Mechanism for the Structural Transformation to the Modulated Superconducting Phase of Compressed Hydrogen Sulfide

Arnab Majumdar,John S Tse,Yansun Yao

doi:10.1038/s41598-019-41607-1

Abstract

A comprehensive description of crystal and electronic structures, structural transformations, and pressure-dependent superconducting temperature (Tc) of hydrogen sulfide (H2S) compressed from low pressure is presented through the analysis of the results from metadynamics simulations. It is shown that local minimum metastable crystal structures obtained are dependent on the choice of pressure-temperature thermodynamic paths. The origin of the recently proposed ‘high-Tc’ superconducting phase with a modulated structure and a diffraction pattern reproducing two independent experiments was the low pressure Pmc21 structure. This Pmc21 structure is found to transform to a Pc structure at 80 K and 80 GPa which becomes metallic and superconductive above 100 GPa. This structure becomes dynamically unstable above 140 GPa beyond which phonon instability sets in at about a quarter in the Γ to Y segment. This explains the transformation to a 1:3 modulation structure at high pressures proposed previously. The pressure trend of the calculated Tc for the Pc structure is consistent with the experimentally measured ‘low-Tc phase’. Fermi surface analysis hints that pressurized hydrogen sulfide may be a multi-band superconductor. The theoretical results reproduced many experimental characteristics, suggesting that the dissociation of H2S is unrequired to explain the superconductivity of compressed H2S at any pressure.

Highlights

Patterns are not by coincidence, raising a question on the origin
Theoretical calculations show that the transverse acoustic phonon modes in the Pc structure start to soften near 140 GPa which gradually develop to an imaginary branch near the boundary of the Brillouin zone (BZ) at higher pressures
The electronic structure, lattice dynamics, and electron-phonon coupling of a new Pc structure of compressed hydrogen sulfide revealed from metadynamics calculations were presented and discussed in detail

Summary

Introduction

Patterns are not by coincidence, raising a question on the origin. It should be noted that Guigue et al.[11] have performed a direct synthesis of H3S from elemental materials but only obtained an orthorhombic Cccm structure up to 160 GPa, rather than the suggested Im-3m structure. The discrepancies in the observed crystal structures obtained from different thermodynamic P-T pathways raised the possibility that the superconducting ‘high-Tc phase’ could be from a kinetically controlled metastable product and may not be the predicted H3S. It was found that H2S does not decompose into H3S and S at the pressure range of interest Instead, it transforms to a modulated structure with calculated diffraction pattern matching well with that of the observed ‘high-Tc phase’[6]. We report results of ab initio metadynamics calculations performed at a broad P–T regime to trace the transformation pathway from the low-pressure H2S precursor to the high pressure modulated structure. The Tc of the modulated structure is estimated to be within 110–220 K at 200 GPa, which compares well to the measured Tc for the ‘high-Tc phase’. The Pc structure is likely to be the underlying structure along the experimental compression path

Methods

Results

Conclusion