A new nitrogen allotrope has been created at last

Carbon is famous for its allotropes, including humble graphite, flashy diamond, and satisfyingly symmetrical buckminsterfullerene. But allotropes of nitrogen, carbon’s neighbor in the periodic table, are rare and usually fleeting. Now chemists at Justus Liebig University Giessen have created and isolated the neutral nitrogen allotrope hexanitrogen, or N₆—a string of six nitrogen atoms held together by four double bonds and one single bond in the molecule’s center. Making hexanitrogen contributes to fundamental scientific knowledge and could offer opportunities for energy storage in the future, say N₆’s creators Peter R. Schreiner, Artur Mardyukov, and Weiyu Qian (Nature 2025, DOI: 10.1038/s41586-025-09032-9).

Until now, neutral nitrogen allotropes were limited to the common nitrogen found in air (N₂), polymeric nitrogen, and molecules that were so short-lived they could be identified only spectroscopically, including the azide radical N₃ and a form of N₄ that has never been structurally characterized. Chemists have spent decades trying to make neutral nitrogen allotropes, but they’ve typically gone after highly symmetric structures, such as an all-nitrogen version of benzene.

The problem with such a molecule, Schreiner says, is that it would be made from alternating single and double bonds between the nitrogen atoms, which would readily convert into triple-bonded N₂. “Just stretch it a little, and boom, it goes kaput in no time,” he says.

Schreiner, Mardyukov, and Qian realized that to make a neutral nitrogen allotrope stable enough to isolate, they had to create a structure that wouldn’t easily break into N₂ units. While hexanitrogen looks like it could dissociate into N₃ by breaking its central bond, doing so has an energetic cost. “That is the magic behind this particular molecule: that it has a high barrier to dissociation into N₃ and N₂,” Schreiner says.

The chemists prepared N₆ at room temperature via the gas-phase reaction of chlorine or bromine with silver azide, and they trapped it in an argon matrix at 10 K. They also made N₆ films in liquid nitrogen.

Lu Ming, who studies high-energy nitrogen compounds at Nanjing University of Science and Technology, says that the discovery fills an important research gap in the field. “The work offers valuable guidance for researchers aiming to synthesize similar compounds through chemical routes,” Lu says in an email.

But Lu points out that the molecule has poor thermal stability. Future efforts on neutral nitrogen allotropes “should focus on overcoming challenges related to synthesis, improving thermal stability, enabling reliable structural characterization, and achieving more complete energy release, all of which are essential for practical applications,” he says.

Thomas M. Klapötke, who works with energetic materials at Ludwig Maximilian University of Munich, agrees that the work is important but has doubts that it will be useful for energy storage applications as the researchers claim in their paper.

Schreiner says that while the team doesn’t know for sure about energy storage, N₆ hangs around in liquid nitrogen for long periods. He says, “The question is, How do you release the energy from there in a controlled way? But that is always the question.” The molecule, he adds, is also a powerful explosive that generates only N₂ as a by-product. Next, he says, the chemists are hoping to make the neutral nitrogen allotrope N₁₀.