Stairlike Aurivillius Phases in the Pseudobinary Bi₅Nb₃O₁₅-ABi₂Nb₂O₉ (A = Ba and Sr) System: A Comprehensive Analysis Using Superspace Group Formalism

We report the possibility of extending the so-called stairlike Aurivilius phases in the pseudobinary Bi₅Nb₃O₁₅-ABi₂Nb₂O₉ (A = Ba and Sr) over a wide range of compositions. These phases are characterized by a discontinuous stacking of [Bi₂O₂] slabs and perovskite blocks, leading to long-period intergrowths stabilized as a single phase. When analyses from precession electron diffraction tomography and X-ray and neutron powder diffraction are combined, the monoclinic incommensurately modulated structure with q = αa* + γc* previously proposed for the ABi₇Nb₅O₂₄ composition could be generalized to the Bi₅Nb₃O₁₅-ABi₂Nb₂O₉ (A = Ba and Sr) compounds. Considering the compositions expressed as (A,Bi)_{1- x}Nb _xO_{3-3 x}, the stacking sequence associated with compositions ranging from x = ²/₅ to ³/₈ is governed by the component γ of the modulation vector and can be predicted following a Farey tree hierarchy independently to the A cation. The length of the steps, characteristic of the stairlike nature, is controlled by the α component and depends on the substitution ratio A/Bi and the nature of A (A = Ba and Sr). This study highlights the compositional flexibility of stairlike Aurivillius phases.