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.

Graphical abstractDistortion 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.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► 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.