Electrical Properties And First-principles Study Of Bismuth Sodium Titanate-based Ferroelectric Ceramics

Na_0.5Bi_0.5TiO₃（NBT）based lead-free ceramics are attracting an increasing amount of attention nowadays in the field of ferroelectric energy storage,ferroelectric catalysis,and ferroelectric optoelectronics.It is expected to be used for transducers,sensors,memories,and the optoelectronic devices.The charge,spin,and polarization are coupled by the oxygen octahedral structure of NBT producing electrical,optical,magnetic,energy storage physics and so on.It is an effective way to tailor the microstructure and performance by doping.Na_0.5Bi_0.5(Ti_1-xM_x)O₃（M=Mo,Nb,Ta,x=0,0.005,0.01,0.015,0.02）ceramics were prepared by traditional solid-phase technique firstly,and then,the microstructure,Raman spectrum,light absorption,optical band gap were tested.The energy band,density of state,Born effective charge and Bader charge were calculated by VASP to study the ion relationship between electrical polarization and properties.（1）Mo-doped on B-site,Na_0.5Bi_0.5(Ti_1-xMo_x)O₃（NBT-Mo_x,x=0,0.005,0.01,0.015,0.02）lead-free ferroelectric ceramics were tested and calculated.The optical band gap were tested to study the influence of Mo-doping on NBT.The results shown that with the increase of Mo doping,the optical band gap was first decreased and then increased.The band structure was changed from indirect to direct by doping,and the introduced impurity level caused the decrease of the band gap.With the increasing of Mo doping,the Fermi level entered in the conduction band,and the CBM was occupied.The Fermi level was pushed to a higher energy by the occupied state of electrons in the conduction band.With the increasing of doping,the Moss-Burstein effect dominated gradually and the optical band gap was increased.These two band gap regulation mechanisms competed with each other.When x=1%,the band gap reached the minimum at 2.33 e V,and the light absorption intensity reached the maximum at 69%.At the same time,Mo ion doping caused distortion of the octahedron,and causing the adjacent octahedrons to form different degrees of rotation/tilting structural distortion.From the calculation result of Born effective charge,it can be seen that the spontaneous polarization of the system mainly came from the distortion of the oxygen octahedron and ion displacement.The results of Bader charge calculation showed that the change of bonding would induce the charge migration between ions,thereby changing the polarization properties of the system.The Jahn-Teller effect would change the rotation form and the angle of the oxygen octahedron,and adjusting physical properties such as ferroelectricity of the material.（2）Nb-doped on B-site,Na_0.5Bi_0.5(Ti_1-xNb_x)O₃（NBT-Nb_x,x=0,0.005,0.01,0.015,0.02）lead-free ceramics,the results showed that when the Nb doping amount at 2%,the light absorption intensity reaches the maximum,and the optical band gap value reaches the minimum at 2.46e V.The theoretical calculation and analysis indicated that Nb-doping intruduced impurity levels in the system forbidden band.The impurity levels were mainly contributed by the Nb:4d orbital.Nb-doping could control the energy band structure of the NBT system and enhance its semiconductivity.Nb ion doping caused distortion of the doped octahedrons,and caused the adjacent octahedrons to form different degrees of rotation/tilt structure distortion.（3）Ta-doped on B-site,Na_0.5Bi_0.5(Ti_1-xTa_x)O₃（NBT-Ta_x,x=0,0.005,0.01,0.015,0.02）lead-free piezoelectric ceramics showed that the light absorption intensity reached the maximum of 46.7%,and the optical band gap value reached the minimum value of 2.41e V with Ta doping amount of 1.5%.Through the theoretical calculation and analysis,Ta-doped NBT lead to impurity levels into the forbidden band.The impurity levels were mainly contributed by Ta:5d orbitals,and the effect was weaker than Mo and Nb.The doping of Ta ions causes distortion of the doped octahedrons,and caused the adjacent octahedrons to form different degrees of rotation/tilt structural distortion simultaneously.Bader charge analysis showed that Ta was weaker than Ti;Na and Bi in NBT had relatively stable effective ionic charges.