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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.

The group-IIB elements, including zinc (Zn), cadmium (Cd), and mercury (Hg) are studied at multi-TPa pressures using first-principle electronic structure calculations coupled to an efficient structural prediction method. The experimental structural transition sequence with increasing pressure is successfully reproduced, and most metallic elements structures, for instance, simple cubic (sc), body-centered cubic (bcc) and samarium-type (Sm-type) are not stable at terapascal pressures of the group-IIB elements. In addition, the high-pressure hexagonal-close-packed (hcp) phase of Zn, Cd and Hg, can remain up to 6 TPa, and double hexagonal-close-packed (dhcp) and faced-centered cubic (fcc) phases transitions have been observed in Zn and Cd. Hg transforms directly from hcp to fcc, with dhcp absent. The calculated results also show that the c/a ratios of Zn and Cd have a similar transformation, and Hg goes towards the ideal value of 1.633 for a hexagonal close packing at high pressures. The calculated electronic properties indicate that Zn are high-pressure electrides with some of the electrons localized at interstitial sites enclosed by the surrounding atoms. The structural changes from hcp to dhcp and fcc are associated with a dynamical instability, and the soft mode phase transition mainly corresponds to the phase transition mechanism. [ABSTRACT FROM AUTHOR]