Crystallographic and morphologic features of quadruple and double superstructures in Bi1-xSmxFeO3 with 0≤x≤0.40 were investigated by x-ray powder diffraction and transmission electron microscopy. The presence of a quadruple superstructure accompanied by antiferroic ionic displacements was found near the morphotropic phase boundary between the R3c phase (0<x<0.10) and the Pnma phase (0.25≤x≤0.40). The quadruple superlattice phase coexists with a ferroelectric R3c phase at x=0.15. These superstructures were found to originate from the freezing of multiple phonon modes related to transverse wave ionic displacements. These findings provide further insight towards understanding the complexity of rare-earth-doped BiFeO3.
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