Structure Control,Electrical Behavior And First-principles Calculations Of BiFeO₃-based Ceramics

BiFeO₃ is the only single phase multiferroic material at room temperature at present with both room temperature ferroelectric?Tc=1098K?and antiferromagnetism?TN=643K?properties,which palys an important role in the research field of multiferroic materials.Its high ferroelectric Curie temperature,large spontaneous polarization??100?C/cm2?and magnetoelectric coupling observed at room temperature have been extensively studied.However,at the same time,there are still some problems in the research and application of BiFeO₃ ceramics at present.Thermal instability of ceramics at high temperature leads to low phase purity and impurity phase.The ceramic is difficult to be sintered,which results in low density.The variable valence of Fe³⁺ results in large leakage current,and it is difficult to abtain saturated magnetolelectric performance.And symmetry of R3 c also restricts the flip of ceramic domain.In view of the above main problems,BiFeO₃ base ceramics with A site doped by rare earth element?Nd?and B site doped by transition element?Mo/Zr/Ti?have been designed in this paper.By constructing morphotropic phase boundary?MPB?structure and adopting solid reaction method,BiFeO₃ base ceramics with high purity and density have been obtained.The evolution of structure and its related magnetoelectric properties have been studied.And the first principle calculation has been carried out for BiFeO₃ ceramics,and the relavent properties have been analyzed and explained by theoretical dates.Firstly,Bi_0.85Nd_0.15Fe_1-yTiyO₃ ceramics with diferent Ti contents have been synthesized,and the effect of Ti content on phase structure and magnetoelectric properties has been studied.XRD and TEM results show that there are polar R3 c phase,anti-polar Pnam phase and non-polar Pnma phase in the same grain,which are the typical MPB structures.Ti doping can reduce the average grain size of the system,and the grain size of 3%?7% Ti doping system is significantly reduced.The group with the best ferroelectric performance is divided into Ti content y=0.01 and the residual polarization Pr is 35.0?C/cm2 at the electric field of 180 k V/cm.Next,the influence of B site ion species on the structure evolution and magnetoelecric properties of Bi_0.85Nd_0.15Fe_0.99B_0.01O₃?B=Mo/Zr/Ti?ceramics has been investigated.XRD rietveld results indicate that the three systems are all MPB structures with three phase coexistence of R3 c,Pbam and Pnma.And the Pbam phase content for Mo/Zr/Ti doping system is 0.16%,13.78%,and 25.95% separately.Ti system has the best ferroelectric performance,followed by Zr system,and the hysteresis loop doped by Mo has the lowest rectangularity.And related analysis results confirm that the leakage current is not the key factor affecting the ferroelectric properties in BiFeO₃ base ceramics.The dielectric constant of Zr system has an obvious frequency dispersion relationship,and its AC conductivity is the largest with relatively high oxygen vacancy and defect contents in XPS spectral line,which indicates that the defect and oxygen vacancy contents in Zr system are comparatively high.Bi_1-x Nd_x Fe_0.99Zr_0.01O₃ ceramics with different Nd contents have also been synthesized.The evolution rule of its physical phase structure is as follows: with the increase of Nd content,the system gradually transitions from R3 c phase to Pbam intermediate phase,and finally evolves into Pnma orthorhombic phase.The ferroelectric and piezoelectric properties first increased and then decreased with the increase of Nd content.The performance is the best under x=0.125 with the residual polarization Pr of 27.7?C/cm2 under the electric field of 150 k V/cm and piezoelectric coefficient d33 of 44.5p C/N.The conductance mechanism of the system can be considered as the overlapped large polaron tunnel?OLPT?conductance,with x=0.125 component having the highest conductivity.The relationship between DC conductivity and temperature T is an Arrhenius law.The optimized cell parameters of BiFeO₃ obtained by the first principle calculation is a=b=c=5.644?,?=?=?=59.333°.The bonding analysis demonstrates that the three shorter Fe-O bonds?1.963??in BiFeO₃ crystal cell are covalent bonds,and the three longer Fe-O bonds?2.156??are ionic bonds.All Bi-O bonds are ionic bonds and O-O bonds are anti-bonds.The energy band structure indicates that it is a direct bandgap semiconductor with a bandgap width of 1.195 e V.The results of state density show that the electrons near the Fermi surface are contributed by the O-2p,Bi-6s and Fe-3d orbitals.And the results of Born effective charge calculation confirm that the covalent bond composition of BiFeO₃ crystal cell is relatively large.The intrinsic polarization of rhombohedral BiFeO₃ is 96.76?C/cm2 by using Born effective charge and ion polarization displacement.