The theory of first order ferroelectric phase transition in a KDP type crystal

The first order ferroelectric phase transition (FT) of a KDP type crystal is studied by the method of Green's functions based upon the pseudo-spin model and the model of coupling of the proton and the anharmonic phonon. Equations determining the spontaneous polarization ps, tunnelling effect x, and quantized axis are derived. From these equations, the transition temperature Tc, the ferroelectric criterion, the temperature dependence of the spontaneous polarization, the temperature dependence of the susceptibility of the paraelectric phase, the soft model frequency and its temperature dependence, the condition of FT, the jumping value of spontaneous polarization at the transition temperature ps (Tc), the absolute saturation value of spontaneous polarization, and the Curie's constant C etc are given.