A unique metallic phase of H 3 S at high-pressure: sulfur in three different local environments.

We propose a new metallic phase (R3[combining macron]m, Z = 24) for H3 S in a large pressure interval (∼108-166 GPa) using an evolutionary crystal structure search algorithm combined with first-principles calculations. This structure consists of SH6 , SH3 and S units which are connected through strong SH hydrogen-bonds. It supports four-types of SH hydrogen-bonds that symmetrize at ∼166 GPa. This is the only phase in the H-S family where sulfur exists in three different local environments. Sulfur in SH6 behaves as a cation, like in the SF6 molecule, whereas other sulfurs behave as anions. Hydrogens in SH6 behave like halogens (anions) whereas other hydrogens behave like alkalis (cations). The new structure has a substantially smaller DOS at the Fermi-level in comparison to earlier structures R3m and Im3[combining macron]m. This implies a lower superconducting temperature (TC ) for the new phase and thus questions the validity of earlier high-TC explanations. We also find that S-H covalent bonds are different from the B-B bonds of MgB2 .