Possible "Magn\'eli" phases and self-alloying in the superconducting sulfur hydride

TL;DR

This paper predicts an infinite variety of metastable, modulated crystal structures in sulfur hydride under pressure, explaining experimental superconducting behavior through phase alloying and structural transformations.

Contribution

It introduces a theoretical framework for long-period modulated structures and phase alloying in sulfur hydride, expanding understanding of its superconducting properties.

Findings

Existence of long-period modulated crystal structures.

Small formation enthalpy at H$_{2}$S--H$_{3}$S boundaries.

Explains pressure-dependent superconducting transition temperatures.

Abstract

We theoretically give an infinite number of metastable crystal structures for the superconducting sulfur hydride H$_{x}$S under pressure. It has been thought that theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low and high $T_{\rm c}$ in the experiment, respectively. The newly found structures are long-period modulated crystals where slab-like H$_{2}$S and H$_{3}$S regions intergrow in a microscopic scale. The extremely small formation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the first-principles calculations suggests possible alloying of these phases through formation of local H$_{3}$S regions. The modulated structures and gradual alloying transformations between them not only explain the peculiar pressure dependence of $T_{\rm c}$ in sulfur hydride observed experimentally, but also could prevail in the experimental samples under various compression schemes.