Ab initio study of the structural stability of fcc-CH x phases

Abstract

It has recently been suggested that carbon nanocrystals obtained by chemical vapor deposition with a gaseous precursor containing CH 4 and H 2, may include hydrogen atoms in the carbon lattice, forming solid fcc- and bcc-CH x phases. In this work, we evaluate the structural stability of five fcc-CH x phases by means of first-principles calculations. The total energy is obtained as a function of the isotropic, tetragonal, and trigonal deformations for the bulk structures. First, we analyze the C 2H(cuprite), CH(zincblende), CH(rocksalt), and CH 2(fluorite) structures. It is found that the four systems show a minimum in the total energy for the isotropic deformation, but are unstable against tetragonal and trigonal deformations. In the second part, we explore the structural stability of CH 2 in the pyrite structure. We find that CH 2(pyrite) with the hydrogen atoms defined by the internal parameter u = 0.35 and a lattice parameter of 3.767 Å is elastically stable, providing a possible explanation for the experimental observation of fcc-carbon in materials prepared in the presence of hydrogen or methane.