Structural transformations in Pb 1 − x Bi 4 + x Ti 4 − x Mn x O 15 ( x = 0.2 and 0.4): a Raman scattering study

The temperature evolution of the Raman scattering of Pb 1 −  x Bi 4 +  x Ti 4 −  x Mn x O 15 ( x  = 0.2 and 0.4) on cooling from 850 to 140–130 K was studied in order to elucidate the effect of B ‐site Mn 3+ doping accompanied by the increase in the A ‐site Bi 3+ content on the structural transformations in four‐layer Aurivillius‐type compound ( A n  − 1 Bi 2 B n O 3 n  + 3 , n  = 4). The ferroelectric–paraelectric transition ( T c ~800 K for x  = 0.2 and ~765 K for x  = 0.4) is well mirrored by the Raman scattering near 60 cm −1 arising from the so‐called rigid layer mode. The temperature dependence of the phonon mode near 42 cm −1 arising from A ‐site Pb/Bi displacements indicates a second structural transformation near 570 K and 400 K for x  = 0.2 and 0.4, respectively, similar to that observed for Mn‐free PbBi 4 Ti 4 O 15 . This structural alteration resembles a thermodynamically second‐order phase transition for all three compounds and the critical temperature ( T a ) decreases with the increase in the A ‐site Bi 3+ amount, related to the heterovalent substitution (Mn 3+ for Ti 4+ ) on the B site; T a  = 600, 570, 400 K for x  = 0, 0.2, 0.4, respectively. The B O 6 tilting and bending mode near 220 cm −1 also shows an anomaly at T a , and, thus, this second structural transformation was attributed to subtle rearrangements of A ‐site cations accompanied by octahedral B O 6 tilting in the perovskite‐like blocks. Copyright © 2016 John Wiley & Sons, Ltd.