Electronic signature of the pentagonal rings in silicon clathrate phases: Comparison with cluster-assembled films

The valence-band spectrum of the silicon Si-34 clathrate phase has been measured by photoemission spectroscopy. In contrast with tetrahedral Si (diamond phase), the clathrate structure presents mainly five-member rings. From the comparison between the tetrahedral Si and the clathrate spectra, we show that the presence of odd-member rings induces the merging between s-like and sp-like bands and a strong reduction of the valence-band width. These results are corroborated by first-principles calculations within an accurate quasiparticle approach.