Ab initio study on Quasi-Binary Acetonitriletriide Sr$_3$[C$_2$N]$_2$

We report using density functional theory (DFT), the ground-state properties of the recently synthesized and characterized Sr$_3$[C$_2$N]$_2$ crystal. The nearly colorless, centrosymmetric Sr$_3$[C$_2$N]$_2$ crystallizes in a monoclinic unit cell with a $P2_1/c$ space group (No.14) and many of its properties remain unknown basing on the fact that it's a latecomer in the field. The goal of this study is to fill this information gap through a theoretical prediction. The calculated structural properties were comparable to those obtained by an experimental group led by Clark and co-workers thus giving us extra confidence in the accuracy of our DFT computations on Sr$_3$[C$_2$N]$_2$. We employed the same approach in calculating mechanical and dynamical stabilities together with the electronic density of states of Sr$_3$[C$_2$N]$_2$. No imaginary phonon modes were observed and thus implying dynamical stability. The thirteen elastic constants calculated passed the stability criteria of a monoclinic system. From the computed Poisson's ratio ($η$=0.27) and G/B=0.54, our calculations predict Sr$_3$[C$_2$N]$_2$ being brittle and not able to withstand high-pressure applications. To analyze the chemical bonding mechanism, the corresponding total density of states (TDOS) and partial DOS were plotted. The top of the valence band (VB) mainly consists of C 2p states N 2p, N 2s and a slight admixture of Sr 5s states. The bottom of the conduction band (CB) shows a strong hybridization between C 2p, N 2p, N 2s, and Sr 5s states, yielding a bandwidth of 7.18 eV in the entire conduction band. We were able to obtain a tunable electronic gap of 2.65 eV in Sr$_3$[C$_2$N]$_2$. The authors herein note that Sr$_3$[C$_2$N]$_2$ falls in an unknown family of pseudonitrides that may possess novel physical and chemical properties.

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