Qiang Zhu, Qingfeng Zeng, Artem R. Oganov
We present a systematic search for low-energy metastable superhard carbon allotropes by using the recently developed evolutionary metadynamics technique. The rationale was to find candidate structures to explain the new allotrope found on cold compression of graphite to 15-20 GPa. Starting from 2H- or 3R-graphite and applying the pressure of 20 GPa, a large variety of intermediate sp^3 carbon allotropes were observed in evolutionary metadynamics simulation. We found four families of sp^3 carbon allotropes based on `6' (diamond/lonsdaleite), `4+8' (bct C4), `5+7' (M/W-carbon), and `5+7' (X-carbon) topology. Our calculation found all the previous proposed candidates for `superhard graphite', and discovered several new allotropes which are energetically even more favorable. These allotropes are predicted to have excellent mechanical and optical properties. This study shows evolutionary metadynamics is a powerful approach to systematically search for low-energy metastable phases.
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http://arxiv.org/abs/1203.0706
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