Study Of Non-metal And Metal Co-doping On The Structure And Properties Of BiFeO₃

Bismuth ferrite(BiFeO3)is a kind of perovskite multiferroic material with rhombohedral distortion.The Curie temperature(Tc)is as high as 1100 K while the Neel temperature(TN)is as high as 640 K,making BiFeO3 coexitance of ferroelectric polarization and ferromagnetism at room temperature.However,due to the special spiral G-type antiferromagnetic structure,the macroscopic magnetic moment of BiFeO3 is often weak,which makes its internal magnetoelectric coupling as well as application in electronic storage and other directions limited.According to previous studies,both the iron-site doping and oxygen-doping of BiFeO3 can effectively change its magnetic structure and enhance the magnetic properties.As a kind of semiconductor structure with narrow bandgap,the energy band structure and optical properties of BiFeO3 also can be effectively adjusted through doping at different sites,making it possible for its application on electronic and photovoltaic devices.Therefore,improving the properties of BiFeO3 by doping is one of the most popular researches currently.With the rapid development of computer and density functional theory,calculating through the first principle has been a common method in the field of multiferroic materials.In this paper,we construct 2×2×1 supercells and simulate the effects of nonmetal element(F,N,S)co-doped with different metal element on structures and properties of BiFeO3 by the first principle.The conclusions obtained are as follows:(1)Co-doping of non-metallic(F,N,S)with Cr will change the Fe-O-Fe bond angle and Fe-O bond length,make the structure of BiFeO3 change from monoclinic to triclinic,so that net magnetic moment of ~5.9 ?B,~6.0 ?B and ~7.8 ?B are produced.F and N doping into BiFe0.75Cr0.25O3 introduced electrons and holes,respectively,resulting in an emerged hybrid state close to the Fermi level and reduction in the energy gap.The N-doped BiFe0.75Cr0.25O3 makes BiFeO3 more sensitive to visible light absorption,which can be applied to photovoltaic devices in visible light reigon.(2)BiFe0.75Mn0.25O3 and F,N,S-doped BiFe0.75Mn0.25O3 will exhibit net magnetic moments of ~8.8 ?B,~8.7 ?B,~7.1 ?B,and ~8.7 ?B,respectively.The net magnetic moments of BiFe0.75Co0.25O3 and F,N,S doped BiFe0.75Co0.25O3 are 4.9 ?B,~7.0 ?B,~6.9 ?B,and ~4.9 ?B,respectively.Due to the fact that both F and N can effectively enhance the local magnetic moment of Co,the F and N doping will make the magnetic moment of BiFe0.75Co0.25O3 increase significantly.In addition,N-doped BiFe0.75Mn0.25O3 and F,N-doped BiFe0.75Co0.25O3 all can make the system show semi-metallic and increased refractive index.Besides,their absorption edges also move towards the energy of 0,significantly extending the range of its absorption spectrum.(3)The non-equivalent M(M=Na+,Mg2+)doped BiFe0.75Cr0.25O3 are metallic and semi-metallic,respectively,which is originated from the movement of electronic states of O 2p,Bi 6s and Cr 3d caused by hole doping.After doping F into M-doped BiFe0.75Cr0.25O3(BFMCO),both the metallic and semi-metallic properties caused by the non-equivalent Na+ and Mg2+ doping disappeared,which made it possible for applitions in spintronics.In addition,the non-equivalent Na+ and Mg2+ doped BiFe0.75Cr0.25O3 exhibit high static dielectric constant,large refractive index,and enhanced photoresponse in the visible region,demonstrating its application in the field of photovoltaic devices.However,after F doping,the absorption edge of the Al3+-doped BiFe0.75Cr0.25O3 shifts towards left and the photoresponse of the visible light region is enhanced.