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

The structural analogy between fullerene and fragments of elemental boron allotropes extends to the structure of MgB2, the simplest bulk high Tc (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.