Low-density framework form of crystalline silicon with a wide optical bandgap

Synthesis of a guest-free clathrate form of crystalline silicon was;achieve d by successive vacuum treatment and density separation of NaxSi136-based m aterials. The new allotrope of silicon has an open framework structure base d upon slightly distorted tetrahedral atoms bound into five- and six-member ed ring structures, and corresponds to a fully saturated and condensed full erane-type solid. Theoretical calculations indicate that the new form of si licon should be a wide bandgap semiconductor. This prediction is borne Out by experiment: electrical conductivity and optical absorption measurements yield a band gap of 1.9 eV, approximately twice the value of "normal" semic onducting silicon.