Indication of Low-Energy BC 5 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 of superhard BC5 (named as I-BC5 and I-BC5), which are 0.28 and 0.27 eV/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.