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 P (1) over bar and Cmca structures and is competitive with the P (1) over bar and Cmca phases at 150-175 GPa. The main difference between the C2/m and P (1) over bar 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 P (1) over bar 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 53-74K at 150 GPa. (C) 2018 The Japan Society of Applied Physics