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

The lattice instability related to ferroelectric phase transitions of and its family has been studied by Raman scattering in the temperature range between 293 K and 958 K. The lowest optical mode of of at room temperature shows remarkable temperature variation towards . Its mode frequency decreases markedly and the damping factor increases rapidly below . The composition dependence of the low-frequency modes has been investigated in the family (A = Ba, Ca, Sr, M = Nb, Ta) and (M = Nb, Ta). All the lowest modes soften upon heating, except (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.