A new superconducting open-framework allotrope of silicon at ambient pressure

TL;DR

This paper predicts a new metallic silicon allotrope with open channels stable at ambient pressure, which can be synthesized from a Na-Si clathrate, potentially enabling new Si-based electronic devices.

Contribution

It introduces a novel open-framework metallic silicon allotrope at ambient conditions, synthesized via a Na-Si clathrate precursor, and confirms its stability and superconductivity.

Findings

Predicted a stable metallic Si allotrope P6/m-Si6 at ambient pressure.

Discovered a Na-Si clathrate P6/m-NaSi6 that can be converted into Si allotrope.

Both structures are superconducting with Tc around 12-13 K.

Abstract

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.