Contrasting Structural Behavior in the Aurivillius Phase Ferroelectrics Bi₄Ti₃O₁₂, BaBi₄Ti₄O ₁₅ and Ba₂Bi₄Ti₄O₁₈

The crystal structures of the Aurivillius phase ferroelectrics Bi ₄Ti₃O₁₂ BaBi₄Ti₄O ₁₅ and Ba₂Bi₄Ti₅O₁₅, containing perovskite-like layers consisting of three, four and five adjacent TiO₆ octahedra, respectively, have been analysed using high-resolution powder neutron diffraction data. At 2K the structure of Bi ₄Ti₃O₁₂ can be successfully modelled in the orthorhombic space group B2cb, with no evidence of the monoclinic distortion in the ferroelectric phase suggested by previous single crystal studies. At 298K BaBi₄Ti₄O₁₅ shows a subtle orthorhombic distortion, but without the octahedral tilting seen in related ferroelectric phases, and is refined in space group F2mm. At 298 K. Ba₂Bi ₄Ti₅O₁₈ adopts a tetragonal polar phase (I4mm), thus displaying a third type of polar distortion within this family. It is suggested that these differences arise from a 'tolerance factor' type mechanism whereby the increasing content of the large Ba₂₊ cation causes a shift from underbonding at the perovskite A-site to underbonding at the perovskite B-site resulting in a change in the mechanism of ferroelectricity from A-site displacements to B-site displacements, akin to the mechanism in BaTiO₃.