Crystal chemistry and domain structure of rare-earth doped BiFeO3 ceramics
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Bi(1−x)RE x FeO3 (BREF100x, RE = La, Nd, Sm, Gd) has been investigated with a view to establish a broad overview of their crystal chemistry and domain structure. For x ≤ 0.1, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFeO3. For Nd and Sm doped compositions with 0.1 < x ≤ 0.2 and x = 0.15, respectively, a new antipolar phase was stabilised similar in structure to PbZrO3. The orthoferrite, Pnma structure was present for x > 0.1, x > 0.15, and x > 0.2 in Gd, Sm, and Nd doped BiFeO3, respectively. For x > 0.2, La doped compositions became pseudocubic at room temperatures but high angle XRD peaks were broad and asymmetric. These compositions have been indexed as the orthoferrite structure. It was concluded therefore that the orthoferrite phase appeared at lower values of x as the RE ferrite, end member tolerance factor decreased. However, the compositional window over which the PbZrO3-like phase was stable increased with increasing end member tolerance factor but was not found as single phase in La doped compositions at room temperature. On heating, the PbZrO3-like phase in BNF20 transformed to the orthoferrite, Pnma structure. T C for all compositions decreased with decreasing A-site, average ionic polarizabilty and tolerance factor. For compositions with R3c symmetry, superstructure and orientational, and translational (antiphase) domains were observed in a manner typical of an antiphase-tilted, ferroelectric perovskite. For the new PbZrO3-like phase orientational domains were observed.
The authors would like to thank Professor C. A. Randall, Professor D.C. Sinclair, and Dr. I. Levin for their useful discussions concerning this manuscript.
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