Possible "Magneli" 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 T-c, 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.