Designing new n=2 Sillen-Aurivillius phases by lattice-matched substitutions in the halide and [Bi₂O₂]²⁺ layer

The chemical and structural flexibility of the perovskite structure, which makes is so ubiquitous in nature and useful in a range of technological applications, extends to layered variants such as Ruddleston-Popper, Dion-Jacobsen and Aurivillius phases. Multi-layered variants such as the Sillen-Aurivillius phases are related to Aurivillius phases by the insertion of an additional halide layer between every second [Bi₂0₂]²⁺ layer. Sillen-Aurivillius phases exist in various AnXm combinations, where n is the number of perovskite layers A and m the number of halide layers X. We have synthesised a new n=2 Sillen-Aurivillius compound Bi₃Sr₂Nb₂O₁₁Br based on Bi₃Pb₂Nb₂O₁₁Cl by simultaneously replacing Pb²⁺ with Sr²⁺ and Cl^- with Br^-. Rietveld refinements against X-ray and neutron powder diffraction data revealed a significant relative compression in the stacking axis (c axis) in contrary to the belief of inserting a significantly larger halide layer in the new compound. We could not stabilise other combinations such as Bi₃Sr₂Nb₂O₁₁Cl and Bi₃Pb₂Nb₂O₁₁Br due to inter-layer mismatch. Sr²⁺ doping reduces the impact of the stereochemically active 6s² lone pair found on Pb²⁺ and Bi³⁺, resulting in a contraction of the c axis by 1.22 % and an expansion of the a-b plane by 0.25 %, improving inter-layer compatibility with Br^-. XANES analysis shows that the ferroelectric distortion of the B-site cation is less apparent in Bi₃Sr₂Nb₂O₁₁Br compared to Bi₃Pb₂Nb₂O₁₁Cl. Variable-temperature neutron diffraction data show no evidence for a ferroelectric distortion.