First-Principles Study of C6M2 (M=B, Al, Mg, Li), C7B and Related Compounds

We calculate electronic and lattice dynamics properties Of C6M2 (M = B, Al, Mg, Li), C7B, AlC2, MgC2, LiC2 and LiB2 compounds. The electronic and lattice properties are optimized automatically by the first-principles molecular dynamics (FPMD) method. The present electronic structure calculation is based on the local density approximation (LDA) in the density functional theory (DFT). C6M2 (M = B, Al, Mg, Li) and C713 compounds are hypothetical hexagonal layered materials. They consist of C-C and M-C layers in a unit cell. The electronic band structures of them are found to be metallic. Several compounds have unoccupied flat bands above the Fermi level along the A-I line. These flat bands are similar to those of MgB2. Lattice dynamics properties (phonon frequencies) of them are calculated in order to investigate lattice stabilities in this study. Calculated phonon frequencies of them are imaginary with the exception of LiB2. Imaginary phonon frequencies imply that it is structurally unstable. Most of calculated compounds are cohesively unfavorable. Although the cohesive energy of LiB2 is positive, the value is quite small with 7.2 meV/LiB2. Cohesive energies of other compounds are positive and larger than 0.17 eV.