Search for high-Tc conventional superconductivity at megabar pressures in the lithium-sulfur system

TL;DR

This study uses ab-initio methods to explore high-temperature conventional superconductivity in the lithium-sulfur system under high pressure, finding limited superconductivity mainly in Li3S at very high pressures due to specific electronic hybridization effects.

Contribution

It predicts new stable phases in the Li-S system at high pressures and identifies the conditions under which high Tc superconductivity may occur in alkali metal compounds.

Findings

Li3S exhibits a sizable Tc above 640 GPa.

Several new phases are stabilized at pressures above 20 GPa.

Electride-like charge localization observed in predicted phases.

Abstract

Motivated by the recent report of superconductivity above 200 K in ultra-dense hydrogen sulfide, we search for high-\tc\ conventional superconductivity in the phase diagram of the binary Li-S system, using {\em ab-initio} methods for crystal structure prediction and linear response calculations for the electron-phonon coupling. We find that at pressures higher than 20 GPa, several new compositions, besides the known Li$_2$S, are stabilized; many exhibit electride-like {\em interstitial} charge localization observed in other alkali metal compounds. Of all predicted phases, only Li$_3$S at P > 640 GPa displays a sizable \tc, in contrast to what is observed in sulfur and phosphorus hydrides, where several stoichiometries lead to high \tc. We attribute this difference to 2$s$-2$p$ hybridization and avoided core overlap, and predict similar behavior for other alkali metal compounds.