Pressure and deuteration effects on the static ferroelectric properties of KH2PO4(KDP) in the four-particle cluster approximation

Using experimental results on the effects of deuteration and applied pressure on the static ferroelectric properties of KDP, we construct a model for the strain dependence of the microscopic parameters which occur in the four-particle cluster approximation of Blinc and Svetina. The free energy calculated numerically from the theory is analyzed in terms of the Landau expansion in the polarization, which facilitates comparison with experiment, and a formal connection is made between the phenomenological and microscopic theories of pressure dependence. We find that the microscopic model is in quantitative agreement with experiment for the case of negative long-range interaction parameter $\gamma$, corresponding to antipolar long-range coupling.