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 cl asses: fully alternant square-hexagon cages and pentagon-paired heterofulle renes that include three N-N and three B-B forced homonuclear bonds. System atic density-functional tight-binding (DFTB) geometry optimisations on (BN) (x) (10 less than or equal tox less than or equal to 35) show the largest b inding energies per atom consistently for the heterofullerenes, with overla pping energy ranges only at the highest nuclearities. The morphologies of s ome fullerene-based BN cages are similar to the alternant octahedral shells that have been conjectured as structures for experimentally observed BN pa rticles. Both classes provide feasible closure for larger structures such a s BN nanotubes and nested shells. (C) 2000 Elsevier Science B.V. All rights reserved.