Transition from exohedral to endohedral geometries of anionic and neutral B4Sin (n = 4–15) clusters: quantum chemical calculations
The growth patterns of anionic and neutral B4Sin (n = 4–15) clusters are investigated by using density functional theory (DFT) calculations combined with particle swarm optimization (CALYPSO) software. The geometries of anionic and neutral B4Sin clusters transform from exohedral to endohedral structures with the increasing cluster sizes. B4Si14ˉ anion is the critical size of forming B4-endohedral structure for anionic clusters, while B4Si15 neutral is the threshold size of forming B4-endouhedral structure for neutral clusters. Both anionic and neutral B4Sin (n = 4–15) clusters are primarily dominated by prism-based or bipyramid-based geometries. The global minima of anionic and neutral B4Sin clusters adopt different geometrical structures, except for anionic and neutral B4Si10. The binding energies of B4Sinˉ clusters show an odd-even alternation with the growing number of Si atoms. The B atoms in B4Sinˉ/0 are keen on occupying the high coordination sites to interact with more Si atoms and exhibit B‒B single bonding and B=B double bonding properties. The B atoms are found to carry more negative charges due to charge-transferring from Sin frameworks to B atoms. Interestingly, B4Si4ˉ anion adopts a C2h symmetric bicapped tetragonal bipyramid with σ plus π double bonding characters and has the highest binding energies among the anionic clusters. The bonding interactions in B4Si4ˉ are in the order of B‒B > B‒Si > Si‒Si.