By squeezing nitrogen and hydrogen above 130 GPa and probing the product atom by atom, researchers tested whether hydrogen could help overcome one of polymeric nitrogen’s biggest limitations.
Paper: High-pressure synthesis of a polymeric N3H2 solid stabilized by hydrogen bonds. Image Credit: Stock_wichel / Shutterstock
*Important notice: This news reports on an unedited version of an accepted paper and is awaiting final editing. Therefore, the paper should not be regarded as conclusive or treated as established information.
A study published in the journal Nature Communications reports the synthesis of a polymeric nitrogen–hydrogen compound, N3H2, under extreme pressure.
The findings provide the first experimental realization of a polymeric nitrogen–hydrogen structure and show that hydrogen bonds cross-link its nitrogen chains. The compound has a calculated ambient-pressure decomposition energy of 7.98 kJ/g, higher than the values cited for TNT and HMX.
Exploring Hydrogen-Stabilized Polymeric Nitrogen
Polymeric nitrogen has attracted considerable interest as a high-energy-density material because single-bonded nitrogen networks can release large amounts of energy when they convert into strongly triple-bonded molecular nitrogen (N2). The large energy difference between these bonding states drives that release.
The formation of nitrogen gas during decomposition also makes polymeric nitrogen compounds of interest for propulsion research.
Practical applications remain limited by the extreme pressures required for synthesis and the difficulty of retaining structural stability during decompression.
Previous studies have suggested that incorporating hydrogen could help overcome this limitation by passivating reactive nitrogen sites and stabilizing the polymeric structure. Experimental investigations of nitrogen–hydrogen mixtures, though, had produced only molecular compounds and amorphous nitrogen–hydrogen solids, without establishing an extended polymeric structure.
To address this gap, the team studied the nitrogen–hydrogen system at pressures of up to 139 GPa. Successful synthesis of N3H2 occurred only at 134(3)–139(1) GPa; laser heating at 110(3) GPa and below instead decomposed the amorphous azane solids. The aim was to characterize an unexplored hydrogen-containing polymeric nitrogen compound and determine how hydrogen contributes to its stability.
High-Pressure Synthesis and Structural Characterization
Four diamond anvil cells (DACs) were loaded with high-purity nitrogen–hydrogen gas mixtures containing different elemental ratios. The samples were compressed to pressures of up to 139 GPa and laser-heated to trigger chemical reactions. The team used ytterbium fiber or CO2 lasers in different experimental setups. The synthesis was reproduced in a second DAC at 134(3) GPa.
To determine the crystal structure, the team conducted synchrotron single-crystal X-ray diffraction (SCXRD) measurements at the European Synchrotron Radiation Facility (ESRF) and Deutsches Elektronen-Synchrotron (DESY). They also used Raman spectroscopy to examine the material's chemical bonding and vibrational properties.
The researchers complemented these experiments with density functional theory (DFT) calculations to relax the crystal structure and investigate its electronic and mechanical properties. They also employed ab initio random structure searching (AIRSS) to identify energetically favorable atomic arrangements.
These experimental and computational approaches enabled a detailed characterization of N3H2 and its properties across a wider pressure range.
Revealing the Polymeric Structure and Its Stability
Structural analysis showed that N3H2 contains a fully single-bonded nitrogen network. The nitrogen atoms form puckered six-membered N6 rings with a chair-like conformation. Nitrogen–nitrogen single bonds connect these rings into infinite one-dimensional chains along the crystallographic c-axis. The nitrogen atoms adopt trigonal pyramidal coordination geometries, consistent with sp³ hybridization.
Hydrogen bonds connect the individual chains and play a central role in stabilizing the structure. Each N6H4 unit donates four and accepts four hydrogen bonds, forming eight hydrogen bonds in total and connecting to six neighboring units in six separate chains. This arrangement creates an extended hydrogen-bonding network that connects the polymeric chains.
The team directly located hydrogen positions in the diffraction data at 139 GPa. Hydrogen scatters X-rays weakly because it has only one electron, making it challenging to detect at extreme pressures.
Recent advances in synchrotron brightness and detector sensitivity allowed the researchers to identify localized residual electron density consistent with hydrogen positions. The N–H bond length was fixed at 1.07(2) Å during refinement, and the assignment was independently supported by crystal chemistry, DFT, and AIRSS calculations.
Incorporating hydrogen atoms into the structural refinement also improved the agreement between the experimental data and the structural model.
The team also examined the electronic and mechanical properties of N3H2. DFT predicted an electronic band gap of approximately 3.75 eV at 139 GPa. The calculated bulk modulus was 31 GPa, indicating that N3H2 is more compressible than pure polymeric nitrogen. The team also evaluated the material's energy-release potential. The estimated decomposition energy was 7.98 kJ/g at ambient pressure for the reaction:
6N3H2 → 7N2 + 4NH3
This value exceeds the reported energy densities of TNT (4.2 kJ/g) and HMX (5.7 kJ/g). The figure is a computational estimate, not an experimentally measured energy release.
Pressure-dependent measurements provided further insight into the compound's structural stability. During decompression, X-ray diffraction data were collected down to 58(1) GPa, while the researchers tracked its characteristic Raman modes down to 26(3) GPa.
The smooth changes in the diffraction patterns and vibrational frequencies showed no evidence of a phase transition over the respective pressure ranges measured by X-ray diffraction and Raman spectroscopy.
Both decompression runs ended when the diamond anvils broke, and the samples escaped.
Calculations comparing the energies of competing phases found N3H2 to be energetically favored relative to those phases at and above 100 GPa. At lower pressures, the NH4–N5 phase, which has the same overall N ratio, becomes energetically preferred down to 20 GPa.
Separate lattice-vibration calculations found the N3H2 crystal stable at 30 GPa but unstable at 0 GPa, where hydrogen bonds and an intrachain N–N bond weaken.
Stability and Possible Applications
The study demonstrates the successful synthesis of a polymeric nitrogen–hydrogen compound under extreme pressure conditions. Its structure combines covalent nitrogen chains with an extended hydrogen-bonding network, providing experimental evidence that hydrogen can stabilize polymeric nitrogen–hydrogen structures at high pressure.
The experiment extends SCXRD localization of individual hydrogen atoms far beyond the previously reported limit of about 25 GPa.
Directly resolving hydrogen positions through synchrotron single-crystal X-ray diffraction at 139 GPa shows that modern synchrotron facilities can characterize hydrogen-containing compounds at pressures that were previously inaccessible. This capability could support investigations of other light-element materials formed under extreme conditions.
Further research should focus on improving structural stability and recoverability at lower pressures. The researchers suggest that interactions stronger than hydrogen bonding might help stabilize polymeric nitrogen materials during decompression and improve their recoverability.
Nitrogen–hydrogen compounds could influence the chemical composition and phase stability of materials within icy planetary interiors, where high pressures and temperatures prevail. The authors caution that N3H2 may be limited in abundance in such interiors because many planets have low N/H ratios and ammonia remains stable across the pressure range studied.
Its demonstrated existence still changes the phase relationships that must be considered in multicomponent planetary materials. This work expands the experimentally established chemistry of the nitrogen–hydrogen system and provides a basis for studying new hydronitrogen materials with stronger interactions and better recoverability.
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Source:
- Ranieri, U., Geneste, G., et al. (2026). High-pressure synthesis of a polymeric N3H2 solid stabilized by hydrogen bonds. Nature Communications. DOI: 10.1038/s41467-026-79382-z, https://doi.org/10.1038/s41467-026-79382-z