‘If we quench to ambient pressure, this could be the best energetic material known,’ says Ma. Thanks to the very large difference in energy between nitrogen–nitrogen single and triple bonds, synthesising polymeric nitrogen and then releasing the pressure would liberate a huge amount of energy as dinitrogen reforms. High pressure polymeric forms of nitrogen are of great interest as high energy density materials for energy storage, propellants and explosives, says Ma. Dioxygen and dihydrogen, by contrast, require pressures of 1920 and 500GPa, respectively, for molecular dissociation. Despite being held together by an extremely strong triple bond, modest pressures of only 150GPa are required to dissociate this bond to generate polymeric structures with three weaker nitrogen–nitrogen single bonds instead. ‘For nitrogen, the cage structure offers very efficient atomic packing,’ Ma adds.ĭinitrogen is already known to behave unusually under pressure. The researchers used an ab initio random structure search to corroborate their CALYPSO result. ‘It was a surprise for me to find a cage structure for nitrogen, especially under high pressure,’ says Ma. The team dubbed this material ‘diamondoid nitrogen’, because of its striking similarity to the 10-carbon adamantane cage structure, which is the basic carbon sub-unit of diamond. The polymer consists of a repeating 10-atom nitrogen cage. Their calculations predict that nitrogen will form a polymer at pressures of 263GPa and above. ‘You give it the chemical composition and it can predict the structure of the material at a given pressure and temperature.’ ‘CALYPSO is designed for just such a purpose,’ explains Ma. The researchers used their CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimisation) algorithm to explore nitrogen polymorphism at high pressure. The research was performed by a team led by Jianfu Li at Linyi University, and Yanming Ma at Jilin University, both in China. What’s more, it would be amongst the most energy-dense materials known. The structure, if confirmed by experimental studies, would be the first stable cage-like material in an element heavier than carbon. A top view of the calculated nitrogen polymorph structure (left) and the diamond-esque N10 structure (right)Nitrogen will form an unusual cage-like structure when subjected to high pressures, an international team of researchers has calculated.
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