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POLYNITROGEN Single-bonded nitrogen allotrope, if harnessed, could serve as a propellant STEPHEN RITTER Scientists have eagerly studied the behavior of molecular nitrogen under high pressure for nearly 20 years, searching for evidence of polymeric nitrogen, a single-bonded form of nitrogen with a structure similar to that of diamond. Several new nitrogen phases have been discovered, including a nonmolecular semiconducting phase, but polymeric nitrogen has remained elusive. Until now. UNDER PRESSURE Single-bonded polymeric phase of nitrogen has a diamond-like structure. NATURE © 2004 In the August issue of Nature Materials (http://dx.doi.org/10.1038/nmat1146), Mikhail I. Eremets of Max Planck Institute for Chemistry, in Mainz, Germany, and coworkers report the preparation of polynitrogen by compressing N2 above 110 gigapascals (about 1.1 million atm) and 2,000 K in a tiny laser-heated diamond anvil cell. Raman scattering and X-ray data indicate that each nitrogen atom in the product is connected to three neighbors by single covalent bonds. The material is stable at room temperature at pressures above 42 GPa, though the researchers were unable to isolate an intact sample at atmospheric pressure. They hope to devise a method to isolate polynitrogen, but doing so might require stabilizing it in the form of compounds containing other elements or by introducing impurities, they note. Nitrogen is unique in that the triple bond holding the diatomic molecule together is one of the most stable chemical bonds known, notes chemistry professor Thomas M. Klapötke of Ludwig Maximilians University in Munich, Germany. In contrast, polymeric nitrogen is expected to be highly unstable because the N–N single bond is relatively weak (160 kJ per mole). Decomposition to N2 and formation of the much stronger triple bond (954 kJ per mole) would be highly exothermic. Polynitrogen should have an energy capacity more than five times that of the most powerful nonnuclear energetic materials and "would seem to be the ideal high-energy-density material," Klapötke says. Further experiments are needed to determine whether polynitrogen could ever be suitable as a propellant or, less likely, as an explosive, he adds. |
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