Phase stabilities and mechanical properties of two new carbon crystals

¹  College of Engineering Science and Technology, Shanghai Ocean University - Shanghai 201306, China
²  State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences - Shanghai 200050, China

Corresponding author: ycliang@shou.edu.cn

Received: 7 July 2009
Accepted: 24 August 2009

Abstract

Density functional calculations are used to systematically evaluate phase stabilities and mechanical properties of two recently proposed carbon crystals (K₄ and M-carbon), along with graphite, cubic diamond and hexagonal diamond. It is found that the K₄ 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.