Controlling structure distortions in 3-layer ferroelectric Aurivillius oxides

Combined Rietveld refinements of x-ray and neutron powder diffraction data were used to understand the subtle structure distortions in 3-layer Aurivillius oxides that yield off-centering displacements in ferroelectric and multiferroic compositions. Ferroelectric phases including Bi2A2Ti3O12 (A=La, Pr, Nd, La/Pr, La/Nd, Pr/Nd), Bi2A2TiNb2O12 (A=Ca/Sr, Sr, and Sr/Ba) and Bi2A2TiTa2O12 (A=Ca/Sr, and Sr/Ba) were studied to separate the effects of cation size and charge on the structure distortions and properties. A new approach to describing the local coordination around the Ti, Nb, and/or Ta ions is presented, where the oxygen octahedra are characterized as containing kinks in three dimensions. The kink angles follow trends with the A-site ionic radius and the ferroelectric polarization. The driving force for extensive cation site mixing between the Bi and A-site cations has been clearly established, with site mixing required to maintain interlayer bonding. Distortion of the oxygen octahedra from planar geometries can be controlled via choice of the perovskite A-site cation, and the kink angle correlates with cation off-centering and ferroelectric polarization. [Display omitted] ► A-site cations define the tilt and distortion of the octahedral. ► Distortions of oxygen octahedra, ignoring the central cation, link to ferroelectric polarization. ► Bi ion occupancy in the perovskite causes distortion of the oxygen sublattice. ► We predict multiferroic behavior from off-centering caused by the Bi ion lone pair.