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

Abstract

Releasing silicon from high pressure will lead to diverse metastable structures depending on the experimental conditions. However, identifying the crystal structure of the metastable silicon phases becomes challenging based on limited diffraction data. In this study, 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. Comparison of the calculated d-spacings of fully-relaxed oP-Si₆ with experimental diffraction data from bulk silicon after irradiation with relativistic laser intensities indicates that oP-Si₆ would have been synthesized in previous experiments, as supported by the energy barrier calculations. This work enriches the silicon metastable allotrope family with novel physical properties and provides further insights into the structural identification of previous experimental data.