Thermodynamics and dynamical properties of the KH2PO4 type ferroelectric compounds. A unified model

Within the framework of the proposed unified proton ordering model for the ferroelectric compounds of the KH2PO4 family, in the four-particle cluster approximation for the short-range interactions and mean field approximation for the long-range interactions, we calculate thermodynamic and longitudinal dynamic characteristics of the KD2PO4 type ferroelectrics and ND4D2PO4 type antiferroelectrics. Calculations for partially deuterated K(H1-xDx)2PO4 type ferroelectrics and N(H1-xDx)4(H1-xDx)2PO4 type antiferroelectrics are performed within the mean crystal approximation. It is shown that at the proper choice of the theory parameters, a good quantitative description of experimental data for the KH2PO4 family crystals is obtained.