The superconductivity in the H-S system at above 144 GPa is reported up to 203 K, meaning that the strong electron-phonon coupling and high hydrogen (H) phonon frequencies are still effective in the traditional superconducting materials. Several theoretical calculations suggest that it is a new polyhydride H₃S that possesses such unusually high Tc.

However, the experimental data on the structure and composition of the superconducting phase have not been reported, which called for more detailed experimental investigation of high-pressure behavior of H₂S.

Professor Alexander Goncharov, a research member in Institute of Solid State Physics (ISSP), Hefei Science of Physical Science conducted a research on the record-high superconducting transition Tc material H₂S. The superconducting transition phase was confirmed to be R3m or Im-3m, and five new H-S compounds with different stoichiometries were firstly discovered at elevated pressures.

In his work, the results of low- (180 K) and room-temperature (295 K) x-ray diffraction (XRD) and Raman spectroscopy investigations to 150 GPa were reported, which revealed an increasing instability of H₂S with pressure and formation of previously unreported HxS compounds at elevated pressures.

At pressures above 55 GPa, the Cccm H₃S and other unknown sulfur-rich crystalline material, such as R3m (Im-3m) H₃S has been recorded above 110 GPa. However, it is found that the formation of this phase is impeded, which explains the previously observed dependence of Tc on the experimental P-T path.

The experiments were combined with theoretical variable-composition searches using the USPEX code that revealed stable H₃S, H₄S₃, H₅S₈, HS₂, and H₃S₅ compounds at 25–150 GPa.

These measurements clearly show the sluggishness of the transition to these phases and possible low crystallinity of materials prepared by compression at low temperatures, which could affect the superconducting properties and can explain why annealing at 295 K can increase the Tc.

The phase diagram of H-S system at high pressures and various temperatures. ( Image by Alexander Goncharov)

Link to the article: Hydrogen sulfide at high pressure: Change in stoichiometry

Contact:

Dr. JIANG Shuqing

Institute of Solid State Physics

Hefei, Anhui 230031, China

Tel:0551-65591159

E-mail: jiangshuqing@issp.ac.cn