Polymerization of nitrogen in lithium azide

Additional electrons can drastically change the bonding trend of light elements. For example, N atoms in alkali metal azides form the linear N₃^- anions instead of N₂ molecules with the introduction of additional electrons. The effect of the additional electrons on the polymerization of N under pressure is important and thus far unclear. Using first principles density functional methods and the particle swarm optimization structure search algorithm, we systematically study the evolution of LiN₃ structures under pressures up to 600 GPa. A stable structure featuring polymerized N under pressures higher than 375 GPa is identified for the first time. It consists of zig-zag N polymer chains that are formed by N₅^- five-member rings sharing N-N pairs. Throughout the stable pressure range, the structure is insulating and consists of N atoms in sp³ hybridizations. Comparing with the atomic and electronic structures of previous phases, our study completes the structural evolution of LiN₃ under pressure and reveals the structural changes which are accompanied and driven by the change of atomic orbital hybridization, first from sp to sp² and then from sp² to sp³. © 2013 AIP Publishing LLC.