Possible “Magnéli” Phases and Self-Alloying in the Superconducting Sulfur Hydride

We theoretically give an infinite number of metastable crystal structures for the superconducting sulfur hydride HₓS under pressure. Previously predicted crystalline phases of H₂S and H₃S have been thought to have important roles for experimentally observed low and high Tc, respectively. The newly found structures are long-period modulated crystals where slablike H₂S and H₃S regions intergrow on a microscopic scale. The extremely small formation enthalpy for the H₂S-H₃S boundary indicated by first-principles calculations suggests possible alloying of these phases through the formation of local H₃S regions. The modulated structures and gradual alloying transformations between them not only explain the peculiar pressure dependence of Tc in sulfur hydride observed experimentally, but also could prevail in the experimental samples under various compression schemes.