无铅四方相钙钛矿短周期超晶格压电效应机理研究

Piezoelectric effect mechanism in lead-free tetragonal perovskite short-period superlattices

超晶格压电行为与内部正离子之间的内在联系尚缺乏相关的研究.本文基于密度泛函理论的第一性原理方法,研究了三种无铅四方相钙钛矿铁电超晶格(BaTiO_3/SrTiO_3,KNbO_3/KTaO_3和BaTiO_3/KNbO_3)中A,B位正离子对整体的极化和压电贡献.通过计算超晶格不同轴向应变条件下原子结构和Born有效电荷,获得了超晶格和各个正离子的极化值和压电系数.结果表明,在轴向压缩应变条件下(-0.15—0 A),无铅超晶格中的正离子位移D(A)和D(B)受到抑制,在拉应变时位移才显著增大,因此极化和压电行为不明显.在轴向拉伸应变作用下(0—0.15 A),无铅超晶格中各原子的极化贡献显著增大,特别是B位原子Ti,Nb和Ta的极化贡献使得总的极化强度也显著提高,并当拉应变达到一定值,超晶格才会出现明显的压电行为.无铅超晶格的极化和压电行为主要由B位正离子贡献.

There is no relevant research on the relationship between the piezoelectric behavior of superlattice and the in- ternal cations. In this paper, by the first-principles method of density-functional theory, we study the polarizations and piezoelectric contributions of cations A and B in three lead-free tetragonal perovskite ferroelectric superlattices （BaTiO3/SrTiO3, KNbO3/KTaO3 and BaTiO3/KNbO3）. By calculating atomic structures and atomic Born effective charges of three superlattices under different axial strain conditions （-0.15-0.15 A）, the polarization and piezoelectric coefficients of superlattices and internal cations are obtained. With the axial compressive strain changing from -0.15 to 0 /~, the variations of displacements D（A） and D（B） of cations A and B in lead-free superlattices are very small, and displacements D（A） and D（B） significantly increase as the axial tensile strain （0 0.15 A） is applied, indicating that the axial compressive strain is not beneficial to the ferroelectric displacement in the tetragonal superlattice, especially in BaTiO3/SrTiO3 nor KNbO3/KTaO3 superlattices. The tetragonal ferroelectric superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3 may be unstable under the condition of the axial compressive strain, and only the axial tensile strain can promote the existence of tetragonal phase in superlattice. As the axial strain is applied, Born effective changes of A-site cations in three lead-free tetragonal superlattices are small, and Z^3（B） gradually declines, and Zxy（B） contin- ually rises. The axial strain induced charges are transferred from the B-site cations to O atoms along the c-axis, and the charges are transferred from O atoms to B-site cations along the xy direction. The variation rate of Born effective charges under the condition of the axial tensile strain is greater than under the condition of the axial compressive strain, especially in superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3, showing that the axial tensile strain is more beneficial to the redistribution of atomic charges in the superlattices. Under the condition of the axial compressive strain, the total polarizations of superlattices BaTiO3/SrTiO3 and KNbO3/KTa03 are close to zero; while polarizations of superlattices BaTiO3/KNbO3 gradually increase with the axial compressive strain varying from -0.15 to 0 A. There are atomic ferroelectric displacements in superlattice BaTiO3/KNbO3, and the interaction between BaTiO3 ferroelectric layer and KNb03 ferroelectric layer contributes to the generation of ferroelectric behavior. When the axial tensile strain （0-0.15 A） is applied, the polarization contributions of B-site cations in superlattices BaTiO3/SrTiO3 and KNbO3/KTaO3 increase significantly, especially the polarization contributions of B-site cations Ti, Nb and Ta, and the total polarization is ob- viously improved. The effect of the tensile strain on polarization of BaTiO3//KNbO3 is smaller than on polarizations of BaTiO~/SrTiO3 and KNbO3/KTaO3. The interaction between two ferroelectric layers in BaTiO~/KNbO3 contributes to the redistribution of atomic charges, and alleviates ferroelectric displacements of atoms to some extent. The polarization contribution of B-site cations is largest, because of their large Born effective charges and ferroelectric displacements. When the tensile strain reaches a certain threshold, tetragonal superlattices will present obvious piezoelectric behavior. With the tensile strain increasing, total piezoelectric coefficient d33 and piezoelectric contributions of A, B-site cations both increase. The piezoelectric behaviors of lead-free superlattices are mainly attributed to the B-site cations.