Indication of Low-Energy BC₅ Structures

The Bond counting rule is proved to be an important and effective criterion for searching low-energy metastable structures for ternary boron carbon nitrogen (B-C-N) compounds. The Bond counting rule, however, has its limitations for the binary diamond-like or ternary B-C-N polymorphs with the same bond ratio. First-principles calculations validate that the Mulliken charge difference may serve as an indication of low-energy crystal structures, and clarify the energy difference among those polymorphs to some extent. For example, we predict two ground state phases ofsuperhard BC₅ (named as I-BC₅ and II-BC₅), which are 0.28 and 0.27eV/formula lower in energy than the P3m1 structure reported recently in the literature Calandra et al. Phys. Rev. Lett. 101(2008)016401, respectively. The charge transfer analysis reveals that the smaller Mulliken charge difference for the same kind of element will result in more stable structures.