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.     

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