Abstract
New polynitrides containing metastable forms of nitrogen are actively investigated as potential high-energy-density materials. Using a structure search method based on the CALYPSO methodology, we investigated the stable stoichiometries and structures of cesium polynitrides at high pressures. Along with the CsN3, we identified five new stoichiometric compounds (Cs3N, Cs2N, CsN, CsN2, and CsN5) with interesting structures that may be experimentally synthesizable at modest pressures (i.e., less than 50 GPa). Nitrogen species in the predicted structures have various structural forms ranging from single atom (N) to highly endothermic molecules (N2, N3, N4, N5, N6) and chains (N∞). Polymeric chains of nitrogen were found in the high-pressure C2/c phase of CsN2. This structure contains a substantially high content of single N-N bonds that exceeds the previously known nitrogen chains in pure forms, and also exhibit metastability at ambient conditions. We also identified a very interesting CsN crystal that contains novel N44− anion. To our best knowledge, this is the first time a charged N4 species being reported. Results of the present study suggest that it is possible to obtain energetic polynitrogens in main-group nitrides under high pressure.
Highlights
New polynitrides containing metastable forms of nitrogen are actively investigated as potential highenergy-density materials
Using a structure search method based on the CALYPSO methodology, we investigated the stable stoichiometries and structures of cesium polynitrides at high pressures
It may strike as a surprise that energetically new stoichiometries, other than the commonly known CsN3 are calculated to be stable at high pressure
Summary
New polynitrides containing metastable forms of nitrogen are actively investigated as potential highenergy-density materials. Polymeric chains of nitrogen were found in the high-pressure C2/c phase of CsN2 This structure contains a substantially high content of single N-N bonds that exceeds the previously known nitrogen chains in pure forms, and exhibit metastability at ambient conditions. The N5− anion[13], neutral N4 molecule[14] have been isolated experimentally These successes led to an increasing interest in polynitrogen compounds and to the search of other stable polynitrongen forms in recent years. The LiN5 crystal is energetically stable only at high pressures, but exhibit mechanical stability at ambient conditions which provides the possibility for its recovery. Previous theoretical studies[23] suggested that the energy density of CsN3 can be further enhanced at high pressure, where the nitrogen atoms form extended structures. The bonding nature in these polynitrogens is of great important to nitrogen chemistry and to the understanding of metal-nitrogen interactions
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