Proton Dynamics in Hydrogen-Bonded Ferroelectrics

A comparison between infrared absorption and neutron inelastic-scattering measurements indicates the existence of a broad low-energy (around 400 cm(-1)) protonic level in KH(2)PO(4) above the Curie point. This level is only weakly discerned in the ir but is seen quite clearly in neutron scattering. The mode is explained as resulting from splitting of the ground level of the proton in a slightly asymmetric double minimum potential well where tunneling of the proton takes place. The asymmetry is caused by the interaction between the different protons, and it changes slowly with time as the result of the collective motions of the protons. Prominent changes in ir spectra in the 400-cm(-1) range were found on cooling through the Curie point, indicating the disappearance of the tunneling mode. This picture allows a simplified treatment of the system, leading to the existence of two correlated phase transition points, and a negative thermal expansion coefficient of the II bond at temperatures between these points. The two transition points are identified as the Curie point and the melting or dissociation point of the crystal. The theory also suggests an increase in the dielectric constant as the melting point is approached, corresponding to the Curie-Weiss behavior as the Curie point is approached.