RAMAN-SCATTERING STUDY OF BISMUTH LAYER-STRUCTURE FERROELECTRICS

The lattice dynamical properties have been studied by Raman spectrosco py in bismuth layer-structure oxides Bi4Ti3O12, and ABi(4)Ti(4)O(15) ( A=Ba, Ca, Pb, Sr), The rigid-layer modes were observed in all compound s in the range 57-62.5 cm(-1). The lowest mode of Bi4Ti3O12 shows mark ed softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi4Ti4 O15 and SrBi4Ti4O15 the underdamped soft modes were also clearly obser ved. In contrast, in BaBi4Ti4O15 only low-frequency overdamped modes h ave been observed. In PbBi4Ti4O15 the low-frequency spectra have a com plicated structure and the softening of the underdamped modes was not observed clearly.