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.