A novel approach to coupled proton-phonon macroscopic system in H-bonded solids with strong low-barrier hydrogen bonds on the examples of KH _2PO _4 and KD _2PO _4

摘要

To describe the proton-phonon coupling (PPC) in the materials at hand, the novel pseudospin Ising-type Hamiltonian is proposed. It is derived by the vibronic theory of heteroligand molecular systems treating the oxygen atoms O(-) and O(?) [linked with hydrogen atoms by covalent (O-H) or by hydrogen (O?H) bond] as substituting ligands in PO _4 tetrahedra for 3d KH _2PO _4-like species. Using the Hamiltonian obtained, we treat several vibrational modes participating in the structural phase transitions, the Ising parameters dependence on vibronic constants, and the PPC influence on the T _c value. ? 2012 Wiley Periodicals, Inc. Using the heteroligand systems vibronic theory approach for KDP and its isomorphs, the pseudospin Ising model is modified to furnish the Ising parameters determined in an explicit analytical form in terms of the electronic and vibronic characteristics for PO _4 units, enabling the use of quantum chemistry. The method is illustrated by the estimation of proton-phonon coupling influence on the T _c of KDP.