Abstract
We report our observations on a three-dimensional modulation that comprises 〈110〉 and [001] components in YBa2(Cu1−xMx)3O7-δ (M ≡ Co, Fe or Al; 0·02 > x > 0·33). The [001] modulation, which was observed for the first time at low dopant concentrations (x > 0·1), was carefully analysed by nanoscale electron diffraction techniques. We found that the periodicity (or the correlation length) of the modulation along the c axis decreases with increasing values of x and was about 500 Å for x = 0·03 (quasiperiodic), and about 12 Å for x = 0·33 (periodic). The cause of the modulation along the [001] direction was attributed to stacking of the domains on top of each other along the c axis. The variation in local oxygen ordering, stacking sequence and lattice discontinuity are expected to be responsible for the incoherence between the overlapped domains. Lattice displacement is the characteristic feature of the modulation in the entire concentration range of x.
