Structure and bonding properties of (Bi2Se3)m(Bi2)n stacks by first-principles density functional theory

We have investigated crystal structure, chemical bonding, and electronic properties of the compounds Bi2Se3, BiSe, Bi4Se3, and Bi2Se by first-principles calculations within the density functional theory. The compounds are members of a general series of stacks (Bi2Se3)m(Bi2)n composed of five-layer blocks Se-Bi-Se-Bi-Se and two-layer blocks Bi-Bi. Both types of blocks can be considered as closed-shell systems. We find that the interaction between two five-layer blocks is of van-der-Waals-type, whereas interactions involving two-layer blocks are of weak covalent nature and stronger. When treating exchange and correlation with the generalized gradient approximation interblock van der Waals bonding is highly underestimated while using the local density approximation yields reasonable results. Bi2Se3, which exclusively consists of five-layer blocks, is a narrow-gap semiconductor, whereas Bi-Bi blocks containing compounds represent semimetals. Formation energies mBi2Se3+nBi2=Bi2m+2nSe3m are close to zero which supports the idea of a potentially continuous series of stacks corresponding to an ordered solid solution of pure Bi in Bi2Se3.