The sulfur-hydrogen system is the first one in which superconductivity at temperatures over 200 K has been reported, albeit at high pressure. The particular phases causing the measured Tc and their structures are not yet firmly identified. Here, synchrotron single-crystal x-ray diffraction studies of S-H samples were performed up to 150 GPa and revealed two previously unobserved and unpredicted sulfur-hydrogen phases—H6±xS5 with x0.4, and H2.85±yS2 with y0.35. The crystallographic data obtained in this work, both for the new phases and for the previously identified H3S polymorphs, provide an unambiguous experimental proof of the chemical richness of the S-H system and the structural diversity of compounds forming at high pressures and high temperatures. Our results have profound implications for the interpretation of the resistance, superconductivity, and other physical properties measurements on the complex S-H system.

Dominique Laniel, Bjoern Winkler, Elena Bykova, Timofey Fedotenko, Stella Chariton, Victor Milman, Maxim Bykov, Vitali Prakapenka, Leonid Dubrovinsky, and Natalia Dubrovinskaia: Novel sulfur hydrides synthesized at extreme conditions. Physical Review B 102, 134109 (2020) DOI: 10.1103/PhysRevB.102.134109. abstract

Structure of the tI140 compound (hydrogen atoms not shown). (a) A single sinusoidal strand of S atoms. (b) The tI140 unit cell. Yellow spheres represent sulfur atoms forming sinusoidal chains, while the blue spheres are sulfur atoms linking the chains together.