Toward a Unified Approach to the Crystal Chemistry of Aurivillius-Type Compounds: II. Bi7Ti4NbO21, a Case Study

In part I of this paper, a generalized structural model of Aurivillius-type compounds has been presented using a 4D superspace group analysis where Aurivillius structures are considered as cation-deficient perovskites with the general formula AB1-xO3. Being essentially composition independent, the model is valid for any Aurivillius-type compounds where x is the only composition-dependent parameter. For any composition, the conventional space groups can be easily derived from a unique superspace group. In this second part, a practical example of a structural refinement using the superspace approach is presented for the compound Bi7Ti4NbO21 and compared with a conventional 3D refinement. Similarities with other compositionally flexible systems are discussed.