Mechanical stabilities of K4 carbon and K4-like NaC2

The mechanical stabilities of K-4 carbon and K-4-like NaC2 have been studied by performing first-principle calculations. Total energies as functions of isotropic deformations and volume-conserving tetragonal and trigonal deformations have been calculated. For K-4 carbon, the total energy shows a minimum for isotropic and trigonal deformations, but exhibits maxima for tetragonal deformation. In contrast, the total energy of K-4-like NaC2 shows a minimum under all three deformations. These results indicate that K-4 carbon is not a metastable phase, but that K-4-like NaC2 is a metastable phase. In addition, the heat of formation of K-4-like NaC2 is discussed. (c) 2012 Elsevier Ltd. All rights reserved.