Anharmonic and Quantum Effects in KDP-Type Ferroelectrics: Modified Strong Dipole–Proton Coupling Model

Abstract

Anharmonic and quantum effects caused by hydrogen bonds appear strongly in ferroelectric crystalsof the KH₂PO₄- or KDP-type. Both effects are closely relatedto the long-standing and still unresolved questions about the ferroelectric phase transition in these crystals:that of the type of phase transition, and that of the origin of strong isotope effects of proton-deuteronreplacement. In this chapter, the most relevant experimental studies devoted to these problems are reviewed.The proton-tunneling model, as widely accepted model of the phase transition in these crystals, was questionedin the 1980s. A development of the alternative model, modified strong dipole–proton coupling(MSDPC) model, is presented. The model assumes that quantum protons in KDP have higher characteristic frequenciesthan the heavy-ion sublattice represented by the system of three-dimensional dipoles. Therefore, protonsbeing in a ground state adiabatically follow dipole dynamics, as opposed to the proton-tunneling model.Proton tunneling is absent in the model, and ferroelectric ordering and anharmonicities in the dipole systemare caused by the proton ground-state energy. Different ground-state energies of protons and deuteronscause isotope effects and make deuterated systems more anharmonic, so that the phase transition in deuteratedcrystal is predicted as more of an order–disorder type than in non-deuterated crystal. Successesand shortcomings of the model in explaining experimental results are commented upon.