Raman Scattering Study of Bismuth Layer-Structure Ferroelectrics

The lattice dynamical properties have been studied by Raman spectroscopy in bismuth layer-structure oxides Bi 4 Ti 3 O 12 , and ABi 4 Ti 4 O 15 (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 Bi 4 Ti 3 O 12 shows marked softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi 4 Ti 4 O 15 and SrBi 4 Ti 4 O 15 the underdamped soft modes were also clearly observed. In contrast, in BaBi 4 Ti 4 O 15 only low-frequency overdamped modes have been observed. In PbBi 4 Ti 4 O 15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.