First-principles Study Of Strucyural Phase Transition And Piezoelectric Properties In Superlattice

Ferroelectric superlattice has attracted a lot of attention for its excellent properties over single bulk material, it has a very wide range of potential applications in terms of microelectronic devices and photovoltaic film, and it has become the international hotspot issue. Chemical composition, component ratio, and strain have a great influence on the performance of the superlattice. Exploring the principle of phase transition and the performance of the superlattice in the equilibrium state is of great significance in practical application. Based on first principles, we investigate phase transition of Pb Ti O₃/Sn Ti O₃（II-IV/II-IV）,Na TaO₃/KTaO₃（I-V/I-V） and PbTi O₃/Sn Zr O₃（II-IV/II-IV） three 1/1 two layers ferroelectric superlattice, And examines trends of polarization and piezoelectric behavior in strained PbTi O₃/Sn TiO₃ superlattice.The initial phase of PbTiO₃/Sn Ti O₃（II-IV/II-IV）, Na TaO₃/KTaO₃（I-V/I-V）superlattice is P4/mmm, and the initial phase of Pb Ti O₃/Sn Zr O₃（II-IV/II-IV）superlattice is P4 mm. By density functional perturbation theory, we calculate the phonon spectrum to determine the stability of the structure. P4/mmm containing double degenerate virtual frequency phonons, and a virtual phonon that vibrate in the z direction. By freezing the double degenerate virtual phonons, we can get Amm2, by freezing virtual phonon that vibrate in z direction, we can get P4 mm. We calculate the phonon spectrum of two phase, by freezing virtual phonon that vibrate in z direction in Amm2, we can get Cm, by freezing double degenerate virtual frequency phonons in P4 mm, we can get Cm. So we can study the phase transition by freezing phonon, the new phase has lower energy than former phase.During the lattice mismatch strain increased from 1.5% to 1.5%,Pb Ti O₃/Sn Ti O₃ superlattice phase change occurs, Cm translate into Amm2. The band gap increase from 1.17 eV to 1.32 eV. The bourne effective charge decrese, it indicate the reduced interactin between ions and charge transporting in superlattice.Spontaneous polarization component and piezoelectric tensor in x and y direction will decrease, it will disappear in z-direction when the Cm translate into Amm2.The angle between the polarized direction and the z axis as a function of the lattice mismatch strain, in the end of 90°. With increasing strain, superlattice dielectric tensor will decline,the largest tensor in x and y axis is 6.629.