We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen—C:(H2S)2H2 and from sulfur in mixed methane-hydrogen fluids—(CH4)x(H2S)(2x)H2 at 4 GPa. X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 58 and 143 GPa, respectively. Both samples show a similar Al2Cu-type I4/mcm basic symmetry, while the hydrogen subsystem evolves with pressure via variously ordered molecular and extended modifications. The methane-bearing sample lowers symmetry to an orthorhombic Pnma structure after laser heating to 1400 K at 143 GPa. The results suggest that C-S-H compounds are structurally different from a common Im3m H3S.

Bykova, Elena and Bykov, Maxim and Chariton, Stella and Prakapenka, Vitali B. and Glazyrin, Konstantin and Aslandukov, Andrey and Aslandukova, Alena and Criniti, Giacomo, Kurnosov, Alexander Goncharov, Alexander F., Structure and composition of C-S-H compounds up to 143  GPa, Phys. Rev. B 103 (14) L140105 (2021) abstract

XRD results on C-S-H compounds at high pressures. (a) Reconstructed reciprocal lattice plane defined by [100] and [001] directions of methane-substituted B2 batch samples at various pressures. (b) The structural model of I4/mcm (H2S)2H2 (see Supplemental Material for the unit-cell parameters and atomic positions). Only the H2 molecules were included in the structural model: those oriented along the c axis and in the ab plane below and above 14 GPa are both shown. The S-S closest distances are labeled. The distances marked as d1 and d4 are located in ab plane, while d2 and d3 distances connect S(C) atoms lying on the neighboring ab planes. Following laser heating at 143 GPa, we find a distortion of the unit cell, which becomes orthorhombic Pnma, but the arrangement of sulfur atoms repeats one in the parent I4/mcm structure; the positions of hydrogen atoms could not be determined. The potential positions of hydrogen atoms forming hydrogen bonds in the S cages can be inferred by analyzing the P dependencies of the d1 to d4 [Fig. 2].