Figure 1. Structural detail of Bi₅Ti₅O₁₇ phases.

Sketch of the Bi-5517 phases discussed in the text, showing relevant structural features. Large violet balls stand for Bi atoms, small red balls for oxygens, green polyhedra for the TiO₆ groups. Most relevant atomic distortions indicated with arrows. For each structure, black lines outline the supercell profile (a) Immm, (b) Pmnn, (c) Pm2₁n. Only one b-oriented slab is shown (there are two such slabs in the simulation cell). Bi types indicated in panel (a). b-oriented mirror plane, and inversion centres at the Bi_c positions, indicated in panels (a,b). In the Pmnn case, this mirror plane has an associated ½(a+b+c) glide translation; hence, the symmetry is not obvious from the figure. Bi_c off-centering displacements indicated in panel (c). The Bi_c displacement along c is compensated by with a symmetric one occurring in the second slab in the cell (not shown); the Bi_c displacement along b adds up with the symmetric one in the second slab, and thus a b-oriented polarization appears.