Thermodynamics and dynamical properties of the

Within the framework of the proposed unied proton ordering model for the ferroelectric compounds of the KH2PO4 family, in the four-particle cluster approximation for the short-range interactions and mean eld ap-proximation for the long-range interactions, we calculate thermodynamic and longitudinal dynamic charac-teristics of the KD2PO4 type ferroelectrics and ND4D2PO4 type antiferroelectrics. Calculations for partially deuterated K(H1xDx)2PO4 type ferroelectrics and N(H1xDx)4(H1xDx)2PO4 type antiferroelectrics are performed within the mean crystal approximation. It is shown that at the proper choice of the theory parame-ters, a good quantitative description of experimental data for the KH2PO4 family crystals is obtained