In Simple Terms
Researchers have created a new material from nitrogen and hydrogen, but only by squeezing them under enormous pressure. Hydrogen atoms help hold long nitrogen chains together, revealing a new way these elements can be arranged. The material has not been shown to survive at ordinary pressure, so it is not ready for practical use.
Inside a tiny chamber surrounded by diamond tips, researchers pushed nitrogen and hydrogen into conditions far beyond anything found at Earth’s surface. The result was an unusual crystal with the formula N3H2: its nitrogen atoms form long chains, while hydrogen atoms act as bridges between them. The study, by Aureliano Ranieri, Jean Genest, and colleagues, was published in Nature Communications and provides the first experimental evidence that nitrogen and hydrogen can form an extended polymeric structure.
Why Scientists Are Looking for Polymeric Nitrogen
Under ordinary conditions, nitrogen is found mostly as N2 molecules, each made of two atoms joined by a strong triple bond. By contrast, in polymeric materials, nitrogen atoms connect through single bonds to form networks or chains. That distinction matters: if a network of single bonds transforms into N2 molecules with triple bonds, it could release a large amount of energy.
For this reason, scientists study polymeric nitrogen materials as potential high-energy-density systems, and their ability to turn into nitrogen gas has also drawn interest in propulsion research. But the idea faces two major obstacles. Creating these structures requires extreme pressure, and keeping them intact after the pressure is reduced is difficult. Hydrogen has previously been proposed as an element that might calm some reactive nitrogen sites and stabilize the network. However, earlier experiments with mixtures of the two elements did not demonstrate the formation of extended polymeric chains.
The Crystal Forms Only Within a Narrow Pressure Range
The team tested high-purity gas mixtures of nitrogen and hydrogen with different elemental ratios, using four diamond-anvil cells. Each cell traps a tiny sample between two diamond surfaces, allowing it to be exposed to immense pressure. The experiments reached 139 gigapascals, and the samples were heated with lasers to trigger reactions. Depending on the experimental setup, the researchers used either an ytterbium fiber laser or a carbon dioxide laser.
The compound N3H2 did not appear across the full pressure range tested. It formed from 134(3) to 139(1) gigapascals and was reproduced in a second diamond-anvil cell at 134(3) gigapascals. At 110(3) gigapascals and below, laser heating instead decomposed pre-existing amorphous nitrogen–hydrogen compounds, rather than producing the desired crystal structure. The result shows that the material forms only within a specific, high-pressure window; simply combining the elements and heating them is not enough.
To determine how the atoms were arranged, the team combined single-crystal X-ray diffraction with Raman measurements. The diffraction measurements used synchrotron light sources at the European Synchrotron Radiation Facility (ESRF) and Germany’s DESY. The researchers also used density functional theory calculations and a method for searching randomly generated atomic structures to compare possible models and test the properties of the structure they had measured.
Nitrogen Rings Link Up into Chains
The results showed that the nitrogen atoms in N3H2 are joined by single bonds and arranged in puckered six-membered rings known as N6 rings. Each ring has a chair-like shape, and the rings connect to form extended chains along the crystal’s c-axis. The sample is therefore not merely a collection of separate, small molecules, but a structure that extends in one dimension. Each nitrogen atom also has three bonds arranged in a pyramid-like shape, consistent with sp³ hybridization.
One of the structure’s most important features is that the nitrogen chains do not stand alone: hydrogen bonds connect them to one another. According to the structural analysis, an N6H4 unit can donate four hydrogen bonds and accept four more, for a total of eight bonds. These links connect it to six neighboring units distributed across six separate chains. The result is an interwoven structure that ties the chains together and adds stability under high-pressure conditions.
Locating the hydrogen atoms was not straightforward. A hydrogen atom has only one electron, so it scatters X-rays far less strongly than heavier atoms do. But the sensitivity of modern detectors, combined with the intensity of synchrotron beams, allowed the team to identify localized electron density consistent with the positions of hydrogen atoms at 139 gigapascals. The researchers used a bond length of 1.07(2) angstroms for N–H when refining the structural model, then supported the assignment by comparing it with crystal chemistry and theoretical calculations. Including the hydrogen positions also improved the agreement between the model and the measured data.
Calculated Properties—and Important Limits
Calculations indicate that the compound has an electronic band gap of about 3.75 electron volts at a pressure of 139 gigapascals. The researchers estimated its bulk modulus at about 31 gigapascals, meaning that the material is more compressible than pure polymeric nitrogen. These properties were calculated from the material’s structure; they were not all directly measured under ordinary-use conditions.
The team also calculated a hypothetical decomposition energy of 7.98 kilojoules per gram at ambient pressure for the reaction: 6N3H2 → 7N2 + 4NH3. That value is higher than the published figures for TNT (4.2 kilojoules per gram) and HMX (5.7 kilojoules per gram). But this comparison does not mean that N3H2 is available or usable. The number is a theoretical estimate of energy, not the result of an experiment that measured energy release, and making the compound required extraordinary pressure.
The stability calculations tell a more complicated story than the energy figure alone. At 100 gigapascals and above, N3H2 was calculated to be energetically favored over the competing phases included in the comparison. At lower pressures, the NH4–N5 phase, with the same overall nitrogen-to-hydrogen ratio, became favored down to 20 gigapascals. Lattice-vibration calculations also indicated that the crystal is stable at 30 gigapascals but unstable at 0 gigapascals: as pressure falls, the hydrogen bonds and one of the N–N bonds within the chain weaken.
What Happened as the Pressure Came Off?
The researchers tracked the sample as they reduced the pressure, using different measurement tools. X-ray diffraction data continued to be collected down to 58(1) gigapascals, while distinctive Raman patterns could be followed down to 26(3) gigapascals. Within the range covered by each method, the recorded changes in diffraction patterns and vibration frequencies showed no clear sign of a phase transition. This means that no transition was observed down to those limits; it does not prove that the structure can survive at ordinary pressure.
Both decompression experiments ended when the diamond anvils broke, destroying the samples. As a result, the researchers could not continue measurements down to ambient pressure or test whether the material could be recovered after removal from the cell. This limitation is consistent with calculations predicting that the crystal is unstable at 0 gigapascals. Whether it can be retained outside a high-pressure environment therefore remains an open question, not an achievement the experiments have already demonstrated.
Why the Discovery Matters Beyond This One Compound
In addition to identifying N3H2, the study stands out because the hydrogen positions were determined directly from X-ray diffraction data at 139 gigapascals. The researchers say this is far beyond the previous limit for locating individual hydrogen atoms by this method, which was about 25 gigapascals. The advance is therefore not just a new chemical formula. It also shows that modern synchrotron tools can analyze materials made of light atoms under conditions that have made those atoms difficult to see.
This capability could help researchers investigate other materials made of light elements that form under high pressure. The next step proposed by the study is to look for stronger interactions than hydrogen bonds that might help support nitrogen chains as pressure is reduced. Creating a new structure under pressure is a structural achievement; making it recoverable or useful is a separate challenge that these experiments have not yet solved.
Nitrogen–hydrogen compounds may also be relevant to studies of materials inside icy worlds, where high pressures and temperatures can occur. The study’s authors caution, however, that N3H2 may be scarce in many such environments because the nitrogen-to-hydrogen ratio is low and ammonia is stable across the pressure range considered. Even so, the existence of this phase adds a new possibility to the chemical maps researchers use to understand multielement materials in these worlds.
The Bottom Line
Ranieri, Genest, and their colleagues demonstrated a polymeric structure made of nitrogen and hydrogen at pressures of 134(3)–139(1) gigapascals. N3H2 consists of nitrogen rings linked into chains, with hydrogen bonds connecting those chains into a network. X-ray diffraction allowed the team to locate hydrogen at 139 gigapascals. The material has a calculated decomposition energy of 7.98 kilojoules per gram, but calculations indicate that it is unstable at ordinary pressure, and the experiments did not recover it. The clearest achievement is a deeper understanding of nitrogen–hydrogen chemistry and a better ability to see light atoms under extreme pressure—not proof of a near-term practical application.