Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar Atomic structure of domain and defect in layered-perovskite Bi 2 WO 6 thin flms Xiaokuo Er a , Yuelin Zhang b , Yupeng Yin a , Chuanshou Wang b , Hongliang Wang a , Jinxing Zhang b, ⁎ , Qian Zhan a, ⁎ a Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China b Department of Physics, Beijing Normal University, 100875 Beijing, China ARTICLEINFO Keywords: Microstructure Domain structure Nanoscale defect Layered-perovskite oxide thin flm Transmission electron microscopy ABSTRACT Layered-perovskite oxide thin flms provide new opportunities to design next-generation electronic devices with their special structure and potentially excellent electrical properties. The microstructure and domain structure in epitaxial Bi 2 WO 6 (BWO) thin flms have been investigated by advanced transmission electron microscopy (TEM) combined with piezoresponse force microscopy (PFM). A slight distortion of oxygen octahedron, which ac- companies the movement of the Bi 2 O 2 fuorite-like sheet relative to the perovskite-like WO 4 blocks, results in the formation of 90° domains. A kind of nanoscale defect is characterized by aberration-corrected scanning trans- mission electron microscopy (AC-STEM). An extra Bi-O layer is suggested to be presented at the Bi 2 O 2 fuorite- like sheet. The present study may help to enrich the understanding of ferroelectric domain switching on the microscopic scale and the efect of defects on the ferroelectric property. 1. Introduction Recently, Aurivillius oxides have stood out for their special structure and potentially excellent electrical properties [1–7]. Bi-con- taining Aurivillius compounds, with the general formula of (Bi 2 O 2 ) 2+ (A n−1 B n O 3n+1 ) 2− , are one of the most important branches, in which perovskite-like layers (A n−1 B n O 3n+1 ) 2− are sandwiched by fuorite-like layers (Bi 2 O 2 ) 2+ with an n value of 1–8 [8–14]. Bi 2 WO 6 (BWO), the simplest Bi-containing Aurivillius compound [15–23], with lattice parameters of a = 5.437 Å, b = 5.458 Å and c = 16.430 Å, has received a large amount of attention in recent years. A frst-principles study has been carried out to better understand the reconstructive ferroelectric-paraelectric transition of BWO [10]. The polarized Raman and infrared (IR) studies of monodomain BWO single crystal have been reported by M. Maczka et al. [17]. By observation under a polarizing microscope, the BWO crystals grown by the fux growth technique were confrmed to be in a single domain state [23]. Most recently, the elastic mono-domain in-plane switching with a non-volatile strain of ap- proximately 0.4% has been achieved in Bi 2 WO 6 thin flms by the pie- zoresponse force microscopy (PFM) measurement [18]. The atomic- scale domain structure of layered-perovskite BWO needs to be further explored for a deeper understanding of the microscopic origin of po- larization switching. Moreover, the efect of defects on the electrical performance has been investigated in perovskite-like ferroelectric thin flms [24–31]. X.B. Ren reported that the restoring force of a large electric-feld-in- duced strain (or piezoelectricity) in the aged BaTiO 3 can be provided by a general symmetry conforming property of point defects (dopant/im- purity ions and vacancies) [24]. The metastable defects, similar to the Aurivillius phase in BiFeO 3 flms, were found to induce the polarization retention failure [26,28]. B.H. Park et al. suggested that the stability of the metal-oxygen octahedral in SrBi 2 Ta 2 O 9 and Bi 4 Ti 3 O 12 Aurivillius compounds should be related to fatigue behaviors. The nature of defects in these compounds was studied to fnd new materials with no fatigue and large residual polarization [29]. In the present study, the microstructure and domain structure of a Bi 2 WO 6 epitaxial thin flm has been investigated by advanced transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM). The [001] orientated 90°domains and nanoscale defects have been reported. We intend to show a correlation be- tween the structure and the ferroelectric property to deepen the understanding of the domain structure on the microscopic scale and the efect of nanoscale defects in Bi 2 WO 6 layered-perovskite oxide flms. https://doi.org/10.1016/j.matchar.2019.06.022 Received 19 April 2019; Received in revised form 3 June 2019; Accepted 14 June 2019 ⁎ Corresponding authors. E-mail addresses: jxzhang@bnu.edu.cn (J. Zhang), qzhan@mater.ustb.edu.cn (Q. Zhan). Materials Characterization 154 (2019) 395–399 Available online 14 June 2019 1044-5803/ © 2019 Published by Elsevier Inc. T