Predicted open-framework crystal structures of sodium-silicon at high pressures

Abstract

Open-framework sodium silicides have been attracted great attention for their outstanding physical properties, as well as potential candidates to obtain fascinating metastable Si allotrope for the practical use. In this work, the pressure-composition phase diagram of the Na-Si system (NaSix, x=1-8) within the pressure range of 0-20 GPa has been performed via the global structure search coupled within the framework of density functional theory. We found eight NaSix structures with open channels that are composed of the polygonal Si lattices and filled with Na atoms. The predicted NaSix compounds are dynamically stable at high pressures by exhibiting metallic behaviors. The finding of new open-framework NaSix structures significantly improves the understanding of the relationship between crystal structures and electronic properties of sodium silicide compounds at high pressures.