Symmetrical Staircase in the Profile of Lattice-Modulation Period versus Pr-Concentration in Bi2Sr2(Ca1-xPrx)Cu2O8+δ

Variation of the lattice-modulation period p a of the “incommensurate” phase Bi 2 Br 2 (Ca 1- x Pr x )Cu 2 O 8+δ (0≦ x ≦0.72) with Pr-concentration x was examined by electron diffraction. The plot of p a versus x exhibits roughly a linear decrease but it has a staircase structure. The period of each stair, p a , agrees with a value expected for a commensurate structure. We observed nine stairs which are symmetrically arranged; the central stair corresponds to the primary structure with a simple modulation, and a stair on the left (or right) corresponds to a structure which is derived from the primary structure by introducing an advanced (or delayed) discommensuration. The observed structures cover a full range of commensurate structures predicted theoretically.