Changes in modulation period of Bi₂Sr₂ (Ca_1-xNd_x) Cu₂O_{a + δ} and their relations to oxygen content

Structural changes of long-period modulated structure (LPMS) of the Bi₂Sr₂(Ca_1-xNd_x) Cu₂O_{a + δ} ceramics with x are investigated by means of electron diffraction. Increasing x, the modulation mode of the mixing of domains of two modulation periods with b=4.5b_o and b=5b_o changes to that with b=4.5b_o and b=4b_o through the modulation mode with only b=4.5b_o at x=0.4-0.5. The wavenumber of the superlattice reflection shows a stepwise increase rather than a linear increase with x. Excess oxygen in a LPMS model with the site of excess oxygen is shown to be consistent with excess oxygen analyzed chemically by iodometry and its change with x.