Unraveling the Crystal Structure of All-Inorganic Halide Perovskites using CBED and Electron Ptychography Roberto dos Reis 1 , Hao Yang 1 , Colin Ophus 1 , Tetiana Shalapska 2 , Gregory Bizarri 2 , Didier Perrodin 2 , Peter Ercius 1 , Jim Ciston 1 , Edith Bourret 2 and Ulrich Dahmen 1 . 1. National Center for Electron Microscopy, Molecular Foundry, LBNL, CA, USA 2. Materials Sciences Division, LBNL, Berkeley, CA, USA. The unique electronic properties observed in halide perovskites originate in their rich structural complexity that allows compatibility with a variety of structural motifs and compounds. Adjustments of the corner-connected BX 6 octahedral network in the ABX 3 structure promote a wide range of optical and electronic properties [1]. For this reason, methods to precisely identify the local symmetry are necessary to unambiguously distinguish different possible structural phases. Here, we determine the crystallography of all-inorganic CsPbBr 3-x Cl x perovskite single crystals, grown via the Bridgman- Stockbarger method, by exploring information contained in electron diffraction patterns. Due to the material’s sensitivity to the electron beam, precise atomistic studies of halide perovskites by transmission electron microscopy (TEM), scanning TEM (STEM) and electron diffraction are relatively underdeveloped. Consequently, X-ray diffraction has been the technique of choice to assign crystal structures, requiring a careful study of the splitting of certain lines and of the presence of superlattice reflections. Effects due to small atomic shifts can easily be missed in X-ray diffraction, which accounts for reports of conflicting structures. Until now, there appears to be no definitive conclusion about the stable structure of these materials as it seems to be largely dependent on the conditions and methods of growth. The possible structures can be narrowed down by following the pathways of distortions from the ideal cubic structure following the group-subgroup relationship shown in Fig. 1a. We use convergent-beam electron diffraction (CBED) acquired at low-temperature (~70K) to identify the symmetry of CsPbBr 3–x Cl x single crystals. High- symmetry CBED pattern containing information on high-order Laue zones (HOLZ), in which the intensities of certain reflections are quite sensitive to small rotations of the octahedral unit, reveal the whole crystal symmetry and positions of Br/Cl anions. Figure 1b presents CBED pattern taken along the pseudocubic [001] axis from CsPbBr 3 sample (x = 0) showing HOLZ reflections. This high-symmetry axis, ([001] pc ) is perpendicular to the alternating layers where the displacements of Br (or Cl) take place during phase transformations from cubic-to-tetragonal-to-orthorhombic. The pattern clearly exhibits a fourfold rotation and two mirror planes, indicated by the arrowheads m1 and m2 and the dotted semi- circle in Figure 1b, with the whole-pattern symmetry denoted as 4mm. Local imaging of the projected atomistic distribution in these crystals is achieved by simultaneous atomic-resolution electron ptychography (a dose efficient diffractive imaging technique [2]) and Z- contrast, using a Gatan K2-IS direct electron detector. A relatively low dose (10 3 e/Å 2 ) is employed to acquire 4D-STEM datasets [3] of 256 x 256 x 1920 x 895 at 800 frames/s with the sample kept at room temperature. The probe-forming aperture is about 17.2 mrad. Figure 2a shows the simultaneously acquired ADF image alongside the ptychographically reconstructed amplitude (Fig. 2b) and phase (Fig. 2c) taken from the same CsPbBr 3 sample, using the Wigner distribution deconvolution (WDD) approach [2,4]. Combination of both techniques, HOLZ-CBED and ptychography, depicts a tetragonal crystal system I4/mcm to CsPbBr 3 and CsPb(BrCl) 3 whereas CsPbCl 3 is found to be orthorhombic [5]. 1468 doi:10.1017/S1431927617008005 Microsc. Microanal. 23 (Suppl 1), 2017 © Microscopy Society of America 2017 https://doi.org/10.1017/S1431927617008005 Published online by Cambridge University Press