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

We synthesized two C-S-H compounds from a mixture of carbon and sulfur in hydrogen—$\mathrm{C}:{\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ and from sulfur in mixed methane-hydrogen fluids—${\left(\mathrm{C}{\mathrm{H}}_4\right)}_x{\left({\mathrm{H}}_2\mathrm{S}\right)}_{\left(2-x\right)}{\mathrm{H}}_2$ 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 $Im\text{−}3m$ ${\mathrm{H}}_3\mathrm{S}$.

Figure 1

XRD results on C-S-H compounds at high pressures. (a) Reconstructed reciprocal lattice plane defined by [100] and [001] directions of methane-substituted B2 batch samples at various pressures. (b) The structural model of $I4/mcm$ ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ (see Supplemental Material for the unit-cell parameters and atomic positions). Only the ${\mathrm{H}}_2$ molecules were included in the structural model: those oriented along the $\mathit{c}$ axis and in the $\mathit{ab}$ plane below and above 14 GPa are both shown. The S-S closest distances are labeled. The distances marked as $d1$ and $d4$ are located in $ab$ plane, while $d2$ and $d3$ distances connect S(C) atoms lying on the neighboring $ab$ planes. Following laser heating at 143 GPa, we find a distortion of the unit cell, which becomes orthorhombic Pnma, but the arrangement of sulfur atoms repeats one in the parent $I4/mcm$ structure; the positions of hydrogen atoms could not be determined. The potential positions of hydrogen atoms forming hydrogen bonds in the S cages can be inferred by analyzing the P dependencies of the $d1$ to $d4$ [Fig. 2].

Figure 2

The unit cell and structure parameters determined in single-crystal XRD experiments. The results for both B1 and B2 batches are shown in blue and red, respectively. (a) The closest S(C)-S(C) distances. (b) The $\mathit{c}/\mathit{a}$ lattice parameters ratio. (c) The volumes per one S(C) atom; the results of this work for batches B1 and B2 (carbon and methane originated) are fit with the Vinet equation of state ($P_0=4.8\mathrm{GPa}$, $V_0=41\left(1\right){\AA }^3$, $K_0=23\left(2\right)\mathrm{GPa}$, ${K_0}^{\prime }=4.3\left(3\right)$). The error bars are smaller than the symbol size. Our data are compared to those of Refs. [19, 20, 21, 22, 30] for Cccm ($I4/mcm$) ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$; the results of Pace et al. [23], which agree well with other data at high pressures but scatter and largely disagree below 60 GPa, are not shown for clarity.

Figure 3

(a) Raman spectra of the B2 batch samples as a function of pressure. Red arrows mark the C-H stretching band peaks of methane molecules, which are redshifted with respect to those of a $\mathrm{C}{\mathrm{H}}_4-{\mathrm{H}}_2$ compound surrounding them. The first-order diamond signal of diamond anvils is masked by a rectangle, while the second-order diamond anvil signal at 71 GPa is marked by an asterisk. (b) Pressure dependence of the C-H Raman modes measured here on the B2 samples in comparison with the results of Snider et al. [7]. The inset shows an enlarged view of the Raman C-H stretch modes, where the data (dots) are fit using a phenomenological model (solid pink lines) and the results of the peak deconvolution are shown (gray lines).

Figure 4

Optical absorption spectra at various pressure of (a) B1 C-doped ${\left({\mathrm{H}}_2\mathrm{S}\right)}_2{\mathrm{H}}_2$ and (b) B2 ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_x{\left({\mathrm{H}}_2\mathrm{S}\right)}_{\left(2-x\right)}{\mathrm{H}}_2$. Dots are the data and lines are guides to the eye. The B2 sample at 143 GPa is probed after the laser heating experiment. The spectra are normalized to the transmission of the medium—${\mathrm{H}}_2$ and mixed ${\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_4$ medium, respectively.