Ab initio study of the structural stability of fcc-CHx phases

It has recently been suggested that carbon nanocrystals obtained by chemical vapor deposition with a gaseous precursor containing CH4 and H2, may include hydrogen atoms in the carbon lattice, forming solid fcc- and bcc-CHx phases. In this work, we evaluate the structural stability of five fcc-CHx 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 C2H(cuprite), CH(zincblende), CH(rocksalt), and CH2(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 CH2 in the pyrite structure. We find that CH2(pyrite) with the hydrogen atoms defined by the internal parameter u=0.35u=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.