First-principles study of C6B2

We calculate the electronic and lattice properties of C6B2 (Na3As type crystal structure) using the first-principles molecular dynamics (FPMD) method. C6B2 is a hypothetical hexagonal compound, which consists of two B-C layers and two C-C layers. All calculated electronic band structures are metallic. C6B2 has unoccupied flat bands close to the Fermi level at the Γ-A line. These flat bands correspond to unoccupied σ-bands of MgB2 at the Γ-A line. Displacements of B atoms along a B-C bond split unoccupied flat bands at the Γ-A line into two subbands. A total energy change with displacements of B atoms indicates anharmonicity of a B-C stretching mode. We investigate C6B2 under various compression conditions. The change in the electronic band structure under a, b-axis compression (Pxy) is small. In contrast, it is quite remarkable under c-axis compression (Pz). We find the lattice anomaly of C6B2 under a, b-axis compression. This anomaly shows a kind of negative Poisson ratios.