The program package VEC (Visual computing in Electron Crystallography) has been revised such that (i) a program converting one-line symbols to two-line symbols of (3+1)-dimensional superspace groups has been incorporated into VEC so that the latter can interpret both kinds of symbols; (ii) a bug in calculating structure factors of onedimensionally incommensurate modulated crystals has been fixed. The correction has been verified by successfully matching the experimental electron microscopy image of an incommensurate crystal with a series of simulated images. The precompiled revised version of VEC and relevant materials are available on the Web at http://cryst.iphy.ac.cn.
The crystal structure of the minor phase, named superstructure II, existing in multiferroic compound BiMnO3 has been studied by electron diffraction and high-resolution transmission electron microscopy. Domains of major and minor phases coexisting in BiMnO3 were observed in high-resolution electron microscope images. The unit cell of minor phase was determined to be triclinic with the size 4×4×4 times as large as the distorted perovskite subcell. The [111] and [101] projected structure maps of the minor phase have been derived from the corresponding images by means of the image processing. A possible rough three-dimensional (3D) structure model was proposed based on the 3D structural information extracted from the two projected structure maps. Since there is no inversion centre in the proposed model, the minor phase may contribute to the ferroelectric property of BiMnO3 .
