First-principle Study On Structures And Properties Of H₂S Under High Pressure

Recently,followed the theoretical prediction of the high pressure superconductors,scientists have found the H₂S of T_C 203K under 155 GPa experimentally.This report has attracted many attentions and caused a great deal of research in the study of high-pressure hydrogen sulfide and other hydrides.In our study,structure researches are performed using a genetic algorithm combinaation with first-principles caculations method based on density functional theory.The structural phase transition,band structure,phonon spectrum and superconductivity of H₂S under high pressure are studied.The results are as follows:By using a genetic algorithm for searching lowest-enthalpy crystal structures,we obtained a stable C2/m phase at 150-175 GPa.The C2/m structure is a monoclinic structure composed of 4 H₂S f.u.in an unit cell.It is found that the enthalpy of formation of these three structures is very close,which shows the same thermodynamic stability.It is suggested that the structure of H₂S may coexist in multiphase at this pressure.By comparing the structures of C2/m,P-1 and Cmca,it can be found that they are all layered structures.The main difference between the C2/m and P-1 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 were twisted.The transition barrier of P-1 structure to C2/m structure and C2/m structure to Cmca structure are further studied.The phase transition barrier of P-1 structure to C2/m structure is only 0.2 meV,and the H3 proton can fluctuate freely during the two phase transition process.The quantum tunneling effect of protons may exist.The electronic property of the C2/m structure at 150 GPa were calculated.It has a large density of states?DOS?near the Fermi level,showing metallic property.The calculated results of phonon frequency show that the structure of C2/m would be dynamically stable.And the superconducting transition temperature of the C2/m structure is 53-74 K at 150 GPa.