Crystal and electronic structure of superhard BC5: First-principles structural optimizations

A thermodynamically stable I4m2 phase of superhard BC5 was predicted from exhaustive structural searches combined with first-principles structural optimizations. From the computed bulk and shear moduli, this phase is expected to be harder than the recently synthesized BC5 with the diamond structure. The stability and strength are attributed to multicenter B-C-B interactions when two B atoms are linked through a common C atom. The I4m2 phase is metallic and superconducting with an estimated superconducting critical temperature (T-c) of 47 K. Another candidate structure with P1 symmetry was obtained and this fits well to the reported x-ray diffraction pattern. B atoms in the P1 structure are not linked together by C atoms which yields higher energy. P1 and I4m2 structures have almost identical predicted hardness.