Superconductivity Of Ternary Metal Boronhydrides Under High Pressure

Superconductivity is the most strategic and promising technology in the 21st century.The efficient transmission capacity of zero resistors and significantly reduced energy consumption makes superconducting materials very largely appreciated in new energy,militors,transportation and medical applications,and superconducting materials are also more and more widely benefiting human society.In recent years,the research of hydrogen-rich compounds has been refreshing the record of superconducting transition temperature,from the theoretically dominated and experimentally synthesized by high pressure to H₃S（T_c～203 K）and La H₁₀（T_c～250-260 K）,both of which fully demonstrate that hydrogen-rich compounds play an important role in high temperature superconducting materials.With the gradual improvement of quantum mechanical theory,the structural characteristics and physical properties of hydrogen-rich compounds under high pressure conditions have now become the key research content of multiple interdisciplinary disciplines such as condensed matter physics,high pressure science and materials science and engineering.In this thesis,we use CALYPSO crystal structure prediction software based on particle swarm optimization algorithm to conduct a global search for the microscopic crystal structures of the typical metal elements ternary borohydride systems,and then we investigate the physical properties in terms of pressure range,structural characteristics,thermodynamic and kinetic stability,electronic properties,and superconductivity of the stable structures existing in conjunction with first-principles calculatin program.The main innovative research results have been achieved as following:（1）The structure of ternary alkaline-metal Ae B₂H₈（Ae=Li/Na/K/Rb/Cs）hydrides was systematically searched using crystal structure prediction software,and the structural stability,electronic properties and superconductivity studies were explored.The research results show that Ae B₂H₈ hydrides can be energetically stable at a pressure of about 10GPa,where the K/Rb/Cs B₂H₈-rich hydrogen compounds are excellent superconductor materials with a high superconducting transition temperature of about 100 K at 25 GPa;The electron-phonon coupling calculations show that the alkaline-metal atoms have a major electronic contribution role in these high-temperature superconductors,which can effectively increase the superconducting transition temperature and significantly reduce the synthesis pressure of this type of complex polyhydrogen compounds.These research results clarify the superconducting mechanism of ternary alkaline metal Ae B₂H₈ hydride,and provide new important ideas and theoretical basis for the design or synthesis of new superconducting materials.（2）A systematic search for the structure of the ternary transition metal elements borohydride Sc/YB₂H_n（n=1-10）was carried out using crystal structure prediction software,and the structural stability,electronic properties and superconductivity were studied.Our calculation results show that the ternary transition metal Sc/YB₂H₁₀hydrogen-rich compound can remain energetically stable under moderate synthesis pressure and Sc/YB₂H₁₀ is predicted to be a superconductor material with excellent performance,in which the superconducting transition temperature of YB₂H₁₀is 93 K at225 GPa;Electronic properties and electron-phonon coupling calculations show that the higher occupancy state of the BH₃ unit at the Fermi level and the high vibrational frequency play a main contribution for superconductivity.The findings of these research results provide a theoretical solution for designing high-temperature superconducting materials.There are 29 figures,6 tables,175 references.