Proton Dynamics in KH2PO4 Type Ferroelectrics Studied by Infrared Absorption

Abstract

The infrared absorption spectrum of ferroelectric crystals analogous to KH2PO4 was measured at several temperatures. Substantial changes in the intensity and width of the ir absorption peaks, especially in the low-energy spectral region, were observed upon crossing the Curie temperature. Assignments of the ir absorption peaks for both phases were made on the basis of a systematic comparison of the spectra of compounds isomorphic to KH2PO4 and containing isotopes of D and 18O. Correlation charts were constructed for both phases, permitting harmonic interpretation of the ir selection rules. The structure of the ir absorption spectrum of KH2PO4 at temperatures above Tc, and comparison of this spectrum with that of KD2PO4. suggests a dynamic hydrogen symmetrizing process along the H bonds. It also indicates the existence of strong coupling between certain PO4 modes and the soft longitudinal hydrogen modes, caused by the extremely anharmonic nature of the potential well along the H bond. The coupling results in a broadening of the PO4 lines involved. The partial broadening that remains in KH2PO4 below Tc indicates that the soft hydrogen modes persist to a small extent even in the ferroelectric phase of this crystal. The ir data are explained on the basis of the fluctuating double-minimum-potential-well theory for the hydrogens.