Determination of space group of BaPb_0.75Bi_0.25O₃ by convergent-beam electron diffraction

The space group of BaPb_0.75Bi_0.25O₃ with distorted perovskite structure was investigated by convergent-beam electron diffraction (CBED) and selected area electron diffraction (SAD). The symmetry of observed CBED patterns was either 1 or m and no CBED pattern with fourfold nor sixfold symmetry was observed, indicating that the crystal system of BaPb_0.75Bi_0.25O₃ is either orthorhombic or monoclinic. The ±G dark field disk of CBED patterns indicated that no inversion center existed in BaPb_0.75Bi_0.25O₃ and that the point group of BaPb_0.75Bi_0.25O₃ was either m or mm2. It was concluded that the point group of BaPb_0.75Bi_0.25O₃ was not mm2 but m because CBED patterns with symmetry of not only m but also 1 were taken at [111] incidence with pseudo-cubic perovskite notation. From CBED patterns taken at [111] azimuth, unique axis of monoclinic BaPb_0.75Bi_0.25O₃ with point group of m was also determined. The lattice type of BaPb_0.75Bi_0.25O₃ was determined to be a body centered one from the extinction rule of SAD pattern. No Gjønnes-Moodie lines, which indicated existence of glide plane or screw axis, were observed in all of the CBED patterns, leading to the conclusion that the space group of BaPb_0.75Bi_0.25O₃ is Im (No. 8).