The geometries and energies of silicon-sodium binary clusters (SinNa, n = 2-7) were investigated with the inclusion of electron-correlation effects using the second-order Møller-Plesset perturbation theory (MP2) with the 6-31G basis set. In addition, the fourth-order (MP4) energies were evaluated to determine the ground-state isomers. Si4Na cluster was calculated with the polarized 6-31G* basis set. For all of the SinNa clusters, the most stable isomers have bridge-site type structures. Apart from the bridge-site type Si6Na, the structure of the most stable isomer of SinNa keeps the frame of the corresponding Sin cluster unchanged, and the electronic structure of SinNa is similar to that of the corresponding negative ion Si-n. The silicon framework of the bridge-site type of Si6Na is distorted from the compressed octahedron to the face-capped trigonal bipyramid.
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