Characteristic features of the modulated structure appearing in the multiferroic material Bi_1-xSm_xFeO₃ around x = 0.15

In Bi_1-xSm_xFeO3 (BSFO) having the multiferroic BiFeO₃ as an end material, when the Sm content increases from x = 0, it has been reported that the ferroelectric-R3c state is changed into the paraelectric-Pnma state around x = 0.14. The R3c/Pnma state boundary around x = 0.14 can be regarded as a temperature-independent morphotropic-phase boundary (MPB). The notable feature in BSFO is that, in addition to these two states, the antiferroelectric PbZrO₃-type state was also found in the vicinity of the MPB. Although the PbZrO₃-type state appears as a modulated structure, its detailed features have not been understood yet. We have thus examined the crystallographic features of prepared BSFO samples around x = 0.14, mainly by transmission electron microscopy. The PbZrO₃-type state was confirmed to be present in samples with x = 0.15 on the basis of x-ray powder diffraction profiles measured from prepared samples at 300 K. On the other hand, the observation made by transmission electron microscopy indicated that the state for x = 0.15 is characterized by a coexistence state consisting of the ferroelectric-R3c and antiferroelectric PbZrO3-type states. In particular, the crystal structure of the PbZrO3-type state could be identified as a modulated structure with two transverse modulation waves, whose wave vectors are given by qi = [1/2 0 0]_o and q₂ = [0 1/2 0]_o in the orthorhombic-Pnma notation. In addition, eigenvectors of these two transverse waves were also determined to be parallel to the same [001]_o direction.