Longitudinal relaxation of mechanically clamped KH₂PO₄ type crystals

Within the framework of a modified proton ordering model of the KH2PO4 family ferroelectric crystals, taking into account a linear over the strain ϵ6 contribution into the proton system energy, we obtain an expression for longitudinal dynamic dielectric permittivity of a mechanically clamped crystal using the four-particle cluster approximation and the dynamic Glauber approach. At a proper choice of the model parameters, we obtain a good quantitative description of available experimental data for these crystals