Complex Low Energy Tetrahedral Polymorphs of Group IV Elements from First Principles.

The energy landscape of carbon is exceedingly complex, hosting diverse and important metastable phases, including diamond, fullerenes, nanotubes, and graphene. Searching for structures, especially those with large unit cells, in this landscape is challenging. Here we use a combined stochastic search strategy employing two algorithms (ab initio random structure search and random sampling strategy combined with space group and graph theory) to apply connectivity constraints to unit cells containing up to 100 carbon atoms. We uncover three low energy carbon polymorphs (Pbam-32, P6/mmm, and I4[over ¯]3d) with new topologies, containing 32, 36, and 94 atoms in their primitive cells, respectively. Their energies relative to diamond are 96, 131, and 112  meV/atom, respectively, which suggests potential metastability. These three carbon allotropes are mechanically and dynamically stable, insulating carbon crystals with superhard mechanical properties. The I4[over ¯]3d structure possesses a direct band gap of 7.25 eV, which is the widest gap in the carbon allotrope family. Silicon, germanium, and tin versions of Pbam-32, P6/mmm, and I4[over ¯]3d also show energetic, dynamical, and mechanical stability. The computed electronic properties show that they are potential materials for semiconductor and photovoltaic applications.