Stacking faults in protonated layered perovskite phases: DIFFaX simulation studies on H2SrTa2O7

H2A0.5nBnO3n+1 (A=Ca,Sr; B=Nb,Ta) layered perovskite phases have been prepared by ionic exchange from the Li2A0.5nBnO3n+1 parent compounds. Their dehydration, via a first step producing a metastable 3D perovskite phase without ordering of A cations and vacancies, leads finally to the well-known A0.5BO3 stable phases. In the special case of H2SrTa2O7, before the first step of its dehydration, two reversible transitions are observed at 116 and 280 °C, occurring between its three α, β and γ forms respectively. They are related to special shifts between adjacent perovskite layers, linked by hydrogen bonds. If the crystal structure at 300 °C of the γ form is well established (I4/mmm, Z=2, a=3.930Å, c=18.515Å) and very similar to that presented by its parent compound Li2SrTa2O7, the structures of the α and β forms reveal to be strongly disordered. If the double octahedra perovskite blocks are present in both cases, the X-ray diffraction patterns of the two compounds can be interpreted in terms of stacking faults occurring along a direction perpendicular to the layers. By using the DIFFaX code to simulate the X-Ray diffraction patterns, we propose for the α and β forms two different schemes of disordered shifts of adjacent layers, based on simple translations vectors along the [110] direction of the parent perovskite cell.