Dynamical Theory of Ferroelectrics with Asymmetrical Hydrogen Bonds. Application to Rochelle Salt‐Type Crystals

A dynamical theory of ferroelectric phase transitions in crystals with asymmetric hydrogen bonds and several proton sublattices is developed. Owing to the presence of the proton‐phonon interaction and the tunnelling of protons, the phase transitions are of displacive type and are induced by the soft phonon‐proton mode. The temperature dependences of the spontaneous polarization and the dielectric constants of Rochelle salt (RS) and deuterated Rochelle salt (DRS) are calculated. The parameters of the theory are determined semi‐empirically. Good agreement with the experiment is obtained. The expected dependences of the frequencies of the soft phonon‐proton mode and pseudosoft proton‐phonon modes on temperature in RS are calculated.