AbstractThe ferroelectric soft mode in the single crystals of the lead germanate system with silicon doping has been analysed by using the following three response functions χ(ω): (1) χ(ω)−1= ω 02− ω2− iγω, (2) χ(ω)−1= ω 02− ω2− ω2− iγ2and (3) χ(ω)−1= ω 02− ω2− iδ2ωτ(1 + ω2τ2)−1. The least squares fitting to the observed spectra showed that the response function of type (3) proposed by Yacobyet al.gave the satisfactory results to the soft mode theory with τ−1≈ 30 cm−1. The temperature dependence of the soft mode frequency and the mode strength has the Curie–Weiss form close to the Curie temperature. The increase of the silicon impurity lowers the soft mode frequency, while the effects on the line width are small. The soft mode frequency seems to be mainly influenced by the change of the short range harmonic force due to the silicon doping. The Curie temperature estimated from the soft mode frequency is con