Crystal Structures And Superconductivity Of Typical Binary Metal Sulfides Under High Pressure

Metal sulfide is a classical system in physics,materials science and electronics,etc.It is possible to obtain novel crystal structures and physical properties by introducing high pressure method into this system.Among the abundant metal sulfides,two kinds of metal sulfides are selected as our main research objects: sulfides composed of VA-group metal and VIB-group transition metal elements.First is the VA-group chalcogenides belonging to the A₂B₃(A = Bi,Sb,As;B = S,Se,Te)family.These sulfides have attracted considerable attention due to their excellent thermoelectric properties even to technological applications.Under high-pressure,these trichalcogenides are prone to undergo several phase transitions,even exhibits the superconducting properties and topological superconductivity characteristics.Therefore,further investigation on VA-group under high pressure is needed to obtain novel superior physical properties.Second is the typical transition metal dichalcogenides MX2(M = Mo,W,V,X = S,Se,Te),these chalcogenides have attracted tremendous attention owing to their rich crystal structures and the consequent potential applications.For instance,large magnetoresistance effect,charge density wave phases at low temperatures,superconducting states,topological phases,etc.These properties make typical transition metal dichalcogenides highly attractive for fundamental studies of novel physical phenomena and for applications in nanoeletronics and other areas.The typical VA-group trichalcogenide Sb₂S₃ and VIB-group transition-metal dichalcogenide 2M-WS2 have been studied systematically under high-pressure.By combining high-pressure X-ray diffraction experiments,electrical resistance measurements and first-principle theoretical calculation,the pressure-induced decomposition and superconducting characteristic were observed in compressed Sb₂S₃.Phase transition and pressure-driven reemergence of superconductivity in 2M-WS2 were presented in our work.Our results provide new insight of the crystal structure and enrich the superconducting phase in chalcogenide materials under pressure.The detailed results are concluded as follows:(1)Through X-ray diffraction measurements and theoretical calculations,the structural stability in Sb₂S₃ under high pressure was detected.Two new compounds Pm(?)m-SbS and P21-SbS₂ were discovered in pressure-induced decomposition of Sb₂S₃.Different from semiconducting precursor Sb₂S₃,both Pm(?) m-SbS and P21-SbS₂ are found metallic.Furthermore,both electrical resistance and the external magnetic field experiments reveal that Pm(?)m-SbS has a superconducting state at low temperatures.The measured Tc exhibits a nearly linear increase with respect to the pressure,starting at about 3.4 K at 42.0 GPa and reaching to 6.6 K at 75 GPa.The tendency of the experimental Tc values is contrary to the values calculated based on BCS theory,indicating the abnormal superconducting behaviour in Pm(?) m-SbS.These findings expand the stoichiometries in Sb-S system and provide new insights for experimental search of novel superconductors.(2)The structural evolution of the recent synthesized sample 2M-WS2 was investigated under high pressure.We found that 2M-WS2 transformed into a semiconducting rhombohedral 3R-WS2 phase starting from 14.7 GPa and finishing at 40.7 GPa.This structural transition results in the contraction of the unit cell volume about 5.1 % at 20 GPa rising from the significant decrease of the interlayer distance.The electrical resistance measurements were conducted to detect the transport properties of the compressed 2M-WS2.Tc in the initial superconducting phase 2M-WS2 at ambient pressure drops from the maximum of 8.8 K to become undetected above17.6 GPa.The successive metallization and superconductivity transitions in 3R-WS2 are observed at 48.8 GPa with Tc 2.5 K.Combined with the theoretical calculations,we propose that the degradation of superconductivity in 2M-WS2 and reemergence of superconductivity in 3R-WS2 mainly attributed to the change of density of states near the Fermi surface driven by the interlayer coupling.We report the first high-pressure experimental research of 2M-WS2 and the observation of superconductivity in 3R phase among VIB-group TMDs.