Dynamical Theory of the Phase Transition in KH₂PO₄-Type Ferroelectric Crystals

Abstract

Dynamical aspects of the ferroelectric phase transition in KH 2 PO 4 -type crystals are elucidated on the basis of the coupling between the proton tunneling mode and the optical mode vibration of the [K-PO 4 ] complexes along the c-axis. Two coupled modes are obtained within the framework of a linear theory. The mode in which both systems oscillate in phase is shown to be responsible for the ferroelectric phase transition. The isotope effect in the Curie point is explained qualitatively and the Curie constant is estimated.