Energetics of the formation of dimers and solids of boron nitride fullerenes

Molecular and solid forms of boron nitride, based on stoichiometric and nonstoichiometric fullerenes, are predicted by means of first-principles calculations. Our study of the energetics of dimer formation indicates that the reactivity of boron nitride fullerenes depends strongly on the stoichiometry. The stoichiometric fullerenes form strong covalent bonds between the tips of neighboring cages, while the tips of nitrogen-rich fullerenes ``repel'' each other, and the latter do not bind at the tips. This is reflected in the nature of bonding in the possible fullerene-based crystalline structures: our calculations predict that stoichiometric fullerenes should form covalent-bound low-density solids with large interstitial channels (5--8 {}), but with hardnesses comparable to those of hard metals. In contrast, nitrogen-rich units (with the exception of B₁₂N₁₆) should form loosely bound molecular solids like the standard carbon fullerenes.