The discovery of high temperature superconductivity (T_­c = 203 K) in the dense sulfur hydrogen system grasped the attention of the high-pressure community.¹ The experimental measurement was made upon compression of H₂S, but prior theoretical predictions found a T_c of 204 K actually emerged from compression of (H₂S)₂H₂ to high densities.² (H₂S)₂H₂ was predicted to undergo a reaction and form molecular H₃S above 1.1 Mbar, which would exhibit high T_­c superconducting properties at 1.8 Mbar – the experimental measurement is therefore thought to arise from the decomposition and recombination of H₂S into H₃S + S. However, direct experimental study of (H₂S)₂H₂ itself has thus far been limited to room temperature.^3,4 We synthesise (H₂S)₂H₂ directly from S and H₂ within diamond anvil cells at low pressures, and study the solid phases up to very high densities at low temperatures, using in-situ Raman spectroscopy and synchrotron x-ray diffraction.

1. A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, and S. I. Shylin, Nature 525, 73 (2015)

2. D. Duan, Y. Liu, F. Tian, D. Li, X. Huang, Z. Zhao, H. Yu, B. Liu, W. Tian, and T. Cui, Scientific Reports 4, 6968 (2014)

3. T. A. Strobel, P. Ganesh, M. Somayazulu, P. R. C. Kent, and R. J. Hemley, Physical Review Letters 107, 255503 (2011)

4. S. Duwal and C.-S. Yoo, The Journal of Physical Chemistry C 121, 12863 (2017)