Effect of Ca doping on modulated structures in multiferroic Bi_1−xCa_xFeO₃

The crystallographic features of Bi₁−_xCa_xFeO₃ samples with x = 0.10, 0.15, 0.20, and 0.30 compositions are investigated by transmission electron microscopy. Perovskite-based modulated structures are found in all the samples except for the x = 0.10 sample where a rhombohedral R3c BiFeO₃-type structure is observed at room temperature. The in situ heating experiment indicates that the modulated structure appears even in the x = 0.10 sample at 673 K or higher. At room temperature, the modulation direction is found to change at approximately x = 0.20. The models of the modulated structures are constructed by introducing periodic antiphase boundaries in terms of the M₃-type rotational displacements of the FeO₆ octahedra. The electron diffraction patterns simulated from the models are in good agreement with the experimental results. The composition dependence of the modulated structures is also discussed.