Boron and MgB₂ analogs of fullerenes and carbon nanotubes: a density functional theory study

Abstract

The structural analogy between fullerene and fragments of elemental boron allotropes extends to the structure of MgB₂, the simplest bulk high T_c (39 K) superconductor, and graphite. Electronic structure studies on equivalent structures in the boron regime relate the chemistry of carbon and boron. The fullerene and nanotube analogs, B₈₄H₅₀, Mg₃₆B₆₀, Mg₃₀B₆₀, Mg₃₂B₆₀ and (6,6) boron and MgB₂ nanotubes were studied using first principles DFT calculations. Structures with all Mg atoms outside the B₆₀ cage are more stable than the structures with Mg atoms inside. The energy difference between MgB₂ sheet and nanotubes is close to that between graphite and carbon nanotubes. The α-boron allotrope is calculated to be about 2.4 eV lower in energy than the boron nanotube. Analysis of band structures indicates that boron and MgB₂ nanotubes are metallic.