Chemical versus steric frustration in boron nitride heterofullerene polyhedra

The energy balance between steric and chemical frustration in boron nitride (BN)x cages is investigated by comparing two isoelectronic structural classes: fully alternant square–hexagon cages and pentagon-paired heterofullerenes that include three N–N and three B–B forced homonuclear bonds. Systematic density-functional tight-binding (DFTB) geometry optimisations on (BN)x (10⩽x⩽35) show the largest binding energies per atom consistently for the heterofullerenes, with overlapping energy ranges only at the highest nuclearities. The morphologies of some fullerene-based BN cages are similar to the alternant octahedral shells that have been conjectured as structures for experimentally observed BN particles. Both classes provide feasible closure for larger structures such as BN nanotubes and nested shells.