The effect of the A-Site cation on the structural transformations in ABi4Ti4O15 (A= Ba, Sr): Raman scattering studies
The effect of the type of A-site cation on the structural transformations of ABi4Ti4O15 (A= 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, BaBi4Ti4O15, 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 A= 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.