Stability of Si70 cage structures

We have performed calculations on the cage structures with and without inner atoms, a stacked structure for Si70 cluster by using FP-LMTO-MD method. It is found that the Si70 perfect cages, including its ionic cages, would distort into puckered hollow balls. But the distorted hollow ball is still less stable than a stacked structure. Further calculations suggest that a distorted cage structure with inner atoms, resulting from Si60 perfect cages by inserting 10 inner atoms, is much more stable than the distorted hollow cage and the stacked structure of Si70.