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

X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room-temperature superconductor is derived from previously established van der Waals compounds found in the H₂S-H₂ and CH₄-H₂ systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique, leading to the superconducting critical temperature T_c of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al₂Cu-type determined for (H₂S)₂H₂ and (CH₄)₂H₂ that differs from those predicted and observed for the S-H system at these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H₂S and CH₄ components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting T_c found in the C-S-H system at megabar pressures.