32-35A simple pseudospin lattice coupledmode model with addition of third and fourth-order phonon anharmonic interactions terms and direct spin-spin interactions terms and external electric field term has been considered for investigation of transition phenomena and dielectric properties of classic ferroelectric Potassium Dihydrogen Phosphate (KDP) crystal. A double-time thermal Green’s function method has been used for derivation of response function. From response function shift, width and soft mode frequency have been derived for KDP crystal. Response function is also related to dielectric constant which has been obtained. Unlike previous authors, some different simple approximations have been used to obtained results quite different to them. By fitting model values of different parameters in the expressions, temperature variations of normal mode frequency, dielectric constant and loss tangent have been calculated numerically for KDP crystal. Our theoretical results are compared with experimental results. It is observed that our theoretical results agree with the experimental results of Kaminow& Harding2.
Therefore, it can be concluded that the simple model with still simplest approximation is quite suitable to explain transition and dielectric properties of classic KDP crystal. Present expressions can be used for similar other crystals also
