Crystallographic features in the vicinity of the morphotropic phase boundary in the multiferroic material Bi_1-xSm_xFeO₃

The multiferroic material Bi_1-xSm xFeO₃ shows ferroelectric and antiferromagnetic properties in the ground state of the Bi-rich side. When the Sm content increases from x = 0 in BiFeO₃, the (ferroelectric-R3c → paraelectric-Pnma) state change occurs around x = 0.14. According to the previous studies on Bi_1-xSm xFeO₃, the state boundary between the R3c and Pnma states can be identified as a morphotropic phase boundary (MPB), which is nearly parallel to the temperature axis in the state diagram. The notable feature of Bi_1-xSm xFeO₃ is that a remarkable piezoelectric response was also found near the MPB. However, the origin of the remarkable response has not been understood sufficiently. In this study, thus, the crystallographic features in the vicinity of the MPB have been examined by x-ray powder diffraction and transmission electron microscopy. It was confirmed that the R3c and Pnma states were present for 0 ≤ x ≤ 0.15 and for 0.16 ≤ x ≤ 0.30, respectively. In addition to these states, there also existed the PbZrO₃-type state around x = 0.15, which was identified as a modulated structure. Based on the analysis of the modulated structure, furthermore, it was suggested that the PbZrO₃-type state could be regarded as a 2q state, which is characterized by two transverse modulation waves with k1 = [1/2 0 0]o and k2 = [0 1/2 0]o in the orthorhombic-Pnma notation.