ABSTRACT

At present, there is a great number of papers on the microwave dielectric dispersion of hydrogen-bonded ferroelectrics. The dielectric dispersion has been investigated mostly below 10 GHz or above 100 GHz. The experimental results below 10 GHz enable analysis of a critical slowing down process in the close vicinity of the Curie temperature, Tc. Submil-limeter or IR spectra give information about only a high frequency tail of fundamental dielectric dispersion. Neither a beginning nor a tail of the dielectric dispersion allows for an unambiguous distinction between various theoretical models. There are practically no complete dielectric spectra of the fundamental dielectric dispersion of H-bonded ferroelectrics in a wide temperature range and at frequencies of 106 to 1011 Hz. As a result, almost 60 years after the discovery of ferroelectricity in KDP-type crystals, the dynamical models of phase transition of KDP (order-disorder, tunneling, coupled proton-phonon or nonlinear anionic polarizability) are still under discussion (Matsubara 1985; Bussman-Holder and Bilz 1984). The phase transition, in accordance with recent Raman studies, suggests an order-disorder behavior of locally distorted PO4 dipoles without proton tunneling (Tominaga et al. 1985). On the other hand, the overdamped soft mode in the IR spectra (Simon et al. 1988) indicates a displacive mechanism.