The pseudo-spin lattice coupled mode model by adding third-and fourth-order phonon anharmonic interactions and extraspin-lattice interaction term has been considered for KDP crystal. Expressions for shift and width ofresponse function, vibrational normal mode frequency dielectric constant and loss tangent have been evaluated. Double time temperature dependent Green function method has been used for derivation. Fitting the values of model parameters in expressions, the temperature dependence of soft mode frequency, dielectric constant and loss tangent have been calculated. Theoretical results are in agreement with experimental results reported by Kaminow and Harding[9] [Phy Rev, 129 (1963)1562].
Ferroelectrics, Green's function, Dielectric constant, Loss tangent, Phase Transition
