Symmetrical staircase in the profile of lattice-modulation period versus Pr-concentration and domain configurations in Bi2Sr2(Ca1-xPrx)Cu2O8+delta

Variation of the lattice-modulation period p(a) of the 'incommensurate' pha se Bi2Sr2 (Ca1-xPrx)Cu2O8+delta (0 less than or equal to x less than or equ al to 0.72) with Pr-concentration x was examined by electron diffraction, a nd the domain configuration by high-resolution transmission electron micros copy (HRTEM). 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 wi th a value expected for a commensurate structure. We observed nine stairs w hich are symmetrically arranged; the central stair corresponds to the prima ry structure with a simple modulation, and a stair on the left (or right) i s associated with an advanced (or delayed) discommensuration. On the other hand, the number of domain configurations i.e. commensurate structures obse rved by HRTEM is seventeen. The observed staircase structure covers a full range of commensurate structures predicted theoretically. The pattern obser ved in the plot of p(a) versus x is consistent with the simulation based on the Frenkel-Kontrova model and an inhomogeneity hypothesis.