Effects Of Ca And Nb Doping On The Crystal Structure,Optical And Magnetic Properties Of BiFeO₃

BiFeO₃,as the only room-temperature?RT?single-phase multiferroic material with ferroelectric and magnetic order,undoubtedly becomes the most widely studied multiferroic functional materials.However,the weak magnetism and the large leakage current seriously impede its practical application.Chemical doping can improve the multiferroic properties of BiFeO₃ but the study of physical mechanism is still under debate.In this work,the effects of Ca and Nb doping on the crystal structure,optical and magnetic properties of BiFeO₃ are studied.Main results are as follows:1.Bi_1-xCa_xFeO₃?BCFO??BiFe_1-xNb_x O₃?BFNO??Bi_1-xCa_xFe_1-xNb_xO₃?BCFNO?samples are well prepared by a sol-gel method.2.In Bi_1-xCa_xFe O₃?BCFO?samples,a phase transition from the rhombohedral R3 c structure?x ? 0.10?to the ideal cubic perovskite structure?x = 0.25?is observed.The Fe-O-Fe bond angle increases toward 180°,and the Fe O6 octahedral rotation distortion is reduced.The particle size does not substantially change with the Ca doping.With the increase of Ca²⁺dopants,the oxygen vacancies increase while the content ratio of Fe²⁺ maintains constant.The optical band gap in the visible region of the specimens decreases as doping amount x increase.The enhancement of remnant magnetization at low doping content x can be attributed to the formation of the magnetic polarons bounded to the impurities.3.In BiFe_1-xNb_xO₃?BFNO?samples,no phase transition is observed as all samples keep R3 c structure without obvious change in FeO6 octahedral rotation distortion.With a small mount of Nb5+ doping,the grain size of the BFNO samples significantly reduce.The size effect induced by grain refinement is considered to be the main reason for the magnetic enhancement of BFNO.4.Similar to Ca-doped samples,the unit cell volume of co-doped BCFNO samples shrinks with the increase of doping amount x.The phase transition from R3 c to Pm3 m is observed.Increasing Ca²⁺ dopants results in the increase of oxygen vacancies.As doping amount x increase,the band gap of BCFNO decreases and the valence band spectra indicatesthat it's a p-type semiconductor.5.The remnant magnetization Mr of BCFNO reaches a maximum value?0.146 emu/g about 15 times compared with pure BFO?at x = 0.10.This enhancement of magneitic properties in BCFNO can be ascribed to the synergistic effect of A and B site ions doping.On the one hand,the size effect induced by Nb ions,on the other hand,the magnetic polarons bounded to the impurities by Ca ions.In this work,the crystal structure,optical and magnetic properties of Ca doped BiFe O₃,Nb doped BiFeO₃ and Ca-Nb co-doped BiFeO₃ are studied in detail.It is expected to provide research reference for later regulation and design of doped perovskite materials.