First-principles study of C 6B 2

Abstract

We calculate the electronic and lattice properties of C 6B 2 (Na 3As type crystal structure) using the first-principles molecular dynamics (FPMD) method. C 6B 2 is a hypothetical hexagonal compound, which consists of two B–C layers and two C–C layers. All calculated electronic band structures are metallic. C 6B 2 has unoccupied flat bands close to the Fermi level at the Γ–A line. These flat bands correspond to unoccupied σ-bands of MgB 2 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 C 6B 2 under various compression conditions. The change in the electronic band structure under a, b-axis compression ( P xy ) is small. In contrast, it is quite remarkable under c-axis compression ( P z ). We find the lattice anomaly of C 6B 2 under a, b-axis compression. This anomaly shows a kind of negative Poisson ratios.