Synthesis and structure of carbon-doped H3S compounds at high pressure
Alexander F. Goncharov, Elena Bykova, Maxim Bykov, Xiao Zhang, Yu Wang, Stella Chariton, Vitali B. Prakapenka, Jesse S. Smith
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.