Vibronic Theory Approach To Ising Model Formalism And Pseudospin-Phonon Coupling In H-Bonded Materials

The investigation of the lowest adiabatic potential energy surface (APES) of the AO(4) structural unit in KDP (KH2PO4)-family materials as a function of its proton surrounding is reported. This approach results in the Ising form of the crystal pseudospin Hamiltonian. Besides the electronic structure characteristics of the AO(4) tetrahedra, the Ising model interaction parameters depend also on the vibronic constants. Numerical estimations of scale for vibronic contributions into Ising parameters (about a quarter of the total value of the maximal Ising parameter V) was obtained by using diffraction data for KH2PO4/KD2PO4 crystal structure change under ferroelectric transition together with the results of quantum-chemical calculation of PO43- force constants. The estimates show the really tangible role of the vibronic terms in the Ising parameters formation, though the physical reasons of these terms remained somewhat unclear. The situation is clarified by revealing the relationship between the above-mentioned approach and the theory of pseudospinphonon (proton-lattice) coupling which leads to the vibronic contributions to the Ising parameters.