Investigation On The Electronic And Magnetic Properties Of Double Perovskite Oxides By First-principle Calculations

Perovskite transition metal oxides have received extensive attention owing to their flexible crystal structure and a series of excellent physical properties.In diverse material fields,there are many perovskite oxides materials,such as multiferroics,superconducting,and magnetic materials.The study of the magnetic and electrical properties of double perovskite oxides,to reveal their intrinsic coupling mechanism and the related physical effects,is an important issue for perovskite research.Density functional methods based on first-principles calculations,being accurate,fast,and not limited by experimental conditions,are widely used to explain various experimental results and phenomena.Therefore,in this thesis,first-principles calculation was used to study the crystal structure,stability,electronic structure,electrical and magnetic properties of double perovskite oxides.The specific work mainly includes the following aspects:?1?The electronic and magnetic properties of double perovskite Ba₂CaIrO₆ have been investigated.The calculated results indicate that Ba₂CaIrO₆ is an antiferromagnetic semiconductor.The partial density of states?PDOSs?and crystal orbit Hamiltonian population?COHP?curves indicate the strong covalent hybridization between Ir 5d and O 2p orbitals,leading Ir deviation from the expected Ir⁶⁺with³₂₂)0)⁰₂)electron configuration.The spin-orbit coupling?SOC?impact on electronic and magnetic properties is further considered.It is found that the existence of SOC reduces the spin magnetic moment of Ir ions,and also induces a small orbital moment.This work confirms the important role of SOC in the electronic and magnetic properties of 5d iridium oxides.?2?The stability of three types crystal structures,electronic and magnetic properties of double perovskite Sr₂NiTeO₆ have been studied by both GGA and GGA+U methods.The GGA results illustrate that Sr₂NiTeO₆ with C2/m symmetry is the most stable structure,while the most energetically preferred one is the P2₁/n symmetry with GGA+U method.The ground state of Sr₂NiTeO₆ perovskite is an antiferromagnetic insulator,independence of GGA or GGA+U method with different magnetic configurations.The magnetic moment of Ni ion calculated by GGA+U method is closer to the experimental observation,which is significantly larger than the magnetic moment calculated by GGA method.In addition,the band gap calculated by the GGA+U method is also larger than the band gap calculated by the GGA method.?3?The effect of B-site cationic arrangement on the electronic and magnetic properties of double perovskite CaCu₃Fe₂Nb₂O₁₂ has been systemically investigated.The results demonstrate the B-site ordered CaCu₃Fe₂Nb₂O₁₂ is a ferrimagnetic insulator with antiferromagnetic coupling between the A?-site Cu and B-site Fe.The calculated total magnetic moment is 7.00?_B f.u.^-1,which is apparently larger than the experimentally measured saturated magnetization because of different degree of B-site disorder.Further electronic structures illustrate the magnetic moments sharply decrease with B-site antisite defects,i.e.,the total magnetic moments obviously reduce with increasing of B-site Fe/Nb disorder and ultimately none magnetism.Interesting,B-site antisite defects not only introduce the Fe-Fe antiferromagnetic coupling,but also induce Cu spins antiferromagnetic arrangement in totally disordered structure.Cu-Fe and Fe-Fe magnetic coupling competition is coupled with antisite defects,and finally Fe-Fe antiferromagnetic coupling turns into the dominating spin coupling in disordered CaCu₃Fe₂Nb₂O₁₂.Moreover,the B-site antisite defects do not alter its insulator nature in spite of the significantly narrowed band gap.Our study opens up a novel avenue to straightforward understanding the effect of cationic ordering on electronic and magnetic properties in double perovskite.