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

Synthesis of a guest-free clathrate form of crystalline silicon was achieved by successive vacuum treatment and density separation of ${\mathrm{Na}}_{x}{\mathrm{Si}}_{136}$-based materials. The new allotrope of silicon has an open framework structure based upon slightly distorted tetrahedral atoms bound into five- and six-membered ring structures, and corresponds to a fully saturated and condensed fullerane-type solid. Theoretical calculations indicate that the new form of silicon 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'' semiconducting silicon.