Proton-Lattice Interactions in Hydrogen-Bonded Ferroelectric Crystals

Abstract

The distortions of the lattice accompanying protonic order-disorder transitions, and the difference between the observed frequency of protonic or deuteronic intrabond motion and the one estimated on the basis of the rigid lattice model, seem to indicate that proton-lattice interactions are not negligible in the case of ferroelectric H-bonded crystals. In order to throw some light on this problem, the energy spectrum and polarizability of an isolated hydrogen-bonded proton, interacting with polar lattice vibrations in the presence of an electric field, have been studied both in the strong- and in the weak-coupling limits. The results obtained show that the intrabond tunneling frequency of a proton, interacting with the lattice, is in fact much smaller than the tunneling frequency of a bare proton in an undeformed lattice. In the case of strong proton-lattice interactions, the polarizability is anomalously large at relatively low field strengths and any small fluctuation in the lattice potential is accompanied by a trapping of the proton in an off-center position. Another specific consequence of proton-lattice interactions is an inherent distribution of dipolar relaxation times.