Dynamical Properties of Protons in KH₂PO₄. I. ---Proton Self-Trapped State and Its Incoherent Tunneling---

Abstract

We investigate a proton-phonon system which represents the hydrogen-bonded ferroelectric crystals, in particular KH₂PO₄(KDP). As a plausible model, `proton self-trapped state' model has been proposed. It is considered that, through the coupling to phonons, the proton on the hydrogen bond tends to be self-trapped at the shifted positions by breaking local symmetry of the surrounding atoms. The same coupling causes the quantum mechanical transition (phonon assisted tunneling) between the degenerated self-trapped states. At higher temperatures the transition is expected to become thermal activation type (incoherent tunneling). The results of neutron incoherent scattering study on KDP have been analyzed based on this model. The scattering function at T=150, K (T>T_c) shows satisfactory agreement with the observed spectrum by taking the relaxation time to cause incoherent tunneling to be 4× 10^-13, sec. The temperature dependence of the relaxation time is also investigated, which confirmed the validity of the thermal activation process with the activation energy of 37.1, meV.