The soft mode dynamics and related properties of order-disorder type crystals have been studied. The phonon width and shift have been calculated, which lead to the renormalization of the relaxation soft-mode. The dielectric properties are directly related to the relaxational soft mode behavior of stochastic motion of H2PO4- groups in order-disorder (KDP-system) crystals. The analysis of the temperature dependence of microwave loss tangent and dielectric constant explain the experimental results.
Relaxational soft- mode, Stochastic motion, Microwave loss tangent