M585, a low energy superhard monoclinic carbon phase

•A monoclinic carbon phase with remarkable stability was proposed.•It consists of ABCA-stacked diamond linked by interfacial zigzag carbon chains.•It contains a new topological manner of 5–8–5 carbon ring.•It possesses remarkable hardness comparable to that of diamond.

A monoclinic carbon crystal, M585, consisting of ABCA-stacked cubic-diamond-segments interlinked by interfacial zigzag carbon chains is proposed based on first-principles calculations. The formed linear 5–8–5 carbon grain boundary shows a totally new topological manner of carbon phase, which is distinct from those in the previously proposed 4+8 and 5–7–5 types. M585 is energetically more favorable than the previously proposed M-carbon, W-carbon, H-carbon and S-carbon and is dynamically stable. The calculations on electronic and mechanical properties of M585 indicate that it is optically transparent and mechanically superhard, which has potential applications in mechanical industry.