Synthesis and structure of carbon-doped H3S compounds at high pressure

Abstract

Understanding of recently reported putative close-to-room-temperature superconductivity in C–S–H compounds at 267 GPa demands a reproducible synthesis protocol as well as knowledge of the compounds' structure and composition. We synthesized C–S–H compounds with various carbon compositions at high pressures from elemental carbon 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 and Raman spectroscopy. We applied a two-stage synthesis of [(CH4)x(H2S)(1−x)]2H2 compounds with various compositions 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 CH4 molecules incorporated into the lattice and allows the determination of the CH4 content, while single-crystal x-ray diffraction results suggest that the methane molecules substitute H2S molecules. The structural behavior of these compounds is very similar to the previously investigated methane-free crystals demonstrating a transition from Al2Cu type I4/mcm molecular crystal to a modulated molecular structure at 20–30 GPa and back to the same basic I4/mcm structure in an extended modification with greatly modified Raman spectra. This latter phase demonstrates a distortion into a Pnma structure at 132–159 GPa and then transforms into a common Im3¯m H3S phase at higher pressures; however, no structural anomaly is detected near 220 GPa, where a sharp upturn in Tc has been reported.