Structural evolution in three and four-layer Aurivillius solid solutions: A comparative study versus relaxor properties

Two solid solutions of three-layer BaxBi4−xNbxTi3−xO12 (0≤x≤1.2) and four-layer Aurivillius compounds (Na0.5Bi0.5)1−xBaxBi4Ti4O15 (0≤x≤1), which both present a ferroelectric to relaxor-like transition with increasing x, were synthesized by solid state reaction. The evolution of their crystal structures, as a function of x, was performed using Rietveld refinements from X-ray powder diffraction data. As x increases, the average crystal structures become less distorted with respect to the archetypal high temperature tetragonal one and the coordination number of Bi3+ in M2O2 layers continuously changes from {4+2} to {4}. The relaxor behaviour which appears in samples for a tolerance factor t>0.96 is associated with a general static disorder in A and M sites together with the presence of some Ba2+ cations in M2O2 layers (less than 10%).