Choe Hyeokmin, Bieker Johannes, Zhang Nan, Glazer Anthony Michael, Thomas Pam A, Gorfman Semën
Department of Physics, University of Siegen, Walter-Flex Strasse 3, Siegen 57072, Germany.
Institute of Electromechanical Design, Technische Universität Darmstadt, Darmstadt, Germany.
IUCrJ. 2018 Jun 1;5(Pt 4):417-427. doi: 10.1107/S2052252518006784. eCollection 2018 Jul 1.
The relationship between crystal structure and physical properties in the ferroelectric NaBiTiO (NBT) has been of interest for the last two decades. Originally, the average structure was held to be of rhombohedral (3) symmetry with a fixed polarization direction. This has undergone a series of revisions, however, based on high-resolution X-ray diffraction, total neutron scattering, and optical and electron microscopy. The recent experimental findings suggest that the true average symmetry is monoclinic (space group ), which allows for a rotatable spontaneous polarization. Neither polarization rotation nor its potentially important real role in enhanced piezoelectricity is well understood. The present work describes an investigation of the average monoclinic distortion in NBT by time-resolved single-crystal X-ray diffraction under external electric fields. The study presents a high-resolution inspection of the characteristic diffraction features of the monoclinic distortion - splitting of specific Bragg reflections - and their changes under a cyclic electric field. The results favour a model in which there is direct coupling between the shear monoclinic strain and the polarization rotation. This suggests that the angle of polarization rotation under a sub-coercive electric field could be 30° or more.
在过去二十年中，铁电体钛酸铋钠（NBT）的晶体结构与物理性质之间的关系一直备受关注。最初，人们认为其平均结构具有菱面体（3）对称性，且极化方向固定。然而，基于高分辨率X射线衍射、全中子散射以及光学和电子显微镜技术，这一观点经历了一系列修正。最近的实验结果表明，其真正的平均对称性为单斜晶系（空间群），这使得自发极化可以旋转。目前，无论是极化旋转还是其在增强压电性方面潜在的重要实际作用，都尚未得到充分理解。本研究通过在外部电场作用下的时间分辨单晶X射线衍射，对NBT中的平均单斜畸变进行了研究。该研究对单斜畸变的特征衍射特征——特定布拉格反射的分裂——以及它们在循环电场下的变化进行了高分辨率检测。结果支持了一个模型，即剪切单斜应变与极化旋转之间存在直接耦合。这表明在亚 coercive 电场下极化旋转角度可能为30°或更大。 （注：原文中“sub-coercive”可能有误，推测应为“sub-critical”之类更合适的词，这里按原文翻译）
