  Diamond Si is a semiconductor with an indirect band gap that is the basis of
modern semiconductor technology. Although many metastable forms of Si were
observed using diamond anvil cells for compression and chemical precursors for
synthesis, no metallic phase at ambient conditions has been reported thus far.
Here we report the prediction of pure metallic Si allotropes with open channels
at ambient pressure, unlike a cubic diamond structure in covalent bonding
networks. The metallic phase termed P6/m-Si6 can be obtained by removing Na
after pressure release from a novel Na-Si clathrate called P6/m-NaSi6, which is
discovered through first-principles study at high pressure. We confirm that
both P6/m-NaSi6 and P6/m-Si6 are stable and superconducting with the critical
temperatures of about 13 and 12 K at ambient pressure, respectively. The
discovery of new Na-Si and Si clathrate structures presents the possibility of
exploring new exotic allotropes useful for Si-based devices.
