Electronic Properties And Structural Stability Of 4d/5d Transition Metal Compounds

The competition between spin-orbit coupling（SOC）,Coulomb correlations together with Hund’s coupling interactions has given rise to a rich set of emergent phenomena and exotic quantum states in 4d/5dtransition metals compounds.Particularly,the seminal discovery of the SOC-induced Mott insulating state in the iridate Sr₂Ir O₄has sparked considerable enthusiasm to explore unprecedented spin-orbital-entangled phases.The spin-orbit entangled J_effstate has become an indispensable physical quantity for understanding the exotic electronic structures and magnetic properties of 4d/5druthenate and iridate compounds.Through first-principles calculations and simulations,several 4d/5druthenate and iridate compounds were selected to study the ground state electronic structure and the structure and electronic structure evolution under the action of pressure/strain.The main work content and conclusions are summarized as follows:（1）The non-magnetic and insulating ground state of the honeycomb lattice compound Na₂Ru O₃with 4d⁴electronic configuration is elucidated,and the evolution of the crystal structure and electronic structure under pressure are discussed.We reveal that individual Coulomb correlation or SOC effect cannot reproduce the experimentally observed nonmagnetic insulating behavior of Na₂Ru O₃,whereas the Coulomb correlation enhanced SOC interactions give rise to an unusual spin-orbital-entangled J_eff=0 nonmagnetic insulating state,which contrasts with the SOC assisted Mott insulating state in⁵ruthenates and iridates.Furthermore,a pressure-induced structural dimerization transition has been predicted around 15-17.5 GPa.The honeycomb lattice of the high-pressure dimerized phase features with parallel pattern of the short Ru-Ru dimers aligning along the crystallographic b-axis direction.Accompanying with the structural dimerization,the electronic structure shows striking reconstruction.At the same time,the cooperation of Coulomb correlation together with SOC can realize a nonmagnetic insulating state in the high-pressure dimerized phase.（2）The intrinsic electronic structure of layered iridium compound K_0.75Na_0.25Ir O₂has explored,and its band structure and band edge adjustability are revealed by uniaxial strain.We theoretically determine the preferred occupied positions of the alkali metal ions from energetic viewpoints and reproduce the experimentally observed semiconducting behavior and nonmagnetic properties.The SOC interactions play a critical role in the band dispersion,resulting in J_eff=0 nonmagnetic states.More intriguingly,our electronic structure not only uncovers the presence of in-gap states and explains the abnormal low activation energy in K_0.75Na_0.25Ir O₂,but also predicts the band edge can be effectively modulated by mechanical strain.The results reveal the unconventional electronic structure of trivalent iridate,indicating its broad application prospects as nanoelectronic materials and thermoelectric materials.（3）The effects of SOC and Coulomb interaction competition on the crystal structure and electronic structure of honeycomb lattice compound Ag₃Li Ru₂O₆with 4d⁴electron configuration under pressure has studied.Both SOC and Coulomb action will increase the pressure required for phase transition,among which Coulomb action has the greatest impact on the pressure response of Ag₃Li Ru₂O₆,and determines whether the dimerization mode of Ag₃Li Ru₂O₆after phase transition is zigzag or parallel.At the same time,distinct structural dimerization behaviors are accompanied by exotic electronic structure reconfigurations,with parallel dimerization patterns leading to molecular orbitals,and zigzag dimerizations are not.In this paper,the J_eff=0 nonmagnetic state of three 4d/5dtransition metal compounds are studied in detail,which provide more theoretical guidance for the research of exciton magnetic materials.