Stable zigzag and tripodal all-nitrogen anion N44- in BeN2

Abstract

All-nitrogen anions have attracted extensive attention because of their unique chemical and physical properties, and potential applications as high-energy density materials. Here, we discovered two N44- anions with planar zigzag and tripodal structures in BeN2 compound using first-principle calculations with structural search. At ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.48 kJ/g, and 2.95 kJ/g relative to Be3N2 and N2 gas). The zigzag N44- anion has P21/c space group in BeN2 compound and is energetically favorite in the pressure range of 0-48 GPa. While tripodal N44- anion has Cm space group and is energetically favorite in the pressure range of 48-100 GPa. Further analysis of chemical bonding pattern and electronic properties show that the Be atoms provide 2s electrons to alter the bonding state of N44- as well as use empty outer shell 2p orbital to accommodate lone pair electrons of N atoms, forming coordinate bonds to stabilize the compounds. More important, the calculated formation of enthalpies indicate that the zigzag and tripodal N44- can be synthesized via compressing Be3N2 and N2 at a modest pressure. Our results provide a scheme to synthesis N44- anions and stabilize all-nitrogen anions by introducing beryllium atoms.