Ammonium azide under high pressure: a combined theoretical and experimental study

Area of Science:

• Materials Science
• High-Pressure Physics
• Computational Chemistry

Background:

• Polynitrogen energetic materials are of significant interest for synthesis and recovery.
• High nitrogen content precursors, such as metal and nonmetal azides, are subjected to elevated pressures.
• Understanding the high-pressure behavior of ammonium azide is crucial for developing novel energetic materials.

Purpose of the Study:

• To investigate the high-pressure behavior of ammonium azide (NH4N3).
• To determine the relative thermodynamic stability of NH4N3 compared to trans-tetrazene (TTZ) and a novel hydronitrogen solid (HNS).
• To experimentally probe phase transitions in NH4N3 under high pressure.

Main Methods:

• Density functional theory (DFT) calculations were employed to assess thermodynamic stability.
• Experimental Raman spectroscopy was used to measure the behavior of NH4N3 up to 71 GPa at room temperature.
• Comparison of experimental results with theoretical predictions and previous studies.

Main Results:

• DFT calculations indicate that the hydronitrogen solid (HNS) becomes stable at much higher pressures (89.4 GPa) than previously predicted (36 GPa).
• Experimental Raman spectra of NH4N3 up to 71 GPa are consistent with prior reports at lower pressures.
• Subtle spectral changes, such as mode splitting, were observed, but no evidence of phase transitions to TTZ or HNS was found.

Conclusions:

• Ammonium azide does not undergo phase transitions to trans-tetrazene or the predicted hydronitrogen solid within the studied pressure range.
• The stability of the hydronitrogen solid is predicted at significantly higher pressures than initially suggested.
• This study provides critical insights into the high-pressure response of ammonium azide, informing the design of novel energetic materials.