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

Abstract

We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen---$\mathrm{C}:{({\mathrm{H}}_{2}\mathrm{S})}_{2}{\mathrm{H}}_{2}$ and from sulfur in mixed methane-hydrogen fluids---${(\mathrm{C}{\mathrm{H}}_{4})}_{x}{({\mathrm{H}}_{2}\mathrm{S})}_{(2\ensuremath{-}x)}{\mathrm{H}}_{2}$ 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 ${\mathrm{Al}}_{2}\mathrm{Cu}$-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 $Im\text{\ensuremath{-}}3m$ ${\mathrm{H}}_{3}\mathrm{S}$.