Raman scattering study of bismuth layer-structure ferroelectrics

The lattice dynamical properties have been studied by Raman spectroscopy in bismuth layer-structure oxides Bi4Ti3O12, and ABi4Ti4O15 (A = Ba, Ca, Pb, Sr). The rigid-layer modes were observed in all compounds in the range 57-62.5 cm-1. The lowest mode of Bi4Ti3O12 shows marked softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi4Ti4O15 and SrBi4Ti4O15 the underdamped soft modes were also clearly observed. In contrast, in BaBi4Ti4O15 only low-frequency over-damped modes have been observed. In PbBi4Ti4O15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.