Ab initio molecular dynamics simulations (Yu et al., 2015) predict that ammonium azide (NH4N3) polymerizes into nitrogen-rich hydronitrogen phases containing one-dimensional chains, branched chains, and five-membered rings at about 60 GPa and 2200 K, or 90 GPa and 1600 K — far milder than the >110 GPa, 2000 K required for polymeric nitrogen from molecular N2 — and that hydrogen-passivated polymeric networks and five-membered rings formed this way survive decompression to ambient conditions in simulation. A decade later this remains entirely computational. Room-temperature compression to 85 GPa shows no reaction, and the only combined high-pressure–high-temperature experiments (up to 30 GPa and 700 K) observe decomposition into N2 and NH3 plus new but structurally unassigned crystalline compounds that persist only down to about 7 GPa on decompression. The open problem is the experimental synthesis and structural characterization of the predicted polymeric nitrogen phases from compressed ammonium azide, determination of the pressure–temperature conditions governing the transformation, and demonstration — or rigorous refutation — of recovery of hydrogen-passivated polymeric nitrogen networks at ambient conditions.

Background

Nitrogen is unique among light elements in having an exceptionally strong triple bond in the diatomic molecule N2 (about 954 kJ/mol), far stronger than the N–N single bond. Materials built from extended networks of single-bonded nitrogen ("polymeric nitrogen") would therefore store large chemical energy (roughly 2.3 eV per atom) that is released when the network collapses to N2, making them attractive high-energy-density materials whose only decomposition product is benign N2. The first polymeric nitrogen phase, cubic gauche nitrogen (cg-N, a three-dimensional network of single N–N bonds with a diamond-like topology), was synthesized from molecular nitrogen only at the extreme conditions of about 110 GPa and 2000 K in a laser-heated diamond anvil cell (DAC — the standard device that compresses micrometre samples between two diamond culets), and it reverts to molecular nitrogen once the pressure is released. Inorganic azides — salts containing the linear azide anion N3−, whose internal N–N bonds are much weaker (about 418 kJ/mol) than the N≡N triple bond — were explored as precursors that could lower the synthesis pressure, since the surrounding cations exert "chemical precompression" on the nitrogen sublattice; sodium azide, for example, forms non-molecular nitrogen near 50–120 GPa, but these polymeric forms also cannot be preserved at ambient pressure. A different stabilization idea is hydrogen passivation: hydrazine-like hydronitrogen molecules are stable at ambient conditions because hydrogen atoms terminate the reactive ends of nitrogen chains. This suggests that a hydrogen-containing azide could yield polymeric nitrogen that is recoverable at ambient conditions. Ammonium azide (NH4N3, "AA"), an ionic crystal of NH4+ and N3− that is over 93 wt% nitrogen, is the prototypical candidate: it contains both azide units to polymerize and hydrogen to passivate the product. Experiments had already established its room-temperature behavior — an orthorhombic (Pmna) phase I transforming near 3 GPa to phase II, with Raman spectroscopy up to 55 GPa showing no polymerization — implying that high temperature is also required. In this context, Yu et al. (2015) performed ab initio molecular dynamics (AIMD — simulation of the ionic motion with electronic structure solved on the fly) and predicted that AA polymerizes into nitrogen-rich hydronitrogen compounds with backbones of one-dimensional chains, branched chains, and five-membered rings, with critical temperatures of about 2200 K at 60 GPa and 1600 K at 90 GPa; annealing at 90 GPa produced a one-dimensional pure-nitrogen periodic chain, and simulated decompression at 300 K suggested that both hydrogen-passivated polymeric networks and five-membered-ring motifs persist to ambient pressure. Related theory (Steele and Oleynik, 2017) additionally predicted that AA mixed with N2 should form ammonium pentazolate (NH4N5), containing the pentazolate cyclo-N5− anion, above about 12.5 GPa. All of these predicted compounds, and above all their survival at ambient conditions, await experimental test.

Problem Statement

Can the predicted pressure- and temperature-induced polymerization of ammonium azide (NH4N3) be realized and characterized experimentally? Specifically: (1) synthesize, under high pressure and high temperature (the predicted window is roughly 60 GPa with about 2200 K, or 90 GPa with about 1600 K, laser-heated diamond anvil cell conditions), polymeric nitrogen phases in ammonium azide — compounds containing extended single-bonded nitrogen backbones such as one-dimensional chains, branched chains, or five-membered rings, with hydrogen passivating the network — and determine their crystal structures by in situ X-ray diffraction and Raman spectroscopy; (2) establish the actual pressure–temperature conditions governing the transformation, including whether polymerization competes successfully against decomposition of AA into N2 and NH3; and (3) determine whether the resulting hydrogen-passivated polymeric networks and five-membered-ring motifs, predicted by decompression simulations to be metastable, can be recovered and remain stable at ambient conditions after pressure release. A complete resolution requires all three parts to be addressed, and each part may be settled either by a positive experimental result or by a rigorous experimental demonstration that the predicted behavior in question does not occur.

References

  1. Hongyu Yu, Defang Duan, Fubo Tian, Hanyu Liu, Da Li, Xiaoli Huang, Yunxian Liu, Bingbing Liu, Tian Cui (2015). Polymerization of Nitrogen in Ammonium Azide at High Pressures. The Journal of Physical Chemistry C 119(45), 25268-25272. DOI: 10.1021/acs.jpcc.5b08595. https://doi.org/10.1021/acs.jpcc.5b08595
  2. Jonathan C. Crowhurst, Joseph M. Zaug, Harry B. Radousky, Bradley A. Steele, Aaron C. Landerville, Ivan I. Oleynik (2014). Ammonium Azide under High Pressure: A Combined Theoretical and Experimental Study. The Journal of Physical Chemistry A 118(38), 8695-8700. DOI: 10.1021/jp502619n. https://doi.org/10.1021/jp502619n
  3. Guozhao Zhang, Haiwa Zhang, Sandra Ninet, Hongyang Zhu, Cailong Liu, Jean-Paul Itié, Chunxiao Gao, Frédéric Datchi (2019). Crystal Structure and Stability of Ammonium Azide Under High Pressure. The Journal of Physical Chemistry C 124(1), 135-142. DOI: 10.1021/acs.jpcc.9b09635. https://doi.org/10.1021/acs.jpcc.9b09635
  4. Guozhao Zhang, Haiwa Zhang, Sandra Ninet, Hongyang Zhu, Keevin Beneut, Cailong Liu, Mohamed Mezouar, Chunxiao Gao, Frédéric Datchi (2020). Transformation of Ammonium Azide at High Pressure and Temperature. Materials 13(18), 4102. DOI: 10.3390/ma13184102. https://doi.org/10.3390/ma13184102
  5. Brad A. Steele, Ivan I. Oleynik (2017). Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure. The Journal of Physical Chemistry A 121(8), 1808-1813. DOI: 10.1021/acs.jpca.6b12900. https://doi.org/10.1021/acs.jpca.6b12900
  6. Miao Zhou, Minghong Sui, Xuhan Shi, Zitong Zhao, Linlin Guo, Bo Liu, Ran Liu, Peng Wang, Bingbing Liu (2020). Lithium Pentazolate Synthesized by Laser Heating-Compressed Lithium Azide and Nitrogen. The Journal of Physical Chemistry C 124(22), 11825-11830. DOI: 10.1021/acs.jpcc.0c03949. https://doi.org/10.1021/acs.jpcc.0c03949