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 sp(3) 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 sp(2) and then from sp(2) to sp(3).