Investigation of the dynamic properties of ferroelectric crystals close to phase transitions

Kiracı, Ali
In this thesis we investigated the dynamical properties of some ferroelectric crystals and ceramics under various temperature and pressure conditions close to the phase transitions. In particular, we focused on the Raman frequencies, damping constant and the activation energy of soft modes with the pseudospin-phonon coupling in some ferroelectric crystals exhibiting phase transitions. We predicted the critical behavior of the frequency related to the order parameter (spontaneous polarization) in the ferroelectric and/or paraelectric phases of potassium dihydrogen phosphate (KDP), barium titanate (BaTiO3), lead titanate (PbTiO3), lead zirconate titanate (PZT), strontium zirconate (SrZrO3), cadmium niobate (Cd2Nb2O7) and lithium niobate (LiNbO3). For these predictions, our calculations have been performed by using the mean field theory (MFT). We analyzed the damping constant as a function of temperature and/or pressure for the ferroelectrics studied in this thesis by using the pseudospin-phonon coupled model and the energy fluctuation model. We also calculated the temperature dependence of the relaxation time of BaTiO3, PZT and LiNbO3 using our results of the order parameter and the damping constant described from both models studied here. We also used the Landau phenomenological theory to describe the observed behavior of the dielectric constant (or susceptibility) in the ferroelectric and paraelectric phases of BaTiO3 and in the ferroelectric phase of LiNbO3. As a part of this thesis, we calculated the temperature dependence of the frequency of Raman modes for NaNO2 through the Grüneisen parameter. By analyzing the experimental data from the literature, the observed behavior of those ferroelectric crystals is described on the basis of the models studied here.