Mechanical stabilities of K4 carbon and K4-like NaC2

The mechanical stabilities of K4 carbon and K4-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 K4 carbon, the total energy shows a minimum for isotropic and trigonal deformations, but exhibits maxima for tetragonal deformation. In contrast, the total energy of K4-like NaC2 shows a minimum under all three deformations. These results indicate that K4 carbon is not a metastable phase, but that K4-like NaC2 is a metastable phase. In addition, the heat of formation of K4-like NaC2 is discussed.

► Mechanical stability of K4 carbon and K4-like NaC2 has been studied.
► K4 carbon is not a metastable phase.
► K4-like NaC2 is a metastable phase.