The Piezoelectric Mechanism Research On The Tetragonal Perovskite Ferroelectric Superlattice

Perovskite ferroelectric materials are widely used in microelectronic devices because of their excellent ferroelectric and piezoelectric properties.With the development of micro-nano manufacturing technology and the improvement of integration of electronic components,the research and demand of high-performance perovskite ferroelectric thin films and low-dimensional structures are growing fast.The perovskite ferroelectric superlattice is a uniqe structure formed by two or more ultra-thin ferroelectric materials,due to the coupling of electrons and atoms between different ferroelectric materials,making the performance of ferroelectric superlattice more special.Besides,by changing the compositions,epitaxial strains and other factors,we can regulate the ferroelectric and piezoelectric properties of the superlattices,which is of great significance for the development and research of new photoelectric materials.In the study of complex solid solution perovskite materials,cation compositional change is the primary tool for design of these materials,a firm understanding of how individual cations contribute to the overall piezoelectric response of the materials would help the rational design of high-performance piezoelectric materials.In this dissertation,we adopt the first-principle theory and the VASP software package,establish a theoretical calculation and analytical method for the research of the polarization and piezoelectric behaviors of the A and B atoms in the tetragonal superlattices.The research subjects are three PbTiO₃-based and three lead-free ferroelectric superlattices.The theoretical calculation model is under the tetragonal compression and expansion,calculate the displacement of A and B atoms deviate from the oxygen dodecahedron center and the oxygen octahedral center,the Born effective charge Z*33 and ?₃₃ along z direction,and volume V of the unit cell.Through the formulas P?A?=Z₃₃^*?A?×D?A?/V and P?B?=Z₃₃^*?B?×D?B?/V,d₃₃?A?=?P?A?/??₃₃ and d₃₃?B?=?P?B?/??₃₃,we get the contribution of A and B atoms to the total polarization and piezoelectricity.Through atomic structure analysis,we found that the D?Pb?in the three PbTiO₃-based tetragonal superlattices is much larger than that of other A and B atoms.In the other three lead-free superlattices,there are no atoms similar to the Pb atoms with a large displacement,especially in the compressive state,D?A?and D?B?are inhibited.By analyzing the polarization contribution of A and B atoms,we found that B atom contributes most to the global polarization of the unit cell,especially the transition metal element?Ti,Nb and Ta?which has a strong covalent bond hybridization with oxygen atom in oxygen octahedron.In addition,among the three PbTiO₃-based superlattices,the contribution from Pb is significent because of the larger D?Pb?.By analyzing the piezoelectric behavior of the superlattices,we found that as the strain increases,the overall piezoelectric coefficient d₃₃ and the piezoelectric contributions d₃₃?A?and d₃₃?B?all increase slowly in the three PbTiO₃-based tetragonal superlattices.While in the other three lead-free tetragonal superlattices,the apparent piezoelectric behavior only appears when the tensile strain increases to a certain extent and the total d₃₃ is mainly come from d₃₃?B?.The sum of the polarization and piezoelectric contributions of the A and B atoms calculated by the above method is very close to the total polarization and piezoelectric coefficient calculated by the Berry-Phase method and other theoretical methods,Further confirms the accuracy of the theoretical calculation model.