Impact Factor Research Of PbTiO₃/KTaO₃Ferroelectric Superlattice With Polar Discontinuity

Ferroelectric superlattice has attracted a lot of attention for its excellentproperties over single bulk material and it has been a hot research topic for itspotential usage in microelectronics apparatus. Researches have shown that mainfactors affecting properties of superlattice are compositions, strain, static electricfield, and interfaces between layers. This paper analyzes a superlattice PbTiO₃//KTaO₃containing polar discontinuity （a kind of interface effect） by VASP andABINIT package with first principle. The analysis targets will be those factorsincluding composition strain and interface（polar discontinuity）.From the minimum of the energy curve of different in-plane lattice constanta, we decide that the balance superlattice parameter; Displacement of Pb K Ti Taatoms tend to be negative as a increases, while O3and O6atoms show oppositedisplacement direction，and the displacements of Pb Ti O atoms in PTO layer arelarger than corresponding K Ta O atoms in KTO layer; Electronic structure studyshows that there is similar distribution of2p electrons of O atoms,6s and5delectrons of Ta atom and3d electrons of Ti atom in area between-5eV and0eV,which implies strong hybridization between Ti and O atoms or Ta and O atoms,the distribution of6s and6p electrons of Pb atom is similar with that of2pelectrons of O atoms, so the interaction between Pb and O atoms is very strong;As the magnitude of a increases, the polarization dramatically decreases, it isbecause that displacements of Pb Ti Ta atoms become smaller, making thedisplacement of positive and negative charge center smaller, thus the polarizationbecomes smaller; As is well known, anomalously larger Born effective chargesresult from strong hybridization of Ti, Ta and O orbits in the Ti–O chain andTa-O chain and are quite sensitive to polar distortion of the chain. Thus, weexpect a significant strain dependence of the Born effective charge for theseatoms, Z*Ti,zzand Z*Ta,zzdrop gradually and Z*O3,zzand Z*O6,zzrise continuouslywith increasing strain, anti-correlation between effective charges of Ti atom andthose of O atoms shows the dynamical charge transfer along the Ti–O and Ta–Obonds as the strain is varied, which affects ferroelectricity of PKT.For （PbTiO₃）m/（SrTiO₃）_n, the polarization results from first principle andelectrostatic mode is similar and the polarization of local polarization does nokeep a constant value through each superlattice, while PTO layer exhibits evenlarger polarization than pure bulk PTO and paraelectric KTO layer shows a smallvalue of polarization. The presence of polar discontinuity leads to an internalelectric field with same direction as polarization direction of PTO, thus making polarization value of PTO larger than regular bulk value. the opposite electricfield to polarization suppresses its polarization behavior, and KTO layer has asmall value around0.2⁰.25C/m2. For （PbTiO₃）_n/（SrTiO₃）_n, we can imply c/2nadecreases and P decreases with bigger n. With more same composition in oneperiod, there are fewer interfaces in a specific lattice. So the effect of interfacesand polar discontinuity become weaker, thus the lattice distortion becomes lessand polarization value declines. With same theory of the polar discontinuity, thepolarization of PTO local layers exhibits a bigger value than bulk PbTiO₃whilethe polarization of KTO local layers has a small value around0.18⁰.23C/m2.