Longitudinal relaxation of mechanically clamped KH_{2}PO_{4} type   crystals
  Within the framework of a modified proton ordering model of the KH_{2}PO_{4} family ferroelectric crystals, taking into account a linear over the strain \varepsilon_{6} 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. 
R. R. Levitskii, I. R. Zachek, A. S. Vdovych
cond-mat.mtrl-sci