Indirect non-electrostatic mechanism of the proton-proton interaction and proton-lattice coupling in KH2PO4 ferroelectrics

It has been shown that the mechanism previously proposed for the effective interaction of protons in MH2 AO(4) ferroelectrics, such as KH2PO4 (KDP), is applicable to the description of the proton-lattice coupling. The expression derived for the total Hamiltonian of the coupled proton-lattice system, along with the single-mode approximation, also allows the consideration of several modes involved in the ferroelectric phase transition. The use of the proposed approach is illustrated by the example of the numerical evaluation of the effect of the proton-lattice coupling on the critical temperature T (c) of the ferroelectric phase transition in KDP and deuterated KDP.