Theoretical Study On The Design And Superconductivity Of Two-Dimensional Planar Hypercoordinate Structures

The remarkable quantum-limited effects of two-dimensional materials give them novel physical properties,which make two-dimensional materials have important application potential and application value in many fields.In recent years,two-dimensional systems containing planar hypercoordinate structures have been continuously designed theoretically,synthesized experimentally and characterized,especially,their unconventional,anti-classical bonding and potential physical properties have attracted increasing research interests.The effective electron delocalization is an important driving force for the stabilization of two-dimensional systems,and therefore main group elements with p-orbital and transition metals with d-orbital electrons are important substrates for the formation of planar hypercoordinate two-dimensional systems.The paper explores the possibility and stabilization mechanisms of s-block atoms in the construction of planar hypercoordinate two-dimensional systems through efficient structure search.Through the particle swarm optimization algorithm,we perform a detailed potential energy surface search for the beryllium-containing system and obtained two planar hexacoordinate monolayer systems,Be₂M（M=Al,Ga）and Be₂TM（TM=Zn,Cd,Hg）,and further systematically analyzed their stability,geometric and electronic structures,and potential superconductivity.The specific results are as follows:1)Two-dimensional planar hexacoordinate monolayers of main group elements are very rare relative to two-dimensional boron monolayer systems.Through systematic structural search,we confirm that two-dimensional Be₂M（M=Al,Ga）monolayer with P6/mmm space group is the ground state structure of the system,where the M atoms and the six nearby Be atoms form the planar hexacoordinate Be and M motifs.Ab initio molecular dynamics simulations,phonon spectra and Young’s modulus calculations indicate that Be₂M（M=Al,Ga）has good thermodynamic,kinetic and mechanical stability and is an excellent candidate for laboratory preparation.The electron-phonon coupling calculations show that two-dimensional Be₂Al and Be₂Ga monolayers are phonon-mediated intrinsic superconductors with superconducting transition temperatures（T_c）of 5.9 K and 3.6 K,respectively.In particular,the compressive strain induced phonon softening to increase the electron-phonon coupling strength in the low frequency region,which can further improve T_c to 8.6 K and 13.0K.Moreover,this work clarifies the feasibility of growing the Be₂M monolayer directly on the electride Ca₂N substrate,while its superconducting properties are well maintained.2)By global structure search,we obtained two-dimensional planar hexacoordinate Be₂TM（TM=Zn,Cd,Hg）monolayer with the P6/mmm space group,where the Be₂Hg monolayer is the global minimum with the lowest energy on the potential energy surface,and each TM atom and the six nearby Be atoms form the planar hexacoordinate Be and TM motifs.Phonon spectra combined with ab initio molecular dynamics simulations demonstrate that Be₂TM has the good kinetic and thermodynamic stability,which is attributed to the good electron delocalization and localization properties.The electron-phonon coupling calculations show that two-dimensional Be₂TM monolayer is a phonon-mediated intrinsic superconductor with superconducting transition temperatures（T_c）of 5.2 K（Be₂Zn）,2.9 K（Be₂Cd）and 3.6 K（Be₂Hg）,respectively.Meanwhile,the considerable adsorption energy proves that the Be₂TM monolayer can be grown directly on the electride Ca₂N substrate,which provides an excellent substrate for the preparation of Be₂TM monolayer in the laboratory.