First-principles studies of structural and electronic properties of hexagonal $\mathrm{B}{\mathrm{C}}_5$

Structural and electronic properties for seven possible stacking configurations in hexagonal $\mathrm{B}{\mathrm{C}}_5$ based on hexagonal BN and graphite structures have been studied using the ab initio pseudopotential density functional method within the local density approximation. From the total energies, structural parameters, and electron density of states calculated after full structural relaxation, the configuration $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$ with $AA$ stacking sequence along the $c$ axis based on the hexagonal BN structure has been shown to be the most stable structure among the seven possible stacking configurations, and both the monolayer and bulk phases of $\mathrm{B}{\mathrm{C}}_5$ are expected to show metallicity. The structural stability of $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$ has been confirmed by calculations of the elastic constants and phonon frequencies.

Figure 1

The atomic arrangement in a monolayer of $\mathrm{B}{\mathrm{C}}_5$ (the solid line outlines the unit cell) and the seven topologically different configurations of bulk $\mathrm{B}{\mathrm{C}}_5$ unit cells. $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$, $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{II}$, and $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{III}$ are constructed from the $h\text{−}\mathrm{B}\mathrm{N}$ structure, while $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{IV}$, $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{V}$, $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{VI}$, and $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{VII}$ are constructed from the graphite structure.

Figure 2

The calculated total DOS for the monolayer and the seven stacking configurations of $\mathrm{B}{\mathrm{C}}_5$ (see Fig. 1) at their equilibrium geometries. The dashed line represents the Fermi level.

Figure 3

The partial DOS for the optimized $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$ configuration and the local DOS for B and C atoms in the optimized $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$ configuration. C1 represents the C atom with a B atom as one of its nearest neighbors in the same layer, while C2 represents the C atom with all of its nearest neighbors being C atoms in the same layer. Dotted line, $s$ state; solid line, $p$ state; dashed line, Fermi level.

Figure 4

The phonon dispersion curves and phonon DOS for the optimized $\mathrm{B}{\mathrm{C}}_5\text{−}\mathrm{I}$ configuration.