Proton Tunneling Model for the Ferroelectric Phase Transition in CsH₂PO₄

Abstract

To understand the isotope effect in the ferroelectric phase transition of CsH₂PO₄ and CsD₂PO₄, a quasi-one-dimensional model is proposed. In contrast to the existing models which consider only either the proton’s degree of freedom or the polarization dipoles, both of them are regarded as essential elements of the system, while the tunneling protons are assumed to have nothing to do with dielectric polarization. The static dielectric constant, calculated on the assumption of no tunneling for CsD₂PO₄ and a finite tunneling frequency for CsH₂PO₄, reproduces well the experimental data by using model parameters consistent with the saturated polarization data. The model predicts absence of soft modes despite proton tunneling, in agreement with the order-disorder behavior of the phase transition as has been confirmed by experiments.