Dynamical susceptibility in KH2PO4-type crystals above and below T-c

The time dependent cluster approximation called the path probability method (PPM) is applied to a pseudo-spin Ising Hamiltonian of the Slater-Takagi m odel for KH2PO4-type hydrogen-bonded ferroelectrics in order to calculate t he homogeneous dynamical susceptibility chi(omega) above and below the ferr oelectric transition temperature T,. Above the transition temperature all t he calculations are carried out analytically in the cactus approximation of the PPM. Below the transition temperature the dynamical susceptibility is also calculated accurately since the analytical solution of spontaneous pol arization in the ferroelectric phase can be utilized, When the temperature is approached from both sides of tile transition temperature, only one of r elaxation times shows a critical slowing down and makes a main contribution to the dynamical susceptibility. The discrepancy from Slater model (ice-ru le limit) is discussed in comparison with some experimental data.