Substitution and Defect Chemistry of La-Cu-O Systems
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SUBSTITUTION AND DEFECT CHEMISTRY OF La-Cu-O systems P. L. GAI, M. KUNCHUR * and E. M. McCARRON Central Research and Development Department, Experimental Station, Du Pont, Wilmington, DE 19880-0356; * Dept. of Physics, University of Viginia, Charlottesville,VA. Abstract Substitutional effects of strontium in La-Cu-O system and defects accommodating stoichiometric deviations have been investigated. The extended shear defects are analyzed using electron microscopy and their role in superconducting transport properties has been examined by magnetic measurements. The initial results suggest that the defects enhance flux pinning.
Introduction All of the high temperature copper oxide based superconductors discovered so far have copper-oxygen (CuO 2) sheets as a common structural feature. These sheets are believed to play a key role in the high temperature superconductivity. Following the discovery of high temperature superconductivity in La-Ba-Cu-O system (1), there have been various reports on the structural, electronic and physical properties of LaCu-O system with partial substitution of Sr2+ for La3+ (2-3). The parent La2CuO 4 has a K2 NiF 4 related structure with an orthorhombic cell: a = 5.36A, b = 5.41A and c = 13.17A. It consists of corner shared octahedra intersected by La-O layers forming intergrowths of single CuO 2 sheets alternating with La20 2 double layers with the rock salt structure. The Cu-O sheets are buckled with the CuO 6 octahedra alternately rotating about the [110] direction causing the distortion from the tetragonal symmetry of the K2NiF 4 structure. Fig. 1 shows the structural projection tilted along the [110] direction from [010]. Partial substitution of Sr for La in the structure oxidizes the CuO 2 sheets and raises the average formal oxidation state of Cu. A tetragonal oxygen deficient K2 NiF 4 structure is obtained for x > 0.05 (4). The material becomes superconducting at Sr concentrations of 0.05 < x < 0.25. Substitution and defect chemistry Experimental procedures A. La2 _xSrxCuO 4 , La2CuO 4 and La1 .85 CUO 4.5: Samples were prepared by standard solid state techniques with stoichiometric quantities of La20 3 , CuO and SrO for La2.xSrxCuO4 and with La20 3 and CuO for La2CuO 4 and Lal. 85 CUO 4.5. Well ground mixtures of binary oxides were placed in platinum boats, fired at 1100 °C in flowing oxygen for 12 hours and then furnacecooled to room temperature. The samples were then reground and fired under the same conditions to ensure complete reaction. A sample of the LaCuO 3 perovskite phase was also prepared at high pressure (3kbar).
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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Local microstructure and stoichiometric variations in the compounds were investigated using a Philips CM30 SuperTwin high resolution electron microscope, integrated with an energy dispersive X-ray (EDX) detector. The relative amounts of flux pinning in each sample was estimated from SQUID magnetic measurements. Results and discussion La2-x SrxCuO4:
The transition temperature Tc of La2
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