Investigation On Structural Evolution And Electronic Properties Of Semiconducting Materials Of ZrS₂ And GaN Under High Pressure

Semiconductor materials have important application value in basic scientific researches,industrial productions and daily life.So exploring the new structures and new properties of such materials is an extremely meaningful subject.The internal structure of a material is a basic factor that determines its properties and the basis for understanding the macroscopic properties and microscopic mechanisms of materials.Most semiconductor materials have a strong covalent three-dimensional network structure.GaN as the star materials of the third generations semiconductors also have a three-dimensional network structure.Investigating the deeper and wider physical properties of these materials become an urgent problem to be solved.In addition,since the discovery of graphene,layered semiconductor materials attracted wide attention because of the unique structural characteristics.It has an orderly stack ability and show a series of novel properties,which expend the application of semiconductor materials.High pressure is an effective way to modify the structure of condensed matter and the distribution of charge in the atoms,thus can further induce structural phase transition.In addition,high-pressure way can also obtain the new structures by reducing the reaction barrier and promoting the chemical reactions.Therefore,it provides us with new ideas for finding new structures and properties of semiconductor materials.The transition metal chalcogenide is a typical two-dimensional layered material.Their chemical formula is MX₂,M is a transition metal,such as Hf,Ti,Zr,Nb,Ti.X is a chalcogen element,such as S,Se,and Te.Due to its unique layered structure characteristics,high pressure has a significant effect on the structural adjustments of this type of materials.It is worthwhile to explore the evolution of its structures and properties under high pressure.Group?nitrides?GaN,AlN,InN?are currently the most promising third-generation semiconductor materials.Such strong covalent network system compounds are not prone to structural phase transition under high pressure,and we consider chemical components and high pressure as common control methods to investigate the emergence of new structures with unconventional ratios.We focus our attention on ZrS₂ with a structure of 1T under ambient pressure.Earlier experiments have proved that ZrS₂ have a phase transition at around 8 GPa.Meanwhile it has been proposed that the resistivity of ZrS₂ decreased with increasing pressure experimentally,but the change was never great enough to render the materials metallic.Thus the structures and properties of ZrS₂ under pressure remain uncertain.We unravel the structural evolution as well as electronic properties of ZrS₂ at high pressure using intelligence CALYPSO structure searching simulations.A systematic analysis of the calculated enthalpy and phonon modes leads to identification of three thermodynamically stable structure P2₁/m,Immm and I4/mmm under high pressure.The ambient-pressure phase of ZrS₂ is trigonal P3?m1.The transition from the trigonal phase to monoclinic structure with a space group of P2₁/m occurs at 2.0 GPa.The consecutive transformation to Immm and I4/mmm occur at 5.7 GPa and 25 GPa,respectively.Then we compare the experimental XRD with that of new structure.We found that the phases at 15.2 GPa are the Immm and P2₁/m phases as the coexisting phases.The calculations of the electronic properties reveal that P3?m1 and P2₁/m are semiconductors.In contract,Immm and I4/mmm structures exhibit metallic behavior.We found two unusual stoichiometries GaN₅ and GaN₁₀ in the Ga-N system except GaN stable phase under high pressure.By analyzing its crystal structures characteristics.It is found that GaN₁₀ contains infinite nitrogen chains and GaN₅contains N-N single bonds.Moreover their energy densities of these two materials are3.8 kJ/g and 4.112 kJ/g,respectively,which have potential as high-energy density materials.Through the calculation of electronic properties indicated that GaN₁₀ exhibits metallic properties and GaN₅ exhibits semiconductors.Then we try to use a series of high-pressure methods to synthesize these two compounds.The Refined analysis of diffraction data by synchrotron radiation technology found that GaN₅ may be synthesized at 50 GPa and 2000 K.