Superhard diamondlike BC5: A first-principles investigation

We perform first-principles density-functional calculations to identify the possible crystal structure of a superhard diamondlike BC5 phase, which was recently synthesized under high-pressure and high-temperature conditions. Interestingly, we find only a small total-energy difference between the energetically most favorable ordered configuration and the fully disordered state of BC5 modeled using a 54-atom special quasirandom structure, indicating a weak ordering tendency. It is thus likely that the BC5 phase synthesized under experimental conditions is disordered in nature. Such a conclusion is further corroborated by the fact that the disordered BC5 structure displays volume-per-atom, bulk modulus and its pressure derivative, and simulated x-ray diffraction spectrum in good agreements with experiments.