Crystal Structure and Stability of Ammonium Azide Under High Pressure

Abstract

Due to its potential applications as a high-energy density material, the high-pressure polymorphs of ammonium azide (AA) have received much attention recently. However, the crystal structure of phase Ⅱ (AA-II), stable above 3.0 GPa, has remained elusive until now. By combining X-ray diffraction and Raman experiments with first principle calculations, we determine that AA-II is a hydrogen(H)-bonded structure of ammonium and azide ions with monoclinic symmetry, space group P2/c. The latter is comprised of alternating molecular layers of ammonium and azide ions and mostly differs from AA-I by its denser packing of molecular planes while preserving the hydrogen bond network. First-principle calculations show that phase Ⅱ has the lowest enthalpy among all other considered structures from 4.9 to 102.6 GPa, pushing the phase transitions to the previously predicted hydro-nitrogen solids to higher pressures. Raman data to 85.0 GPa at room temperature confirm the absence of phase transition and agree very well with the pressure evolution of the Raman modes of AA-II predicted by our calculations.

Transformation of Ammonium Azide at High Pressure and Temperature

“…We first investigated AA embedded in N compressed up to 30 GPa at room temperature using Raman spectroscopy. We observed no sign of phase transition in the Raman spectrum, the latter being consistent with previous measurements on pure AA and AA embedded in argon [ 12 , 14 ]. We then prepared three samples at pressures of about 15 GPa, 20 GPa, and 30 GPa, and studied them as a function of temperature in the range 300–700 K. The followed P – T paths are illustrated in Figure S2 of the SI.…”

“…A membrane diamond anvil cell (MDAC) was used to generate high pressures [ 19 ], equipped with diamond anvils of culet size of 300 m. Re gaskets of initial thickness of 200 m were preindented to a thickness of 35 m, and a hole of diameter of 95 m was laser-drilled at the center of the indent in order to serve as the sample chamber. A piece of AA sample of lateral size 40 to 50 m was loaded in the gasket hole and embedded in N by cryogenic loading of N , in the same fashion as our previous study using liquid argon [ 14 ]. The Raman spectrum of the AA sample after loading was compared to those that were measured in our previous study [ 14 ] and confirmed the absence of impurities, as shown in Figure S1 of the Supporting Information file (SI).…”

“…A piece of AA sample of lateral size 40 to 50 m was loaded in the gasket hole and embedded in N by cryogenic loading of N , in the same fashion as our previous study using liquid argon [ 14 ]. The Raman spectrum of the AA sample after loading was compared to those that were measured in our previous study [ 14 ] and confirmed the absence of impurities, as shown in Figure S1 of the Supporting Information file (SI).…”

“…Figure 2 shows the evolution of the Raman mode frequencies with pressure at 300 K, and compares them to those of the pure AA [ 14 ], N [ 32 ], NH [ 33 , 34 ] and hydrazine (N H ) [ 35 ] solids obtained from literature. We recall that NH (and possibly heavier, unidentified azanes) and N H (below 10 GPa on decompression) were identified as reaction products of N + H mixtures at high pressure [ 3 , 6 , 8 , 9 ], which justifies the present comparison.…”

“…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.…”