A Two Doublet Model for the Phase Transition in KDP

Abstract

By assuming that each of the ground and lowest excited levels of the proton (deuteron) on the hydrogen bond in KDP composes a doublet, the pseudospin hamiltonian for the proton (deuteron) system with tunneling motion, which is assumed to occur in the doublet of excited level, is presented to investigate the ferroelectric phase transition in KDP. On the basis of this hamiltonian, the first and second order transitions, which depend on the pressure, and therefore the existence of a tricritical point are explained on the basis of a mean field approximation, including the isotope effect in the transition. These are discussed in terms of the energy separation between the two doublets, the tunneling frequency at the excited doublet and the parameters of coupling between protons (deuterons) on different bonds.