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 Bi(2)A(2)Ti(3)O(12) (A=La, Pr, Nd, La/Pr, La/Nd, Pr/Nd), Bi(2)A(2)TiNb(2)O(12) (A=Ca/Sr, Sr. and Sr/Ba) and Bi(2)A(2)TiTa(2)O(12) (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. (C) 2012 Elsevier Inc. All rights reserved.