2-ROOT-2 A(P)X2-ROOT-2 A(P) PHASE IN SUPERCONDUCTING CERAMICS

By means of electron diffraction the 2 root 2 a(p) x 2 root 2 a(p) pha se, well-known in YBa2Cu3O7-delta was observed in two other perovskite -based materials (Y0.75Ce0.25)(2)(Sr0.85Y0.15)(2)AlCU2O9 and Bi1.8Pb0. 4Sr2Ca2Cu3O10+x. Highly correlated ordering is observed in the ab-plan e, the correlation along the c-direction being weak. The plane group o f the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results un ambiguously rule out oxygen ordering as a possible origin of the super structure. Experimental evidence points out that the superstructure is associated with the CuO2 layers, that are the only structural element s common to the three compounds studied. A model is proposed where the CuO2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO2 sheets. Comparison of simulated an d experimental [001] zone diffraction patterns strongly supports our m odel.