Aberration-corrected HRTEM of the Incommensurate Misfit Layer Compound (PbS) 1.14 NbS 2
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1026-C10-01
Aberration-corrected HRTEM of the Incommensurate Misfit Layer Compound (PbS) 1.14NbS2 Magnus Garbrecht1, Erdmann Spiecker1, Wolfgang Jäger1, and Karsten Tillmann2 1 Mikrostrukturanalytik, Institut für Materialwissenschaft, Kaiserstrasse 2, Kiel, 24143, Germany 2 Ernst Ruska-Centrum und Institut für Festkörperforschung, Helmholtz-Zentrum Jülich GmbH, Jülich, 52425, Germany ABSTRACT The development of tunable spherical aberration (Cs) imaging correctors for mediumvoltage transmission electron microscopes (TEM) offers new opportunities for atomic-scale investigations of materials. A very interesting class of microstructures regarding a variety of different physical properties are the transition metal dichalcogenide misfit layer compounds exhibiting a high density of incommensurate interfaces due to their stacked nature. In the present study, the benefits coming along with the set-up of negative CS imaging (NCSI) conditions (in TEM) are demonstrated by means of different examples regarding local inhomogeneities in (PbS)1.14NbS2 crystals that can not be dissected in such detail by averaging x-ray techniques. INTRODUCTION Incommensurate crystalline materials give rise to a variety of striking phenomena, e.g. structural modulations [1], phase transitions [2] and superlubricity [3]. In order to understand their origin it is important to have atomic structure information of the incommensurate interfaces available. The transition metal dichalcogenide (TMDC) misfit layer compounds (MX)nTX2 (M = La, Sn, Pb or rare earths; X = S, Se, Te; T = Ta, Ti, Nb, V; n = 1.08….1.19) are ideal model systems for studying such interfaces, because they consist of an alternating stacking of double atomic layers of distorted rocksalt MX and sandwich layers of TX2[4]. For (PbS)1.14NbS2 , which is studied in this work, the two substructures are coherent in the first interface direction, but incommensurate in the perpendicular one, as shown schematically in Fig. 1. X-ray diffraction (XRD) techniques combined with structure refinement procedures are generally employed for the determination of the atomic structure of such compounds (e.g. [4, 5]). A prerequisite for obtaining meaningful results with such averaging techniques is, that the crystal under investigation is uniform and free of bending on a length scale of millimeters. Inhomogeneities, like the coexistence of orientation variants [6], orientation anomalies [6], stacking disorder (e.g. Fig. 3), and the formation of layer undulations (e.g. Fig. 4) are nevertheless local and, in contrast to most XRD techniques, transmission electron microscopy (TEM) and electron diffraction are well suited for structural investigations of small sample volumes. While conventional high-resolution TEM (HRTEM) is a powerful tool to image atomic features, it suffers from relatively large contrast delocalization due to the effect of spherical aberration of the objective lens. As a result the direct imaging of atomic columns close to an incommensurate interface, where the translation symmetry of the perfe
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