Stable global tubular boron clusters in Na 2 B 18 and Na 2 B 18 .

The electron deficiency and strong bonding capacity of boron have created numerous species with unusual structural and electronic properties in the form of pure and hetero-atom doped boron clusters. Here we identified D 9d -symmetry Na 2 B 18 and Na 2 B 18 - tubular boron clusters as global minima, whose stability is significantly enhanced by two doped sodium atoms. The doped Na atoms trigger strong charge transfer from Na to the boron motifs, resulting in salt complexes (Na 2 2+ B 18 2- and Na 2 2+ B 18 3- ). In particular, the optimal electrostatic interactions arising from the doping effect play a crucial role in stabilizing the tubular structure against the planar and quasi-planar preferences of the negatively charged boron clusters.