Structural investigation of ferroelectric BiFeO₃-BaTiO₃solid solutions near the rhombohedral-pseudocubic phase boundary

The structural properties of single crystals of BiFeO3-BaTiO3 (BF-BT) ferroelectric oxide solid solution with a slightly BF-rich composition (72BF-28BT; toward the rhombohedral-phase side) from the rhombohedral-pseudocubic phase boundary were investigated by transmission electron microscopy (TEM) and reciprocal space mapping (RSM), including three-dimensional (3D) RSM. According to the TEM results, the 72BF-28BT specimen had a domain structure similar to that of a rhombohedral crystal and the polarization direction appeared to be approximately «111», corresponding to a ferroelectric crystal with rhombohedral symmetry, although some distorted behavior suggested that the crystal symmetry may not have been strictly rhombohedral. Fine and complex domain structures were also observed inside the rectangular domains. The RSM results also indicated that the crystal had rhombohedral-like symmetry, although several discrepancies were observed, such as unexpectedly split diffraction peaks and distortion-related diffraction distribution in the reciprocal space. The structural behavior observed in the TEM images and 2D and 3D RSM images suggested that the symmetry of the 72BF-28BT specimen was lower than rhombohedral and may have been monoclinic. Structural disorder was detected along the distorted direction from rhombohedral symmetry, which could interfere with obtaining structural information via powder diffraction experiments and lead to inaccurate identification of the crystal structure as a higher-symmetry one.