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 HS-H and CH-H systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique, leading to the superconducting critical temperature T 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 AlCu-type determined for (HS)H and (CH)H 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 HS and CH components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting T found in the C-S-H system at megabar pressures.