[en] 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 ABi₂M₂O₉ (A=Ba, 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. (author). Letter-to-the-editor