Research On Microscopic Mechanism Of PMnN-PZT Ferroelectric Materials By First Principle

Lead zirconate titanate(Pb(ZrxTi1-x)O3,PZT)ferroelectric material has been widely used in ferroelectric memory,dielectric filter and piezoelectric sensor because of its high Curie temperature,favorable characteristics of ferroelectricity,dielectricity and piezoelectricity.With the development of micro-nano technology and tunable microwave device technology,traditional binary PZT ferroelectric materials is not able to meet the needs of application.Mixing or doping modification of PZT-based ferroelectric materials to obtain high Curie temperature,high mechanical quality factor and high electromechanical coupling coefficient,especially the microscopic mechanism of doping modification have become a research hotspot in the field of ferroelectric materials.The first-principles method based on density functional theory was adopted to simulate and investigate the modification of PZT(50/50)ferroelectric materials doped with different proportions of lead lanthanum manganate(Pb(Mni/3,Nb2/3)O3,PMnN).Theoretical models of ternary system of xPMnN-(1-x)PZT(50/50)(x=0%,4%,6%,8%,10%,15%,20%,30%)were constructed respectively by supercell method.The lattice constant,electronic structure,dielectric properties,optical properties and piezoelectric coefficient of xPMnN-(1-x)PZT(50/50)were calculated.The modification rules and microscopic mechanism of PZT(50/50)doped with PMnN were analyzed and summarized as follows:(1)The calculation and analysis of latti:ce constant,electronic structure,dielectric properties,optical properties and piezoelectric coefficient of the intrinsic PZT(50/50)crystal,show that the relative error of the lattice constant,crystal axis ratio compared with experimental value is less than 2.82%.So error range is reasonable,and modeling method and calculation parameters selected are reasonable and feasible,too.Meanwhile,PZT(50/50)crystal is a kind of anisotropic indirect band gap semiconductor with the band gap value of 1.53eV.(2)The geometric and electronic structures of ternary xPMnN-(1-x)PZT doped with PMnN were calculated and analyzed.The results show that ternary PMnN-PZT(50/50)is closer to the morphotropic phase boundary than the binary PZT(50/50),where the tetragonal phase and trigonal phase coexist.With the PMnN doping ratio increasing,Nb5+ and Mn2-gradually replaced Zr4+ and Ti4+ at B position,and the number of electrons distributed near the Fermi energy level increases.This result in the energy band at the high energy gradually approaches the low energy,narrows band gap,and enhances conductivity.(3)It was found that the dielectric constant gradually increases with increasing PMnN doping proportion by calculating and analyzing the dielectric properties of xPMnN-(1-x)PZT(50/50),especially with the doping ratio x=30%.Combined with the analysis of differential charge density,the distortion of electron cloud caused by Mn2+/Nb5+ doping around B-site atom and O atom results in relative displacement of the center of positive and negative ions,which enhances the distortion of B-O octahedron(consisting of B-site atom and O atom).The distortion leads to the appearance of electric dipole moment,and enhances spontaneous polarization,the dielectric constant of the system is correspondingly elevated.(4)The calculation results of the piezoelectric coefficients(e3i,e33)of xPMnN-(1-x)PZT(50/50)system show that when x<15%,as a kind of soft doping,Nb5+distorts the lattice of the crystal system,which leads to the feasible movement of the domain walls in the grains,and enhances polarization and piezoelectric coefficient(e3i,e33).When x>15%,excessive Nb5+/Mn2+will accumulate at the grain boundary,and form some local head to head or tail to tail defect polarization arrangement,which will restrict and nail the movement of domain wall,thereby it reduces the piezoelectric activity and piezoelectric coefficients(e3i,e33).In this paper,ternary systems xPMnN-(l-x)PZT(50/50)with different doping ratios are calculated(x=0%,4%,6%,8%,10%,15%,20%,30%),including geometrical configuration,electronic structure,optical properties and piezoelectric coefficient.And the relatively systematic explanation of microscopic mechanism for the multi-component doping modification of PZT(50/50)doped with PMnN is given,which is of great significance to the research on the doping modification of PZT(50/50)-based ferroelectric materials.Besides,it can provide reliable theoretical guidance for the fabrication and development of multicomponent ferroelectric materials,and is expected to make high-performance ferroelectric materials widely used in field of tunable microwave devices,expand the application of ferroelectric materials.