A first-principles study on crystal structures and metallization of sodium-rich sulfides under high pressure

We performed systematical theoretical simulations on phase diagrams, crystal structures, electron properties, and phonon features of Na-S system under high pressures. NaS, Na₂S, and Na₄S, were found to be stable under pressures. The superconducting transition critical temperature was estimated to nearly 0 K at 100 GPa in Na₃S due to the weak electron-phonon coupling. Furthermore, by the comparison on the structures, the electron features, and alkali metal ions of stoichiometric proportion, we found that not only the pressure but also the number of sodium atoms in the formula unit of alkali metal atoms can promote the insulator-metal transformation in the alkali metal sulfides, such as Li-S, Na-S, and K-S systems.