Electron microscopy of the Pb-Sr-Ca-Er-Cu-O superconductor
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Y.Kim AT& T Bell Laboratories, Holmdel, New Jersey (Received 14 April 1989; accepted 15 September 1989) The structure and microstructure of a solid state processed Pb-Sr-Ca-Er-Cu-O superconductor have been investigated by transmission electron microscopy. In addition to the majority superconducting phase, at least two other impurity phases have been observed. The superconducting phase is a layered structure similar to the Bi2Sr2CaCu2O>, compound, with an extra Cu atom between the two PbO layers. Stacking defects inside the grain have been observed. A grain boundary amorphous phase has also been observed. The steps in the resistivity-temperature plot and the consequent absence of TCjo above 8 K are attributed to either the presence of the Pb2Sr2Cu2Oy unit cell at the grain boundary and/or a local enrichment of oxygen at the grain boundaries. Superlattice spots in the [100] zone axis diffraction patterns from regions enriched in Ca and Er with respect to the nominal composition are interpreted as due to ordering of Sr and Ca/Er in the Sr sites.
I. INTRODUCTION
The superconducting cuprate, Pb-Sr-Ca-Er-Cu-O, was discovered by Cava et al.l with results confirmed by Subramanian et al.2 This system, although similar to the Bi-Ca-Sr-Cu-O (BCSCO) in terms of the CuO2 perovskite units, is structurally different from the BCSCO polytypoids in two ways. First, there is no glide plane between two halves of the unit cell, leading to an unshifted stacking along the c-direction. The second is the presence of an additional Cu atom between the PbO layers. This Cu atom is reported to have a +1 formal valence and is not in a perovskite-type environment. Superconductivity is observed only when part of the Ca is replaced by Er(+3). In the case of the BCSCO superconductor, it was shown that the transport properties are very sensitive to the nature of grain boundaries and the phases that form near the boundaries.3"5 Thus, small deviations of the composition (even though it may occur only locally) from the optimum composition lead to the formation of lower Tc polytypoids at the grain boundaries. Consequently, steps appear in the resistivitytemperature plot. The transport properties are also very sensitive to magnetic fields. Since the new superconducting phase is a derivative of both the BCSCO polytypoid and the Y-Ba-Cu-O (1-2-3) phase and is prepared under reducing conditions, there is a high likelihood that the oxygen content will be altered under ambient conditions. This could also alter the properties of the phase. In order to understand the role of defects and grain boundaries on the electrical properties, it is essential to characterize them. In this paper, results J. Mater. Res., Vol. 5, No. 2, Feb 1990
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of high resolution transmission electron microscopy (HREM) and electron diffraction studies of these compounds are reported. II. EXPERIMENTAL
The Pb-Sr-Ca-Er-Cu-O superconductor was prepared by the conventional solid state processing route. AC susceptibility was obtained usin
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