First-principle Studies On The Structures And Properties Of Several Chalcogenide Compounds Under High Pressure

Intermetallic compounds have attracted a lot of attention because of their advantages as high temperature structural materials,and this anomalous temperature-strength relationship has sparked a new wave of research.Subsequently,many intermetallic compounds with different excellent properties were found in people's continuous research,for example,CeCo₅ and Re₂Co₁₇ are materials with permanent magnetic properties;TiAl and Ni₃Al are materials with high temperature and heat resistance;Nb₃Sn andV₃Ga are materials with high superconductivity;TiNi and CuZn are materials with shape memory function;GaAS is material with excellent semiconductor properties and so on.We systematically studied telluride Li-Te,Na-Te,Ga-Te and Li-Al-Te based on first principles.The high-pressure structure in several systems is predicted,and it is speculated that P4/nmm Li₂Te may be a fast ion conductor.The band gap of Imma Na₂Te increases with the pressure.Ga-Te system is semiconductor at ambient state,and covalent bonds are formed.At high pressure,it is metallic and ionic bonds are formed.In the Li-Al-Te system,the covalent bond between Al and Te atoms disappear under high pressure,and there is a weak interaction between Li and Al atoms.We believe that the high pressure effect and the increase of Li content are the main reasons for the disappearance of the covalent bond between Al and Te in the Li-Al-Te system.Pressure is an effective means to change the state and performance of substances.Increasing pressure can change gaseous into liquids,and most liquids can turn into solids in 1-2 GPa.Nitrogen oxides are diverse and exhibit fascinating structural and electronic properties under pressure.For example,N₂O₄ and N₂O₅ are ionic phases under high pressure.The first principles density functional theory is used to study the N₂O₄,N₂O₅ under high pressure,we find that the well-known structure Im3 N₂O₄ can stabilize between 2-35 GPa,then transforms to P4₂2₁2 phase.N₂O₄ is insulated in the whole pressure we study,and there is a kind of weak covalent bond between the two N atoms.The high-pressure structure P2₁/m N₂O₅ is stable from 29 to 120 GPa,and it is insulated in the entire pressure ranges.The studies on mechanical properties find that it is more likely to be compressed along the b-axis,which could be due to the weak force among the layers.Zinc oxide has attracted extensive attention due to its photoelectric application.On the one hand,ZnO is n-type semiconductor,the realization of p-type ZnO is the focus of current research.On the other hand,the ZnO energy band engineering is also a hot topic at present.It has been found that the isoelectronic doping,S,Se,Te in the same group as oxygen atom doping and substitution will bring excellent properties.We systematically research the ZnOS and ZnOTe alloys by first-principle calculations,in the ZnOS alloys,we predict a stable structure P2₁/m Zn₄OS₃ and several metastable phases,and find that pressure can promote to form stable ZnOS alloy.The variation of band-gap of P21/m Zn₄OS₃ is increasing first then decreasing with pressure.At ambient pressure,the band gap of ZnOS alloy increases first and then decreases with the S increases.We presum that the possible reason is the movement of Zn s state and the S p state.For ZnOTe alloy,we confirmed it can't form the thermodynamiac stable structure,can only form metastable phase.At the same time we also predicted a few metastable phase.C2/m Zn₅O₂Te₃ is a metal phase,while the other structures are all semiconductor phases.It is also found that under normal pressure,with the addition of Te,the band gap of the semiconductor phase first decreases and then increases,which is similar to the results of previous studies.The reason of band gap changes are explored,we believe that the changes in the valence state near the Fermi surface and the weakly hybridization between Te p state and Zn d state may be the reasons of the band gap variation.It was found in the study of optical properties that with the increase of Te content,the polarization ability of the material was enhanced,and the peak shift characteristics of the imaginary frequency part of the dielectric function confirms the change trend of bandgap of the material.