Optical mode softening of ferroelectric SrBi/sub 2/(Nb/sub x/Ta/sub 1-x/)/sub 2/O/sub 9/ system

The vibrational properties of the ferroelectric phase transitions of the SrBi/sub 2/(Nb/sub x/Ta/sub 1-x/)/sub 2/O/sub 9/ ceramic system (x=0, 1/3, 2/3, 1) have been studied by Raman spectroscopy in the temperature range from room temperature up to 500/spl deg/C. For all compositions the lowest optical modes indicate the remarkable temperature dependence towards T/sub c/ and disappear above T/sub c/. Their damping factors increase rapidly towards T/sub c/, while the softening of mode frequency is relatively small. The composition dependence of the rigid-sublattice mode amid the symmetric stretching mode of oxygen octahedra has been discussed.