Polymerization of Nitrogen in Nitrogen-Fluorine Compounds under Pressure

A wide range of polynitrogen species have attracted much attention because of their potential applications as high-energy-density materials. Until now, predicted polynitrogen was found to be negatively charged, with charge transfer from introduced atoms to nitrogen in nitrogen-bearing compounds. Using an evolutionary algorithm combined with first-principles calculations, stoichiometries and structures in nitrogen-fluorine compounds at pressures ranging from 0 to 200 GPa are investigated. In addition to two fluorine-rich compounds NF₃ and NF₅, two other compounds, NF and N₆F, emerge with increasing pressure. N₆F, as a nitrogen-rich compound, will become stable at pressures greater than 180 GPa with a positively charged nitrogen network. Above 120 GPa, the NF compound with polymeric zigzag nitrogen chains is discovered, and it is quenchable to the ambient conditions, acquiring the highest energy density of 5.38 kJ/g among reported binary covalent nitrogen compounds. These newly predicted N-F compounds are useful in understanding the chemistry of polynitrogen.