Study On Properties Of Transition Metal Dichalcogenides PdX₂（X=Se,S） Under High Pressure

Two-dimensional（2D）transition metal dichalcogenides have received extensive attention because of their novel physical and chemical properties and broad application prospects in the future.They are a class of materials that may affect the future application of nanoelectronics and optoelectronic technology.Designing and finding new functional materials in micro-nano size is an important and particularly interesting subject.At present,researchers have discovered a variety of polymorphs PdX₂（X=S,Se）materials,especially its pentagonal layered structure has attracted extensive attentions.Based on first-principles calculation and simulation,we explored the properties of the transition metal dichalcogenides PdX₂（X=Se,S）under high pressure or stress,and carried out research mainly in three aspects:（1）Theoretical prediction a new 2D layered structure material PdSe₂ with high mobility and anisotropic carriers.Under hydrostatic pressure,the monoclinic phase of PdSe₂（I2/a phase）transforms into two similar 2D layered monoclinic C2/m and hexagonal3?8)1 phases,and the phase transition pressures are 4.5 GPa and 17.5 GPa,respectively.The new 2D monolayer material PdSe₂ shows a low exfoliation energy,which can be exfoliated from the monoclinic C2/m bulk phase with kinetic and thermodynamic stability.This new 2D single-layer PdSe₂ material has a band gap value of about 1.10 eV,exhibits excellent visible light absorption,and can be applied as photovoltaic material.In addition,the effective mass of electrons and holes and the carrier mobility show unusual anisotropy,which indicates that monoclinic monolayer PdSe₂ is a promising 2D material that can be used as an effective electron/hole separation material in high performance nanoelectronic devices.（2）Study on the structural properties and electronic structure changes of the orthogonal phase of PdSe₂ material under uniaxial stress.It is revealed for the first time that orthogonal phase PdSe₂ is a ferroelastic material,and the stress drives 90°lattice rotation in the layer stacking direction.The ferroelastic phase transition originates from the bond reconstruction in the unusual square planar（PdSe₄）^2-structural unit.It has a particularly low phase transition energy barrier and big ferroelastic strain,and can be used in the shape memory field.Moreover,under the uniaxial compressive stress,the ferroelastic phase transition is accompanied by the semiconductor-metal-semiconductor transitions and can be applied to electronic switching devices.In addition,the band gap is closely related to the interlayer spacing and can be controlled by uniaxial tensile stress.These excellent stress engineering characteristics indicate that the orthogonal phase PdSe₂ shows potential application prospects in microelectronic and nanoelectronic devices.（3）Study on the crystal structure,electronic structure and transport properties of quasi-2D layered PdS₂ materials under uniaxial stress and hydrostatic pressure.Under uniaxial compressive stress,the PdS₂ material shows ferroelastic phase transition with lattice reorientation.The phase transition results from the reconstruction of the chemical bond in the unusual square planar（PdS₄）^2-structural unit.Under hydrostatic pressure,the layered orthogonal structure transforms into a three-dimensional cubic pyrite structure.The experiment believes that the intermediate phase is a layered PdS₂type structure coexisting with the cubic pyrite structure.Our theoretical calculations propose that the compression-induced intermediate phase has the same structural symmetry with that under environmental conditions,but the interlayer spacing is reduced.The coordination environment of Pd²⁺ions not only plays a key role in the structural transformation,but also leads to changes in the electronic structure and transport properties.In the intermediate stage,the distorted octahedron transforms to regular octahedron,resulting in bandwidth widen and orbital selective metallization.In addition,the superconductivity in the cubic pyrite structure comes from the topological nodal state and strong election-phonon coupling.