Structure and composition of C-S-H compounds up to 143 GPa*

We synthesized two C-S-H compounds from a mixture of amorphous nano-carbon and sulfur in fluid hydrogen and from sulfur in mixed methane-hydrogen fluids by gentle laser heating at 4 GPa (c.f. [1]). X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 57 and 143 GPa, respectively. While both samples show similar structural properties (the structure will be reported at the meeting) reflecting the lattice properties of the heavier elements, Raman spectroscopy probing the local hydrogen structure and ordering show a sequence of phase changes from a plastic to orientationally ordered molecular crystal and, finally, to an extended solid. The equations of state of these materials are slightly different reflecting either a different carbon composition or a difference in the hydrogen local structure detected also by Raman spectroscopy. The basic structure remains unchanged even after laser heating to 1400 K at 143 GPa suggesting that the structure of superconducting C-S-H compound [1] is different from a common cubic H₃S reported superconducting with T_c=203 K at 150 GPa [2].
[1] Sneider, E., et al., Nature (2020); https://doi.org/10.1038/s41586-020-2801-z
[2] Drozdov, A. P., et al., Nature (2015); doi:10.1038/nature14964