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-C : (H 2 S) 2 H 2 and from sulfur in mixed methane-hydrogen fluids-(CH 4 ) x (H 2 S) (2−x) 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 Al 2 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-3m H 3 S.

Lower pressure phases and metastable states of superconducting photo-induced carbonaceous sulfur hydride

“…The overall general volume trends for all of the C-S-H crystals measured are equal or lower than that of our own measurements on pure H 2 S+H 2 , which in turn is noticeably lower than that reported for C-S-H prepared from from mixtures of molecular H 2 S+CH 4 +H 2 gases. [26] At low pressures (18 GPa), SC-XRD measurements confirm phase I [1] as the Al 2 Cu-type structure (I4/mcm) previously identified in molecular mixtures of CH 4 and H 2 , [18] and in H 2 S and H 2 . [19,26] The I4/mcm phase is inferred between 4 to 9 GPa as no observed change occurred in Raman spectroscopy.…”

“…[26] At low pressures (18 GPa), SC-XRD measurements confirm phase I [1] as the Al 2 Cu-type structure (I4/mcm) previously identified in molecular mixtures of CH 4 and H 2 , [18] and in H 2 S and H 2 . [19,26] The I4/mcm phase is inferred between 4 to 9 GPa as no observed change occurred in Raman spectroscopy. Due to insufficient C concentration or unique crystallographic placements, SC-XRD measurements are unable to resolve between C and S on the 8h Wyckoff positions, thus Figure 3a displays only H 2 S units on the 8h sites.…”

“…Synthesis beginning from gas phase molecular precursors presents a promising path to greater control over the chemical homogeneity of C-S-H samples. [20,26] However, based on the path dependence metastability reported here, it needs to be confirmed that this route can produce superconducting C-S-H samples. Ambient conditions superconductivity will eventually be achieved with hydrogen dominant alloys produced with precise chemical control.…”

“…The pressure-volume relations of 8 different C-S-H crystals across three different DACs that were studied in this work are shown in circles. Also included are previously reported values from Bykova et al [26] in triangles, as well as pure hydrogen sulfide in squares. A 2nd Order Birch-Murnaghan equation of state was fit across all data resulting in V0 = 400.573 Å 3 and K0 = 11.028 GPa fit parameters.…”

X-ray diffraction and equation of state of the C–S–H room-temperature superconductor

“…those for 020o and 201o are estimated from the observed spread in the data. 31 Figure 5 shows the P-V relations obtained from diffraction data as a function of pressure from 124 to 178 GPa. We also compare the measured volumes reported for the C-S-H material by Bykova et al 31 and those reported for S-H phases.…”