Micron and Microscopica Ac/a. Vol. 22, No. 1/2, pp. 183—184, 199!. (/739-626(1/9! $3.01) +1)0(1 Printed in Great Britain. Pergamon Press plc HREM OF DEFECTS IN HIGH Tc SUPERCONDUCTORS G. Van Tendeloo*, T. Krekels*, S. Amelinckx*, J. Karpinski°, E. Kaldis°and S. Rusiecki° *Centre for Electron Microcopy for Materials Research, University of Antwerp (RUCA), Groenenborgerlaan 171, B2020 Antwerp, Belgium. o Laboratorium für Festkorperphysik, ETH-Hongerberg/HPF, CH-8093 Zurich, Switserland INTRODUCTION New high Tc superconductors are not only of great technical significance in view of their potential use in superconducting devices, they are also interesting from purely crystallographic point of view. Many of them contain a large number of (complex) defects which can strongly influence the superconducting properties. High resolution electron microscopy has proven to be an excellent technique for the atomic scale analysis of these defective structures. Only two recent examples will be considered here. (001) PLANAR DEFECTS IN Y-Ba-Cu-O Y-Ba-Cu-O is by far the most widely studied superconducting system; its general formula can be written as Y2Ba4Cu6+~O14+~ known members of this series are YBa2Cu3O7 with a critical temperature Tc of 93K; Y 2Ba4Cu7O15 with a Tc value of 75K and YBa2Cu4O8 with a Tc of 80K. The three superconductors are structurally very related; in the 1-2-3 compound there is a single CuO layer between two BaO layers, in the 1-2-4 compound all single CuO layers are replaced by double CuO layers, while for the 1-2-3.5 composition only one in two single layers is replaced by a double layer. These stable compounds (x=0, x=l and x=2) are clearly three out of a systematic family; other members of the series however have been observed in samples with intermediate compositions, but none of them could be prepared in large enough sizes to deserve the name “new phase”. Some examples have been shown by Chaillout et al.[1] and will be fully discussed in this lecture. A different type of polymorphic structure might be associated with the number of CuO planes between successive BaO layers. The 1-2-3 structure this is always 1; for the 1-2-3.5 structure this is alternating 1,2,l,2,...while for 1-2-4 it is always 2. A lot of effort has been put in making compounds with more than two CuO layers. In samples prepared at the laboratory for festkorperphysik in Zurich we found relatively wide areas with macroscopic composition 1-2-4 but where instead of two CuO layers between BaO layers three CuO layers were alternating with a single CuO layer. An example of a such a polymorphic structure is shown in fig. 1. Similar observations were made by Ramesh et al. [2] LONG PERIOD SHEAR STRUCTURE IN La2CuO4.~. In La2CuO4, the first “high” Tc superconductor, a new long period superstructure is formed in slightly non stoichiometric, oxygen deficient material; the long period modulation accommodates the local non- stoichiometry. The basic high temperature phase of the stoichiometric compound is tetragonal, while the room temperature phase is orthorhombic. A high resolution image along [l01]~ of the defective structure is shown in fig.2. The crystallographic shear planes show up prominently in this image. The corresponding diffraction pattern is shown in the inset, together with a higher magnification, allowing indentification of the heavy metal columns as well as the CuO6 octahedra. The resulting complex structure can be found in our recent paper [3], and is a nice example of the possibilities of structure determination by HREM, once the underlying structure has been determined by other means. Diffraction information together with the HREM information allowed us to determine the space group of the room temperature phase as Plc 1. REFERENCES [1] C. Chaillout, P. Bordet, J. Chenavas, J.L. Hodeau, M. Marezio, J. Karpinski, E. Kaldis, S. Rusiecki, Solid State Comm. 70 (1989) 275 [2] R. Ramesh, S. Jin, P. Marsh, Nature 346 (1990) 420 [3] G. Van Tendeloo, S. Amelinckx, Applied Physics A.(in the press) 183