Researchers compressed nitrogen and hydrogen mixtures between 134 and 139 gigapascals to synthesize a polymeric nitrogen–hydrogen solid, tackling a longstanding instability barrier in high-energy materials research, AZoM reported.
Polymeric nitrogen networks release substantial energy when converting into molecular nitrogen due to a large bonding state difference. Practical use has stalled because these materials require extreme synthesis pressures and lack structural stability during decompression. Prior experiments with nitrogen-hydrogen mixtures produced only amorphous solids and molecular compounds rather than extended polymeric chains.
Synchrotron X-Ray Diffraction Resolves Hydrogen Positions
To overcome past limitations, the team loaded four diamond anvil cells with high-purity gas mixtures and applied laser heating at pressures up to 139 GPa. Using synchrotron single-crystal X-ray diffraction at the European Synchrotron Radiation Facility and Deutsches Elektronen-Synchrotron, the researchers mapped the resulting N3H2 crystal structure.
“Here, we report the high-pressure synthesis of polymeric N3H2 at 134(3)–139(1) GPa from N2–H2 mixtures,” the study published in Nature Communications stated.
The diffraction data allowed the team to directly resolve hydrogen positions at 139 GPa, extending single-crystal X-ray diffraction localization far beyond the previous limit of about 25 GPa. Raman spectroscopy and density functional theory calculations supported the structural findings.
Hydrogen Bonds Stabilize Single-Bonded Nitrogen Chains
Structural analysis showed that N3H2 features a fully single-bonded nitrogen network arranged in puckered six-membered rings with chair-like conformations. Nitrogen single bonds link these rings into infinite one-dimensional chains, where atoms adopt trigonal pyramidal coordination geometries consistent with sp³ hybridization.
Hydrogen bonds connect the individual chains, with each N6H4 unit donating four and accepting four hydrogen bonds. This extended network cross-links the polymeric chains and provides the structural stability previously absent in pure polymeric nitrogen allotropes.
Decomposition Energy Estimates Exceed Conventional Explosives
Calculations show the compound has an estimated ambient-pressure decomposition energy of 7.98 kJ/g based on the breakdown reaction into molecular nitrogen and ammonia. That calculated figure exceeds reported energy densities for standard materials like TNT at 4.2 kJ/g and HMX at 5.7 kJ/g.
The material also displayed an electronic band gap of approximately 3.75 eV at 139 GPa and a bulk modulus of 31 GPa. Decompression measurements down to 58 GPa via X-ray diffraction and 26 GPa via Raman spectroscopy showed smooth changes without phase transitions until the diamond anvils broke.
Synthesizing N3H2 at High Pressure
What pressure range successfully produced the N3H2 compound?
Successful synthesis of N3H2 occurred between 134 and 139 gigapascals using diamond anvil cells and laser heating. Laser heating at 110 GPa and below only decomposed the amorphous azane solids rather than forming the polymeric structure.
How do hydrogen bonds affect the crystal structure?
Hydrogen bonds cross-link the one-dimensional chains formed by single-bonded nitrogen rings. Each N6H4 unit is part of a framework where chains are cross-linked through hydrogen bonds.
Can N3H2 be recovered at ambient conditions?
Lattice-vibration calculations indicate the crystal is stable at 30 GPa but unstable at 0 GPa, where the hydrogen bonds and intrachain nitrogen bonds weaken. Further research is required to improve recoverability at lower pressures.
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