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Abstract
The lattice dynamics of ferroelectric KDP-type crystals are treated in a model with coupled double-well potentials. It is assumed that either a local non-linear potential at the protonic lattice site causes the phase transition or the non-linear polarizability of the anionic cluster. It is shown that similar to other ferroelectrics again the anionic polarizability drives the phase transition1. The isotope effect on T and the increase in the Curie constant upon deuteration are quantitatively calculated.