Dilute carbon in H₃S under pressure

Recently, room temperature superconductivity was measured in a carbonaceous sulfur hydride material whose identity remains unknown. Herein, first-principles calculations are performed to provide a chemical basis for structural candidates derived by doping H₃S with low levels of carbon. Pressure stabilizes unusual bonding configurations about the carbon atoms, which can be six-fold coordinated as CH₆ entities within the cubic H₃S framework, or four-fold coordinated as methane intercalated into the H-S lattice, with or without an additional hydrogen in the framework. The doping breaks degenerate bands, lowering the density of states at the Fermi level (N_F), and localizing electrons in C-H bonds. Low levels of CH₄ doping do not increase N_F to values as high as those calculated for Im3 ¯ m-H₃S, but they can yield a larger logarithmic average phonon frequency, and an electron–phonon coupling parameter comparable to that of R3m-H₃S. The implications of carbon doping on the superconducting properties are discussed.