An electron diffraction study of Bi2Sr2(Gd/x/Ca/1-x/)0.8Cu2O(8+delta)

The compound Bi2Sr2(Gd/x/Ca/1-x/)0.8Cu2O(8+delta) is examined by electron diffraction for several values of x. The incommensurate modulation which has been reported for x = 0 persists for all values of x with the wave vector increasing as x increases. A secondary modulation with a wave vector equal to one-half that of the primary is observed for x equal to or greater than 0.85. This modulation manifests itself through satellite reflections that are highly streaked normal to the (001) planes and that flank certain select positions in reciprocal space. The appearance of this secondary modulation is accompanied by a change in the average crystal structure from B-centered to F-centered orthorhombic.