Polynitrogens are the ideal rocket fuels or propellants. Due to strong triple N=N bond in N-2, the direct polymerization of nitrogen is rather difficult (i.e. extreme high temperature and high pressure). However, the use of nitrides as precursors or the reaction of N-2 with other elements has been proved to be an effective way to obtain polynitrogens. Here, with assistance of the advanced first-principles swarm-intelligence structure searches, we found that P1 (1) over bar -BeN4, containing infinite zigzag-like polymeric nitrogen chains, can be synthesized by compressing the mixture of Be3N2 and N-2 at 25.4 GPa, which is greatly lower than 110 GPa for synthesizing cubic gauche nitrogen and other polynitrogen compounds (e.g. bulk CNO at 52 GPa and SN4 at 49 GPa). Its structural stability can be attributed to the coexistence of ionic Be-N and covalent N-N bonds. Intriguingly, this phase has high kinetic stability and remains metastable at ambient pressure. The exceptional properties, including high energy density (3.60 kJ g(-1)), high nitrogen content (86.1%), high dynamical stability, and low polymerization pressure, make Pl (1) over bar -structured BeN4 a promising high energy material. Infinite nitrogen chains in P1 (1) over bar -BeN4 transform to N-10 rings network in P2(1)/c phase at 115.1 GPa. P1 (1) over bar -BeN4 is metallic, while P2(1)/c-BeN4 is an insulator. (C) 2017 Elsevier B.V. All rights reserved.