S. P. Dolin

Quantum-chemical modeling of ferroelectric solids with hydrogen-bond networks of different dimensionalities and their deuterated analogs

The application of quantum chemistry methods in the microscopic theory of H/D-bonded ferroelectrics and antiferroelectrics is illustrated using a comparative analysis of low-temperature order-disorder structural phase transitions in zero-dimensional materials of the K3H(SO4)2 family, zero-dimensional crystals of 5-halo derivatives of 9-hydroxyphenalenone (5Hal-9HPO), and three-dimensional crystals of potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) as examples. In the framework of the Ising model with tunneling, it is demonstrated that (in agreement with experimental data) the transition to the low-temperature ordered phase in zero-dimensional materials is possible only in the case of their deuteration, whereas the quantum paraelectric behavior is characteristic of undeuterated samples. This behavior for KDP crystals is impossible due to the sharp increase in the Ising parameters as compared to zero-dimensional materials. The factors responsible for the increase in these parameters are considered.

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 MH2AO4 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 Tc of the ferroelectric phase transition in KDP and deuterated KDP.

Contribution of vibronic interactions to the thermodynamics of the phase transition in H-bonded ferroelectrics: Proton-lattice coupling effects in crystals of the KDP family

A model Hamiltonian for microscopic description of structural phase transitions in KDP type ferroelectrics was considered in terms of the vibronic theory of heteroligand systems (VTHS). The Hamiltonian contains three terms that describe the proton interactions, the potential energy of lattice oscillators, and the relationship between these subsystems. In the second order perturbation theory, the configuration energies of the Bethe cluster method retained the Ising form, with the pseudospin Hamiltonian parameters explicitly depending on the electronic structure characteristics and the vibronic constants of the AO4-containing structural units. The Ising version of the theory was used for obtaining the numerical estimations of the thermodynamic properties, in particular, the critical temperature Tc as a function of all the parameters of the pseudospin Hamiltonian.

Quantum chemistry of hydrogen-bonded materials. Ferroelectrics and antiferroelectrics

Some aspects of application of quantum chemical approaches and methods in the microscopic theory of hydrogen-bonded ferroelectrics and antiferroelectrics, as well as their deuterated analogs, were reviewed in the context of original works of the authors. Calculations of the spontaneous polarization of KH2PO4 (KDP) and charge transfer “channels” in transition of KDP and other H-bonded crystals from paraphase to an ordered phase were discussed, as well as the factors responsible for difference in the ferroactive behavior between KDP and NH4H2PO4 (ADP). In terms of the dynamic Ising model and in the mean field approximation, the potentialities of quantum chemical approaches were analyzed as applied to examination and prediction of the results of order-disorder structural phase transitions in M3H(AO4)2 family materials, 5X derivatives of 9-hydroxyphenalenone, and crystalline chromous acid α-HCrO2 in relation to deuteration of the H-bonds in these crystals.