The Investigation Of Barium Halides On Structure And Electronic Property Under High-Pressure

High pressure as a scientific methodology can effectively change the interaction between two adjacent atoms,and it will induce forming a series of new high-pressure phases with novel structure,physical and chemical properties.Under high pressure,a series condense matters have showed new phenomenon in mechanics,heat,light,electricity,etc.For example,metal sodium becomes a transparent insulator and hydrogen-rich compounds are transformed into high temperature superconductors under high pressure.Thus,it is significant for developing theoretical prediction method.Carrying out high-pressure research for typical material is remarkable for searching its most stable structures,ant it also can broad concerning on the synthesis and the fundamental structural properties of materials with implications for an entire family of similar materials.Such work can represent the frontier problem in the field of condensed matter physics.Thus,it is of great significance for the development of physics,chemistry and material science to carry out the structural evolution and physical properties investigation under high pressure.Barium halides BaX₂（X=F,Cl,Br,I）are the simplest and most representative ionic solids with great potential application value in medical treatment,electronics,chemical industry and other fields.At ambient pressure,BaF₂ shares Fm-3m（Z=4）structure with many fluoride compounds AF₂（A=Pb,Ca,Sr,et al.）.BaCl₂,BaBr₂,Ba I₂,lead chloride（Pb Cl₂）and stannic chloride（Sn Cl₂）adopt the same structure at ambient condition.Through the systematic literature research,we found that BaF₂has twice high pressure phase transitions up to 133 GPa and BaCl₂,BaBr₂ and Ba I₂have one high-pressure phase up to 40 GPa.Thus,the high pressure structure evolution of BaX₂（X=F,Cl,Br,I）between 0 to 200 GPa remains undetermined which directly limited the investigation of physical properties.According to the reported superconducting mechanism of Cs I,the increasing electrons of Cs is responsible for superconductivity and Ba atom with more electrons is potentially superior to Cs atom in the electron-phonon coupling.Thus,it is an interesting topic to in-vestigate the conduction characteristic at lowering pressure which imply much more ap-plication values.In this work,we carried out a structural search of BaF₂,BaCl₂,BaBr₂ and Ba I₂ using CALYPSO methodology which is based on density functional theory of first-principles calculation.The main results of the thesis are as follows:1.BaF₂ as the most simple barium halide implies significant high-pressure research value,which shows the special high-pressure behavior of typical compounds with low electron number and high protohymenium stress atoms.The high-symmetry cubic barium fluoride（BaF₂）with a space group of Fm-3m（Z=4）is the prototypical fluorite-type compounds at ambient condition,which is shared with many alkaline-earth fluorides.The study of high-pressure evolution of the BaF₂phase is of fundamental importance in helping to understand the structural sequence and principles of crystallography.In this work,we here have systematically investigated the high-pressure structural transition of BaF₂ up to 200 GPa using an effective CALYPSO methodology.Strikingly,two thermodynamically favored phases with orthorhombic Pnma and hexagonal P6₃/mmc symmetry are found at 3.6and 19.2 GPa,respectively.Distinguishingly,P6₃/mmc phase remains stable up to90.5 GPa,and then transform to Pnma structure.Further electronic calculations indicate that BaF₂ maintains insulating feature until 200 GPa.Our current results have broad implications for the special high-pressure behavior of typical compounds with low electron number and high protohymenium stress atoms at high pressure.2.Cs Cl and Cs Br have been reported appearing pressure induced metallization under high-pressure.BaCl₂ and BaBr₂ as the similar compound,they are potential metallic materials.As one of the most prototypical AX₂-type compounds,barium halide of BaCl₂ shared same orthorhombic Pnma structure with BaBr₂ at ambient pressure.In this work,we explored the crystal structures of BaCl₂ and BaBr₂ under high pressure.We found a thermodynamically more favored structure with orthorhombic Cmcm symmetry for both BaCl₂ and BaBr₂,at 74.2 and 47.4 GPa,respectively.Our simulations reveal that the metallic feature of Cmcm BaCl₂ and Cmcm BaBr₂ under high pressure.The present results improve the understanding of high-pressure structures and metallization of AX₂ compounds at extremely high-pressure conditions.3.Iodine compounds with superconductivity have been widely reported.Barium iodide（Ba I₂）as one of the simplest and most prototypical iodine compounds,implied substantially high-pressure investigation value,which can be new member of superconductivity-iodine-compound family.In this work,we explored the crystal structures of Ba I₂ at a wide pressure range of 0-200 GPa using a global structure search methodology.A thermodynamical structure with tetragonal I4/mmm symmetry of Ba I₂ is predicted to be stable at 17.1 GPa.Further electronic calculations indicate that I4/mmm Ba I₂ exhibits the metallic feature via an indirect band-gap closure under moderate pressure.We have also found that the superconductivity of Ba I₂ at 30 GPa is much lower than Cs I at 180 GPa based on our electron-phonon coupling simulations.Our current simulations provide a step toward the further understanding of high-pressure behavior and superconductivity of iodine compounds at extreme conditions.