THE CRYSTAL-CHEMISTRY UNDERLYING FERROELECTRICITY IN BI4TI3O12, BI3TINBO9, AND BI2WO6

The recent rerefinements of the crystal structures of Bi4Ti3O12, Bi3TiNbO9, and Bi2WO6 differ substantially from earlier reported structure refinements. A comparison of the major structural distortions present in each of these materials is presented and the main structural cause of ferroelectricity (in Bi4Ti3O12 and Bi3TiNbO9) is shown to be the a-axis displacement of Bi atoms in the perovskite A sites with respect to the chains of TiO6 octahedra and not the perovskite B atoms moving toward an octahedral edge. By using a modulated structure approach, the driving force for this displacement and other structural features can be simply understood in terms of the need to satisfy bond valence requirements. © 1991.