First-principle Calculation Of Structural Distortion And Charge Disproportionation Of Perovskite AFeO₃(A=Ca/La/Sr)

Transition metal oxides generally exhibit antiferromagnetic or nonlinear helical magnetic properties at low temperature due to their strong orbital correlation.Perovskites AFeO₃show complex spin orders at low temperature,and sometimes the spin order and charge order coexist.Experimentally,with the increase of the radius of the A-site cation,the charge order is suppressed.However,the effect of A-site cations with different valence states on charge orders has not been systematic theoretically studied so far.In this paper,the influence of radius and valence state of A-site cations on charge orders and magnetic properties is studied by the first principle calculation software VASP.It is of great significance for studying the low-temperature phase transition of the transition metal perovskites,the formation mechanism of charge order,and the nature of electron transport.The main research results are as follows:1)CaFeO₃,SrFeO₃and LaFeO₃crystal structure and magnetic properties are studied by the first principle base on density functional theory.The structural stability is determined by calculating the energy as a function of volume curve of the magnetic structure.The results show that the orthogonal structure Pnma and monoclinic structure P2₁/n for CaFeO₃are the most stable,the energy of the ferromagnetic state is the lowest.At low temperature,CaFeO₃will undergo metal-insulator transition with the charge disproportionation,resulting in two inequivalentFe.For Ca²⁺,Sr²⁺and La³⁺,as the radius of ionic increases,Néel temperature increases continuously.Replacing Ca²⁺with Sr²⁺and La³⁺to destroy the charge order phase of CaFeO₃,so the charge disproportionation does not exist in SrFeO₃or LaFeO₃.The bandwidth of AFeO₃can be adjusted by the substitution of A-site cation.As the radius of the A-site cation increases,the bandwidth increases,resulting in the electron delocalization,antiferromagnetism is weakened and ferromagnetism is enhanced,so that the charge ordered phase can be destroyed.2)The crystal structure and magnetic properties of A-site cation-doped Sr_2/3La_1/3FeO₃and Ca_2/3La_1/3FeO₃are studied by first-principles calculation.For Sr_2/3La_1/3FeO₃,the monoclinic structure P2₁/n is the most stable and has the charge disproportionation,showing metallic properties.The calculation of the Pnma crystal structure of Ca_2/3La_1/3FeO₃also has charge disproportionation and exhibits metallic properties.After doping,the radius of the A-site cation is inconsistent,resulting in the difference in structural distortion.Fe forms two differentFeO₆octahedrons at different positions in the crystal,which are expressed in the form of charge disproportionation.Both metallic character before the emergency of charge disproportionation and structural distortion are required for the formation of charge disproportionation.