相场法探究铁电体涡旋拓扑结构与准同型相界

Phase-field method explored ferroelectric vortex topology structure and morphotropic phase boundaries

钙钛矿晶体结构决定了铁电陶瓷铁电性的出现和极化方向的确定,当极化方向具有一定规律的排序时,不同电畴结构会组合形成具有特定形貌的多粒子系统,即存在于铁电体中的拓扑结构.本研究通过相场法,模拟了不同迟滞电场和厚度下铌酸钾钠(K_(0.5)Na_(0.5)NbO_(3))薄膜的电畴结构并进行观察,根据电场下电畴结构的不同翻转路径将电畴翻转分为快速和慢速翻转阶段,基于此提出先确定所需实验的电畴翻转状态再进行定向观测的手段.通过对电畴结构结合极化矢量的分析,首次在铌酸钾钠薄膜中观察到明显多畴组合而成的涡旋-反涡旋对拓扑结构.对涡旋结构进一步分析其翻转过程,观察到这种涡旋拓扑微观结构可以使电畴更容易发生翻转,从而使更多小范围极化矢量进行排序,以形成所需的多粒子系统拓扑结构.这种极化矢量排序对铁电材料介电性能改善机制,是与准同型相界两侧特定极化方向所形成的微观相界有着异曲同工之处.

The perovskite crystal structure determines the appearance of ferroelectricity and the polarization direction of ferroelectric ceramics.When the polarization direction has a certain order,different domain structures will combine to form a multiparticle system with a specific morphology,i.e.the topological structure existing in ferroelectrics.In this study,the domain structures of potassium sodium niobate(K_(0.5)Na_(0.5)NbO_(3))thin films under different hysteresis electric fields and thickness are simulated and observed by the phase field method.According to the different switching paths of the domain structure under the electric field,the domain is divided into fast and slow switching process.Based on this,a method is proposed to first determine the domain switching state of the desired experiment and then conduct directional observation.Through the analysis of the domain structures combined with the polarization vector,a clear multi-domain combined with vortex-antivortex pair topological structure is observed for the first time in K_(0.5)Na_(0.5)NbO_(3) film.The vortex structure is further analyzed for its switching process,and it is observed that this vortex topological microstructure can make the domain more likely to switch,so that more small-scale polarization vectors can be ordered,forming the desired multiparticle system topology.The mechanism of improving the dielectric properties of ferroelectric material by this polarization vector ordering is similar to that of the microscopic phase boundary formed by the specific polarization directions on both sides of the quasi morp ho tropic phase boundary.