Bond Geometry and Phase Transition Mechanism of H-Bonded Ferroelectrics

Taking into account the bilinear coupling between the tunneling protons and the displacements of the electron shells of the surrounding PO₄ groups, we present a new model of hydrogen-bonded ferroelectrics. The model is an extension of both the tunneling model and the nonlinear polarizability model and includes the geometrical aspects of the hydrogen bond. It leads to a structural phase transition and describes the isotope effect due to the substitution H/D in KH₂PO₄ and the pressure dependence.