The effect of the type of A-site cation on the structural transformations of ABi4Ti4O15 (A¼ Ba, Sr) was studied by a
comparative analysis of the Raman scattering of BaBi4Ti4O15 and SrBi4Ti4O15 in the temperature range 120–850
K. The results are also compared with those previously reported on PbBi4Ti4O15. Similar to PbBi4Ti4O15, BaBi4-
Ti4O15, and SrBi4Ti4O15 exhibit an additional structural transformation at Ta < Tc and Ta decreases with the
increase in the ionic radius of the A-site cation. This structural alteration consists of rearrangements of the A-site cations, which for A¼ Ba is accompanied by readjustment of the octahedral tilting and a change in the BO6 geometry. The results presented here confirm that the development of spontaneous polarization in four-layer
Aurivillius-type ferroelectrics is triggered by the rigid-layer phonon mode, i.e., vibrations of the Bi2O2 planes relative to the perovskite-like blocks, rather than by cationic vibrations only within the perovskites blocks
