Optical mode softening of ferroelectric SrBi2Ta2O9 and related bismuth layer-structured oxides

The lattice instability related to ferroelectric phase transitions of SrBi2Ta2O9 and its family has been studied by Raman scattering in the temperature range between 293 K and 958 K. The lowest optical mode of SrBi2Ta2O9 of 29 cm(-1) at room temperature shows remarkable temperature variation towards T-C = 608 K. Its mode frequency decreases markedly and the damping factor increases rapidly below T-C. The composition dependence of the low-frequency modes has been investigated in the family ABi(2)M(2)O(9) (A = Be, Ca, Sr, M = Nb, Ta) and Bi3TiMO9 (M = Nb, Ta). All the lowest modes soften upon heating, except BaBi2M2O9 (M = Nb, Ta). The nature of the damping of the soft modes is discussed in relation to the ionic radius of the A site of a pseudo-perovskite unit.