncalculated theoretically agrees with that observed experimentally\nthough differences are found that indicate the need for improved treatment\nof the intermolecular interactions including hydrogen bonding under\npressure.