Structure prediction for metastable materials: Sn$_{\mathrm{\mathbf{2}}}$\textbf{N}$_{\mathrm{\mathbf{2}}}$

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. An interesting example of a metastable material is Sn$_{\mathrm{2}}$N$_{\mathrm{2}}$, a mixed valence Sn(II)/Sn(IV) tin nitride, which, due to its metastability relative to metallic Sn, N$_{\mathrm{2}}$, and Sn$_{\mathrm{3}}$N$_{\mathrm{4,}}$ remained elusive until recently [1]. This metastability presents a challenge for computational structure prediction, as common ground state search strategies are guided by energy minimization, which will eventually lead to a phase-separated configuration (Sn$+$N$_{\mathrm{2}})$ instead of the desired Sn$_{\mathrm{2}}$N$_{\mathrm{2}}$ compound. Initial structure sampling has identified a number of candidate structures [1], but did not lead to an unequivocal assignment. We present the results of a new hybrid structure sampling approach predicting a bilayer structure with the possibility of polytypism. [1] C.M. Caskey \textit{et al.}, J. Chem. Phys. 144, 144201 (2016).