Abstract
By means of electron diffraction the 2√2 a p × 2√2 a p phase, well-known in YBa 2Cu 3O 7−δ, was observed in two other perovskite-based materials (Y 0.75Ce 0.25) 2(Sr 0.85Y 0.15) 2AlCu 2O 9 and Bi 1.8Pb 0.4Sr 2Ca 2Cu 3O 10+ x . Highly correlated ordering is observed in the ab-plane, the correlation along the c-direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO 2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO 2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.
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