Phase stability and lattice dynamics of ammonium azide under hydrostatic compression

Abstract

We have investigated the effect of hydrostatic pressure and temperature on phase stability of hydro-nitrogen solids using dispersion corrected Density Functional Theory calculations. From our total energy calculations, Ammonium Azide (AA) is found to be the thermodynamic ground state of N 4 H 4 compounds in preference to Trans-Tetrazene (TTZ), Hydro-Nitrogen Solid-1 (HNS-1) and HNS-2 phases. We have carried out a detailed study on structure and lattice dynamics of the equilibrium phase (AA). AA undergoes a phase transition to TTZ at around ∼ 39-43 GPa followed by TTZ to HNS-1 at around 80-90 GPa under the studied temperature range of 0-650 K. The accelerated and decelerated compression of a and c lattice constants suggest that the ambient phase of AA transforms to a tetragonal phase and then to a low symmetry structure with less anisotropy up on further compression. We have noticed that the angle made by Type-II azides with c-axis shows a rapid decrease and reaches a minimum value at 12 GPa, and thereafter increases up to 50GPa. Softening of the shear elastic moduli is suggestive of a mechanical instability of AA under high pressure. In addition, we have also performed density functional perturbation theory calculations to obtain the vibrational spectrum of AA at ambient as well as at high pressures. Further, we have made a complete assignment of all the vibrational modes which is in good agreement with the experimental observations at ambient pressure. Also the calculated pressure dependent IR spectra show that the N-H stretching frequencies undergo a red and blue-shift corresponding to strengthening and weakening of hydrogen bonding, respectively below and above 4 GPa. Intensity of the N-H asymmetric stretching mode B 2u is found to diminish gradually and the weak coupling between NH 4 and N 3 ions makes B 1u and B 3u modes to be degenerate with progression of pressure up to 4 GPa which causes weakening of hydrogen bonding and these effects may lead to a structural phase transition in AA around 4 GPa. Furthermore, we have also calculated the phonon dispersion curves at 0 and 6 GPa and no soft phonon mode is observed under high pressure.

Transformation of Ammonium Azide at High Pressure and Temperature

“…Indeed, the double bond of the azide ion N is weaker than the triple bond of N and, thus, easier to break. Computer simulations have predicted the polymerization of nitrogen in various forms in AA compressed to pressures in excess of 60 GPa [ 11 , 12 , 13 ]. So far, experimental works have investigated AA compressed to 85 GPa at room temperature and did not report any evidence of polymerization [ 12 , 14 ].…”

Crystal Structure and Stability of Ammonium Azide Under High Pressure

“…Various works have investigated the possibility that AA converts to hydro-nitrogen solids (HNS) under high pressure [11][12][13][14] . At ambient pressure, AA adopts a distorted CsCl-type orthorhombic structure with the space group Pmna [15][16][17] , noted AA-I or AA-Pmna hereafter.…”

High pressure structural, elastic and vibrational properties of green energetic oxidizer ammonium dinitramide

“…As illustrated in figure 11a, the far IR lattice mode frequencies which include both NH 4 and N(NO 2 ) 2 oscillations, NO 2 twisting, NH 4 translation, and rotation of NH 4 and N-NO 2 fragments of the ADN molecules are increasing with pressure below 5 GPa and these vibrational modes Due to the presence of hydrogen bonding, it can be expected that the N-H strengthening frequency decreases (red-shift) with increasing pressure and this red-shift leads to a strengthening of hydrogen bonding. 47 Also, our previous studies 48,49 GPa may suggest a structural transition in ADN. Strengthening of hydrogen bonding above 5 GPa reveals that the high-pressure phase has stronger hydrogen bonding nature when compared to the ambient phase.…”