Ammonium Azide under High Pressure: A Combined Theoretical and Experimental Study

Abstract

Efforts to synthesize, characterize, and recover novel polynitrogen energetic materials have driven attempts to subject high nitrogen content precursor materials (in particular, metal and nonmetal azides) to elevated pressures. Here we present a combined theoretical and experimental study of the high-pressure behavior of ammonium azide (NH4N3). Using density functional theory, we have considered the relative thermodynamic stability of the material with respect to two other crystal phases, namely, trans-tetrazene (TTZ), and also a novel hydronitrogen solid (HNS) of the form (NH)4, that was recently predicted to become relatively stable under high pressure. Experimentally, we have measured the Raman spectra of NH4N3 up to 71 GPa at room temperature. Our calculations demonstrate that the HNS becomes stable only at pressures much higher (89.4 GPa) than previously predicted (36 GPa). Our Raman spectra are consistent with previous reports up to lower pressures and at higher pressures, while some additional subtle behavior is observed (e.g., mode splitting), there is again no evidence of a phase transition to either TTZ or the HNS.

Phase stability and lattice dynamics of ammonium azide under hydrostatic compression

“…Recent Raman spectroscopic measurements reveal that the HNS-1 will be formed at even much higher pressures (> 71 GPa) and hence the transition pressures obtained using NCPP including dispersive interactions are in good agreement with the experimental observations. 21 Medvedev et al 17 determined the pressure at which polymorphic structural phase transition from phase I to II occurs at around 3 GPa and phase II consists of NH 4 and N 3 ions similar to phase I with a difference that both of the azide groups must occupy equivalent crystallographic positions to be consistent with observations from Raman spectroscopy. This is further confirmed by the single crystal X-ray diffraction study and the phase II is temporarily assigned as tetragonal phase.…”

Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure

“…Gas Phase Figure 6: Calculated Raman spectra of (NH 4 )(N 5 ) as a function of pressure, the mode assignments are given at 28.6 GPa. synthesis, the Raman spectra are calculated for both crystals as a function of pressure, see for ammonium azide at high pressures 23 , where the symmetric mode is much more intense than any other NH 4 stretching modes. The calculated frequencies are also slightly higher than those measured by Crowhurst et al These differences in the frequency and intensity of the H-N stretching modes is likely due to the different crystalline environment of ammonium pentazolate.…”

“…However, a conclusive identification of the type of new nitrogen oligomers or extended networks as well as the determination of the crystal structure have not been made. A hydronitrogen precursor ammonium azide (NH 4 )(N 3 ), containing both H and N, has not shown any signs of chemical transformation upon compression up to 70 GPa 23,27 .…”

Transformation of Ammonium Azide at High Pressure and Temperature

“…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]. Synthesizing polynitrogen compounds from the sole compression of AA would thus require extreme pressures in the 100 GPa range and is, therefore, not a practical route.…”

“…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 ].…”