Phase stabilities and mechanical properties of two new carbon crystals

Density functional calculations are used to systematically evaluate phase stabilities and mechanical properties of two recently proposed carbon crystals (K(4) and M-carbon), along with graphite, cubic diamond and hexagonal diamond. It is found that the K(4) carbon, which can be referred to as a twin of the cubic diamond crystal, is mechanically unstable, implying that it cannot be formed. Remarkably, our calculations not only substantiate the M-carbon phase is a highly incompressible and potentially superhard material but also show that it exhibits mechanical stability. Moreover, a surprisingly small activation barrier (about 0.018 eV/atom) for the transformation of M-carbon to graphite explains some paradoxical experimental observations. Copyright (C) EPLA, 2009