Internal friction and dielectric loss related to domain walls

The internal friction in potassium dihydrogen phosphate (KDP) crystal and the dielectric loss in KDP and triglycine sulfate (TGS) crystals are measured near Tc in the kHz frequency range. An internal-friction peak (PM2) and a dielectric loss peak (PD2) appear at about several degrees below Tc, which are found to be associated with the viscous movement of domain walls. With the temperature dependence of the density and the viscosity of domain walls, the order parameter, and the interaction among the domain walls taken into account, the internal friction and the dielectric loss due to the viscous movement of domain walls are calculated and compared with the experimental data.