The crystal structure of the 1212 nonsuperconductor phase (Pb 0.71 Cu 0.29 )Sr 2 (Y 0.73 Ca 0.27 )Cu 2 O 7
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I. INTRODUCTION Since the initial discovery by Bednorz and Miiller of a material with a superconducting transition temperature near 30 K,' the rate of discovery of compounds having transition temperatures above 77 K has been accelerating. In all of them the presence of CuO2 square coplanar layers appears to be the essential feature that imparts this unusual physical phenomenon. Beginning with the YBa 2 Cu 3 0 7 (123) crystal structure,2 which can be considered to have a central core consisting of the sequence CuO 2 -Y-CuO 2 that is flanked by Ba ions and CuO3 chains, related compounds have been synthesized in which T1O6 or BiO6 single or double sheets of octahedra flank the square coplanar C u - 0 layers.3"5 Barium or strontium ions are in oxygen layer interstices; Ca or Y are situated between the square coplanar Cu-O nets. The presence of the latter ions between the CuO2 square coplanar layers removes an oxygen ion from the Cu coordination sphere and reduces it from an elongated octahedron to an elongated square pyramid. If Ca2+ flanks the CuO2 layer on both sides, then the Cu ion coordination is reduced to square coplanar.5 The recently discovered Pb2Sr2LnCu3O8 with Tc near 70 K6 can also be considered as a variant of the 123 structure where the square coplanar CuO2 layers are sandwiched between the PbOJ. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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CuO-PbO units. The (Tl, Bi)Sr 2 CaCu 2 0 ;c compound 7 (1212) and LnBa 2 Cu 4 0 8 are further variants of the 123 structure. While exploring the Pb-Sr-Ca-Y-Cu system we found single crystals in a reaction product for which semi-quantitative energy dispersive x-ray spectroscopy showed a composition near Pb 0 ^S^ 3(Y, Ca),Cu 2 . We undertook the structure determination of this phase and report the result here. II. EXPERIMENTAL The starting mixtures consisted of PbO, Sr 2 Cu0 3 , Ca 2 Cu0 3 , Y2O3, and CuO. The Sr 2 Cu0 3 and Ca 2 Cu0 3 compounds were prepared from the respective carbonates and CuO. All starting materials were of reagent grade purity. Weissenberg and oscillation x-ray diffraction diagrams of the previously mentioned single crystal showed that it was tetragonal, a = 3.85 A, c = 11.85 A. The lattice constants reported for {T\,m)Sr2C&C\x20x (1212) are essentially identical to these values,7 and therefore a mixture with the composition PbSr2Ca0 5Y0 5Cu2 was prepared from the respective oxides and heated at 960 °C in air for 12 h. The powder x-ray diffraction pattern could be ascribed to the 1212 phase with a few lines from an impurity that could be indexed as a cubic phase with 1989 Materials Research Society
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J.Y. Lee, J.S. Swinnea, and H. Steinfink: 1212 nonsuperconductor phase
a = 4.32 A. When all of the Y was replaced by Ca in the lPb:2Sr: lCa:2Cu proportion in the starting mixture the product, after heating at 960 °C, showed a very complex powder x-ray diffraction pattern and no evidence of the 1212 phase. A starting mixture lPb:2Sr:0.25Ca:0.75Y:2Cu reacted at 960 °C
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