Molecular dynamics simulation of the soft mode for hydrogen-bonded ferroelectrics

Numerical simulations of dipole dynamics have been studied by employing a recently proposed modified strong dipole proton coupling (MSDPC) model of the phase transition in the hydrogen-bonded type of ferroelectrics. The obtained results in the form of the spectral density of the polarization fluctuation along the polar axis and along the lateral direction are simulated and correlated with known experimental data obtained primarily by Raman and infrared spectroscopy on KDP and DKDP lattices. A central peak (CP) appears in the calculated spectral density of the longitudinal polarization fluctuation in the ferroelectric phase of KDP and DKDP. The simulated CP indicates an order-disorder mechanism of excitation in the paraelectric phase near $T_c$ in both crystals. By increasing the temperature to 460 K, the crossover to the displacive behavior for KDP lattice is predicted. The model also predicts a low-frequency flat anomaly for KDP and a CP for DKDP in the spectral density of transversal polarization fluctuations, as was earlier detected by infrared and Raman spectroscopy. The superiority of the MSDPC model is additionally tested in the description of the experimentally obtained pressure-induced change of the CP line shape for KDP.