Atomic Resolution Electron Microscopy of Bismuth Cuprates
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ATOMIC RESOLUTION ELECTRON MICROSCOPY OF BISMUTH CUPRATES R.Ramesh, B.G.Bagley and J.M.Tarascon Bellcore, Red Bank, NJ 07701 C.J.D.Hetherington and G.Thomas National Center for Electron Microscopy Lawrence Berkeley Laboratory, Berkeley CA 94720. S.M.Green and H.L.Luo Department of Electrical and Computer Engineering University of California, San Diego, LaJolla, CA 92093. INTRODUCTION Since the discovery of superconductivity in certain classes of cuprates, there has been much research activity aimed at understanding the structure, microstructure, electrical and other physical properties of these materials. In this paper, we present a summary of the current status of on-going research on structural and microstructural characterization of the Bi cuprates and cationically substituted derivatives of these superconducting compounds. High resolution structural imaging together with conventional and analytical electron microscopy is used to examine the structure[l]. The overall aim of these studies is to characterize the structural and microstructural evolution and ultimately to determine the effect on the superconducting properties. EXPERIMENTAL The experimental details for sample preparation are reported in ref. 1. Thin foils for electron microscopy were prepared by slicing a 5001tm foil, grinding to about 50prm and finally Argon ion milling to electron transparency in a liquid nitrogen stage at 6kV. High resolution electron microscopy (HREM) was carried out in the Berkeley Atomic Resolution Microscope(ARM) at 800kV, while the diffraction contrast experiments were carried out in a Philips 400TEM/STEM at 100kV. Image simulations were carried out using the MacTempas program. RESULTS AND DISCUSSION Structural details of the Bi2Sr2Can-ICunOy [(BSCCO) (where n=1,2,3 and sometimes 4)] system have been elucidated in earlier papers[see for example references cited in ref. I]. When examined along the [1101 zone axis of the orthorhombic crystal structure the different atomic layers project as columns. Fig.1 shows a typical atomic resolution structural image of the n=3 phase along the [1101 zone axis. The interpretation of this image, in terms of the projection of the n=3 phase along the [110] zone, is verified by carrying out image simulations for suitable values of objective lens defocus and foil thicknesses. Details are published elsewhere[l]. One interesting structural feature of the BSCCO compounds is an incommensurate modulation that is observed as satellite spots in an electron diffraction pattern and can be seen as an almost periodic intensity modulation in those images obtained in the [001] or [100] zone axes orientations. In Fig.2 is shown a typical [100] high resolution image of the structural modulation in the n=3 phase. In this image, a significant modulation in the intensity distribution close to the BiO double layers is apparent. Upon careful examination of the image, the modulation in the perovskite blocks can also be discerned. (This is best observed if the image is held at the level of the eye, along the a-directi
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