Crystallographic features in the vicinity of the morphotropic phase boundary in the multiferroic material Bi 1-x Sm x Fe
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Crystallographic features in the vicinity of the morphotropic phase boundary in the multiferroic material Bi1-xSmxFeO3 Masashi Nomoto,1 Takumi Inoshita,1 Yasuhide Inoue,2 Yoichi Horibe,3 and Yasumasa Koyama1,2 1 Department of Electronic and Physical System, Waseda University, Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan 2 Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo, 169-0051, Japan 3 Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui, Tobata-ku, Kitakyushu, Fukuoka, 804-8550, Japan ABSTRACT The multiferroic material Bi1-xSmxFeO3 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 Bi1-xSmxFeO3, 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 Bi1-xSmxFeO3 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. INTRODUCTION The simple perovskite oxide BiFeO3 is one of multiferroic materials showing ferroelectric and antiferromagnetic orders in the ground state. When the Bi3+ site in the end oxide BiFeO3 are partially substituted by Sm3+ ions, in Bi1-xSmxFeO3 (BSFO), the ferroelectric R3c state with the rhombohedral symmetry was found to change into the paraelectric Pnma state with the orthorhombic symmetry around x = 0.14 [1,2]. The notable feature in BSFO is that the state boundary between the R3c and Pnma states is regarded as a morphotropic phase boundary (MPB), which is basically temperature-independent. In addition, a remarkable piezoelectric response was also found near the MPB, just as in the case of (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT). According to the previous studies on the piezoelectric response in PZN-xPT, the remarkable response is understood to be directly associated with a rotation of a polarization vector in the ferroelectric monoclinic state, which is present around x = 0.08 [3,4]. Although there exists the PbZrO3-type state near the MPB in BSFO, a relation be
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