Orthorhombic Si6: Prediction of a metastable sp3 silicon semimetal with pseudo-Dirac nodal line

Abstract

Releasing silicon from high pressure will lead to diverse metastable structures depending on the experimental conditions. However, identifying crystal structure of the metastable silicon phases becomes challenging based on limited diffraction data. In this paper, an orthorhombic silicon structure (named oP-Si6) has been proposed through structural searches. The oP-Si6 structure is a pseudo-Dirac nodal line semimetal, but could become superconducting by applying uniaxial compression. When comparing the calculated d-spacings of fully-relaxed oP-Si6 with those of experimental diffraction data from bulk silicon irradiated with relativistic laser intensities, it indicates that oP-Si6 is possible to have been synthesized in the previous experiments, as is supported by the energy barrier calculations. This work enrich the silicon metastable allotrope family with novel physical properties and provide more explanation on the structural identification of previous experimental data.