Structures And Superconductivity Of Typical Metal Chalcogenides And Hydrides Under High Pressure

Superconductive materials with zero resistance and the Meissner effect have broad application prospects.After the discovery of superconductivity,the search for new superconductors has been one of the hot spots in the field of condensed matter physics.High pressure synthesis new materials are an important method.Pressure is one of the fundamental thermodynamic parameters.Pressure can shorten the interatomic distance and increase the overlapping of electronic orbitals between neighboring atoms,thus inducing electronic phase transitions and even structural phase transition.In addition,pressure helps to overcome the chemical reaction barrier between different materials and promotes the occurrence of chemical reactions.Therefore,high-pressure technology has become an important method to obtain new superconductors.Novel quantum states and high-temperature superconductors are two important directions in the exploration of new superconductors.As typical representatives of these two aspects,chalcogenides and hydrides have attracted extensive attention in recent years.In this thesis,based on the diamond-anvil cell,the in-situ high temperature and high-pressure experimental technology were used to conduct in-depth and systematic high-pressure experimental studies on the typical semiconductor materials Ge Te and Sb₂S₃and Rare Earth metal hydrides Y-H and Yb-H systems.The results are as follows:1.As a narrow-bandgap semiconductor,the behavior of Ge Te at high pressure has been widely concerned,but its crystal structure and superconductivity remain under debate for decades.Herein,we systematically investigated the pressure dependence of superconductivity and the structural evolution of the Ge Te via X-ray diffraction,Raman scattering and electric resistance measurements.The experimental results show that Ge Te underwent two structural phase transitions at 4.1 and 13.4 GPa and finally transformed into an orthorhombic Pnma phase.Furthermore,the metallization occurred at around 11.0 GPa,where superconductivity could also be observed.With increasing pressure,the superconducting transition temperature increases monotonically from 5.7K at 12.2 GPa to 6.4 K at 23.0 GPa and then is independent of pressure.These results are of great significance for further understanding the structure and superconductivity of group?-?semiconductors at high pressure.2.The group?-?metal chalcogenides have many novel properties under high pressure,attracting extensive research interest in the academic circle.We systematically investigated Sb₂S₃ based on the diamond-anvil cell.X-ray diffraction experiments show that Sb₂S₃ develops ultimately into a highly symmetric cubic solid solution alloy phase at 28.2 GPa.In this structure,the Sb and S atoms are distributed randomly on the bcc lattice.The pressure dependences of the lattice-spacings,c/a ratio and the frequency of Raman vibration mode exhibit abrupt changes in their slopes reveals that the presence of an electronic topological phase transition in Sb₂S₃ near 5.0 GPa.The results of the electric transport study showed that no superconductivity transformation was observed when the sample was cooled to 7 K at 33.0 GPa.This study clarified the phase transition sequence of Sb₂S₃ under high pressure,and disordered body-centered cubic structure was first found in group?-?metal sulfides.3.Yttrium superhydride has abundant unconventional ratio materials at high pressure,and superconductivity has been observed in the clathrate YH₆ and YH₉.However,the superconductivity of the YH₄ and YH₇ have not been reported experimentally,and detailed studies on all yttrium superhydride are lacking in the field.Based on high temperature and high-pressure experimental technology,the Y-H system was systematically explored by using yttrium metal and ammonia borane as precursors.We have successfully synthesized I4/mmm-YH₄ at 157.8 GPa,with T_cof 82.2 K.We investigated the suppression of magnetic field on superconductivity of YH₄ at 170 GPa,and GL and WHH models are fitted to the experimental data yield upper critical field for YH4 are 14.9 T and 18.7 T,respectively.Furthermore,by adjusting the temperature and pressure of the synthesis,we have successfully synthesized Im(?)m-YH₆ at 165 GPa,Imm2-YH₇ at 162 GPa and P6₃/mmc-YH₉ at 300 GPa,with T_csof 218 K,29 K and 230K,respectively.This research will advance the search for high-temperature superconductors in the ternary or polynary system based on the yttrium superhydrides.4.Existing experimental studies of lanthanide superhydrides have shown that with the increase of f electrons,the magnetism becomes stronger,which inhibits superconductivity.However,recent theoretical predictions indicate that the superconducting transition temperatures of ytterbium superhydrides exceed 100 K.Based on high temperature and high-pressure experimental technology,we systematically investigated ytterbium superhydrides under high pressures using ytterbium and ammonia borane as precursors.We have successfully synthesized Fm(?)m-Yb H₃and Pm(?)n-Yb H₅ in the pressure range below 150 GPa and the thermodynamic and kinetic stability of Yb H₃ and Yb H₅ were simulated by the first-principles calculation method.Resistance measurements in the pressure range of 100-170 GPa revealed a slope change in the resistance temperature curve,and the magnetic ordered phase transition was speculated based on studies of lanthanide elements and lanthanide metal hydrides.The results provide support for further research on the electric transport properties of strongly correlated systems.