Synthesis and cation distribution in the new bismuth oxyhalides with the Sillén-Aurivillius intergrowth structures

About 20 new compounds with the Sillén-Aurivillius intergrowth structure, Me(1)Me(2)Bi3Nb2O11X (Me(1) = Pb, Sr, Ba; Me(2) = Ca, Sr, Ba; X = Cl, Br, I), have been prepared. They are composed of stacking of [ANb2O7] perovskite blocks, fluorite-type [M2O2] blocks and halogen sheets. The cation distribution between the fluorite and perovskite layers has been studied for Ba2Bi3Nb2O11I, Ca1.25Sr0.75Bi3Nb2O11Cl, BaCaBi3Nb2O11Br and Sr2Bi3Nb2O11Cl. The smaller Me cations tend to reside in the perovskite block while the larger ones are situated in the fluorite-type block. The distribution of the elements was confirmed for BaCaBi3Nb2O11Br using energy dispersive X-ray analysis combined with scanning transmission electron microscopy (STEM-EDX). An electron diffraction study of this compound reveals a local symmetry lowering caused by weakly correlated rotation of NbO6 octahedra. Based on our findings, we suggest a new stability criterion for mixed-layer structures, which is that net charges of any two consecutive layers do not compensate for each other and only the whole layer sequence is electroneutral.