A stable C2/m phase of H2S at 150–175 GPa.

High temperature superconductivity of H₂S at high pressure is fascinating, and there is still controversy about crystalline structures and stoichiometry responsible for high T_c. Determining the crystal structures of H₂S at high pressure is a tremendous challenge in experiments. We found a previously missing structure with C2/m space group symmetry by genetic algorithm and first-principles calculations, which is an intermediate structure between the and Cmca structures and is competitive with the and Cmca phases at 150–175 GPa. The main difference between the C2/m and structure is the S–H3 bond lengths in the two structures. The C2/m and Cmca structures consist of similar structural units (H₄S₆ rectangular units). In the C2/m structure the chains formed by the H₄S₆ rectangular units are parallel, while in the Cmca structure the H₄S₆ rectangular units are twisted. The possible structural transformations from to C2/m to Cmca structure were also discussed. By calculating the electronic structure, phonon spectrum and electron–phonon coupling parameters, the C2/m structure was predicted to have a superconducting transition temperature T_c of 53–74 K at 150 GPa. [ABSTRACT FROM AUTHOR]