Determination of space group of BaPb 0.75Bi 0.25O 3 by convergent-beam electron diffraction

Abstract

The space group of BaPb 0.75Bi 0.25O 3 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 0.25O 3 is either orthorhombic or monoclinic. The ± G dark field disk of CBED patterns indicated that no inversion center existed in BaPb 0.75Bi 0.25O 3 and that the point group of BaPb 0.75Bi 0.25O 3 was either m or mm2. It was concluded that the point group of BaPb 0.75Bi 0.25O 3 was not mm2 but m because CBED patterns with symmetry of not only m but also 1 were taken at [1 1 1] incidence with pseudo-cubic perovskite notation. From CBED patterns taken at [1 1 1] azimuth, unique axis of monoclinic BaPb 0.75Bi 0.25O 3 with point group of m was also determined. The lattice type of BaPb 0.75Bi 0.25O 3 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 0.25O 3 is Im (No. 8).