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Synthesis and structure of carbon doped H3S compounds at high pressure

Understanding of recently reported putative close-to-room-temperature superconductivity in C-S-H compounds at 267 GPa demands reproducible synthesis protocol as well as knowledge of its structure and composition. We synthesized C-S-H compounds with various carbon composition at high pressures from elemental C and methane CH4, sulfur S, and molecular hydrogen H2. Here we focus on compounds synthesized using methane as these allow a straightforward determination of their structure and composition by combining single-crystal X-ray diffraction (XRD) and Raman spectroscopy. We applied a two-stage synthesis of ((CH4)x(H2S)(1-x))2H2 compounds by first reacting sulfur and mixed methane-hydrogen fluids and forming CH4 doped H2S crystals at 0.5-3 GPa, and then by growing single crystals of the desired hydrogen rich compound. Raman spectroscopy applied to this material shows the presence of the CH4 molecules incorporated into the lattice and allows to determine the CH4 content, while single-crystal X-ray diffraction results suggest that the methane molecules substitute H2S ones. The structural behavior of these compounds is very similar to the previously investigated methane free compounds demonstrating a transition from Al2Cu type I4/mcm structure to a modulated structure at 20-30 GPa and back to the same basic structure in an extended modification with greatly modified Raman spectra. This latter phase demonstrates a distortion into Pnma structure at 132-159 GPa and then transforms into a common Im-3m H3S phase at higher pressures, however, no structural anomaly is detected near 220 GPa, where a sharp upturn in Tc has been reported.