Novel ultrahard monoclinic carbon allotropes from crystal structure engineering and first principles

Two novel monoclinic (space group C2/c) carbon allotropes, m-C8, with ths and dia topologies, were proposed through crystal structure engineering. The ths allotrope is built by three-dimensional tiling of trigonal carbon units, which is different from tiling of C4 tetrahedra in the case of dia m C8. Structural studies were supported by density functional theory (DFT)-based calculations of ground state structures and energy-derived properties. Extensive investigations of cohesive energies, energy-volume equations of state, mechanical (elastic constants and moduli, hardness) and dynamical (e.g., phonons) properties revealed the new allotropes to be cohesive and stable both mechanically and dynamically. Although they are metastable compared to diamond, their formation is possible under non-equilibrium conditions as a result of alternative metastable behavior.