Contrasting Structural Behavior in the Aurivillius Phase Ferroelectrics Bi_4Ti_3O_12,BaBi_4Ti_4O_15 and Ba_2Bi_4Ti_5O_18

摘要

The crystal structures of the Aurivillius phase ferroelectrics Bi_4Ti_3O_12,BaBi_4Ti_4O_15 and Ba_2Bi_4Ti_5O_18,containing perovskite-like layers consisting of three,four and five adjacent TiO_6 octahedra,respectively,have been analysed using high-resolution powder neutron diffraction data.At 2K the structure of Bi_4Ti_3O_12 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 BaBi_4Ti_4O_15 shows a subtle orthorhombic distortion,but without the octahedral tilting seen in related ferroelectric phases,and is refined in space group F2mm.At 298K Ba_2Bi_4Ti_5O_18 adopts a tetragonal polar phase (14mm),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 incresing content of the large Ba~(2+) cation causes a shift from underbonding at the perovskite A-site to underbonding at the perovskite B-site displacements,akin to the mechanism in BaTiO_3.