Lattice modes in PbHPO4 and H2C4O4

A microscopic model has been proposed which gives an explanation of the observed anomalies in the phonon spectra of hydrogen-bonded ferroelectrics. Using a Green's function technique me have obtained a nonlinear temperature dependence of the frequencies of the phonon modes in PbHPO4 and H2C2O4 in agreement with the experimental data. The importance of the proton ordering effects, i.e. the pseudo-spin-phonon interaction on the lattice modes is shown.