First-principle Studies On High-pressure Phase Transitions And Electronic Structures Of Metal Chalcogenides

Metal chalcogenide compounds,as a new type of quasi-two-dimensional materials,due to their unique properties including tunable electronic,optical and transport properties,fine chemical stability and high mechanical flexibility,have a broad range of application prospects in two-dimensional electronic devices,energy storage materials,catalysts etc.On the other hand,applying pressure has been considered as an effective and clean way to tune lattice as well as electronic states,which is used widely in the condensed matter physics.In this thesis,we focus on three ordinary layered metal chalcogenide systems,and study the structural phase transitions and the relevant electronic properties of layered materials under high pressure,by using first-principles calculations which is based on the density functional theory.First,using first-principles calculations and crystallographic structure search techniques,we predict two stable phases under high pressure,the 1T' phase and the 2H phase,of which the pressure ranges are 5?10 GPa and 10?30 GPa,respectively.The 1T' phase of WTe2 is identified through high-pressure synchrotron radiation X-ray diffraction(XRD)experiments,with a critical pressure of 4 GPa.Seeing that the 1T'structure has spatial inversion symmetry,there is an electronic topological phase transition from Weyl semimetal to trivial semimetal,company with the structural phase transition from the ambient Td phase to the 1T'.Through electron-phonon coupling calculations,it is suggested that the pressure-induced superconducting transition may be attributed to the softening of vibrational modes resulting from the enhancement of interlayer Te-Te interaction.However,the 2H phase of WTe2 is not observed in the XRD measurement,which may account for the large transition barrier due to the distinction between the 1T' and the 2H phase in structure.Next,we discuss a robust topological Weyl semimetal(WSM)state,which is protected by C2T symmetry and can be realized in non-magnetic materials without spatial inversion symmetry.In these systems,each Weyl point carries a chiral charge of±2 if spin-orbit coupling(SOC)is omitted,and it splits into two single spin-1/2 WPs with equal chiral charge when SOC is switched on.We propose a prototype for realizing such a kind of WSM state,rooted in the high-pressure phase of TiS2(space group:P-62m).The separation between opposite chiral charges is rather large(much larger than that of the known WSM TaAs and MoTe2),suggesting the robustness of the WSM state in this system.Finally,we focus on the famous thermoelectric material SnSe,and discuss the evolutions of its physical properties with respect to the pressure.Previous studies imply that SnSe undergoes a continuous structural phase transition at around 10 GPa,from the ambient a phase to the high-pressure ? phase.Using the first-principles calculations combined with high-pressure synchrotron radiation XRD,we identify a structural phase transition occurring at a much higher pressure(-27 GPa),from the ? phase to a CsCl-type phase.Through electrical transport measurements under high pressure,we find that CsCl-type SnSe exhibits superconductivity with a maximum critical temperature of around 3.2 K(?39 GPa).Moreover,electronic band structure calculations suggest that CsCl-type SnSe is a topological nodal line semimetal,with non-trivial surface states.Moreover,we also obtained some interesting results on pressure-induced structural and superconducting phase transitions in TaAs,TiS3,LaRu2P2,SrFe2As2,etc.Our studies imply that applying high pressure can effectively induce structural/electronic phase transitions,and is an important way to synthesize new materials and realize exotic electronic states.