钙钛矿超晶格SrTiO_(3)/BaTiO_(3)的挠曲电效应

Flexoelectric effect of perovskite superlattice SrTiO_(3)/BaTiO_(3)

挠曲电效应是应变梯度与电极化之间的机电耦合,存在尺寸效应,在许多微纳米尺寸结构中起着非常重要的作用.本文采用密度泛函理论,对交错层SrTiO_(3)/BaTiO_(3)超晶格进行了系统的挠曲电效应研究,通过探究超晶格在给定的应变梯度下的力电响应,独立得到其纵向挠曲电系数、横向挠曲电系数和剪切挠曲电系数.结果表明:超晶格的横向、剪切挠曲电系数和纵向挠曲电系数较其组分材料有不同的变化,其中超晶格的横向和剪切挠曲电系数分量分别较块体BaTiO_(3)提升约6倍,较块体SrTiO_(3)提升4.2倍和1.3倍;纵向挠曲电系数较其组成材料基本不变;这种挠曲电系数分量不同程度提升的综合效果能够使超晶格SrTiO_(3)/BaTiO_(3)较其单一组分材料的挠曲电效应产生数倍提升.本文对寻找性能优异的复合挠曲电材料具有一定理论指导意义.

The flexoelectric effect describes the coupling of polarization to strain gradient,which has increasingly attracted interest in perovskite oxide materials.The perovskite oxide superlattice containing epitaxial relaxation or intrinsic surface tension or curvature,together with its high dielectric constant,is a highly desirable candidate for high flexoelectricity.In this work,the flexoelectric coefficients of^(1)SrTiO_(3)/^(1)BaTiO_(3) superlattice,which is composed of alternating single atomic layers of SrTiO_(3) and BaTiO_(3),are systematically investigated with first principle density functional theory calculations.Various supercell sizes are used to minimize the discrepancy between the gradient values of the fixed atoms and relaxed atoms.It is found that the strain gradients of the constrained A-site atoms and the relaxed B-site atoms are almost the same when the supercell sizes are 1×1×24 for longitudinal flexoelectric coefficient,7×1×16 for transverse flexoelectric coefficient and 3×1×28 for shear flexoelectric coefficient.Calculation results demonstrate that the transverse flexoelectric coefficient and shear flexoelectric coefficient of^(1)SrTiO_(3)/^(1)BaTiO_(3) superlattice are about one order of magnitude larger than its longitudinal flexoelectric coefficient.Even though its longitudinal flexoelectric coefficient decreases slightly compared with its constituent compounds,both transverse coefficient and shear flexoelectric coefficient are about several times higher than the counterparts of its constituent compounds,respectively.Hence,the overall flexoelectric coefficient of^(1)SrTiO_(3)/^(1)BaTiO_(3) superlattice is enhanced several times in magnitude.There exist a large number of interfaces inside the perovskite oxide superlattice with alternating single atomic layers of SrTiO_(3) and BaTiO_(3),which potentially stimulate the redistribution of charge carriers,orbitals and spins of the atoms at the interface and promote the interfacial strain gradient.The stacking order of the superlattice atoms has a profound influence on the flexoelectric properties.These studies present an alternative approach to fabricating better flexoelectric materials for the applications of electromechanical equipment.