Determination of crystal symmetry for Bi₄Ti₃O₁₂-based ferroelectrics by using electron diffraction

In recent years, inconsistent space groups of monoclinic B1a1 and orthorhombic B2cb have been reported for the room-temperature ferroelectric phases of both Bi₄Ti₃O₁₂ and lanthanide-substituted Bi₄Ti₃O₁₂. In this article, the electron diffraction technique is employed to unambiguously clarify the crystal symmetries of ferroelectric Bi₄Ti₃O₁₂ and Bi_3.15Nd_0.85Ti₃O₁₂ single crystals at room temperature. All the reflections observed from the two crystals match well with those derived from B1a1, but the observed reflections 010, 030, 10 and 30 should be forbidden in the case of B2cb. This fact indicates that both the ferroelectrics are of the space group B1a1 rather than B2cb, which is confirmed by convergent-beam electron diffraction observations. On the basis of the monoclinic space group B1a1, the lattice parameters of both the ferroelectrics were calculated by the Rietveld refinement of powder X-ray diffraction data.