I. V. Stasyuk

Hydrogen transport in superionic system Rb3H(SeO4)2: A revised cooperative migration mechanism

We performed density functional studies of electronic properties and mechanisms of hydrogen transport in Rb3H(SeO4)2 crystal which represents technologically promising class M3H(XO4)2 of proton conductors (M=Rb,Cs, NH4; X=S,Se). The electronic structure calculations show a decisive role of lattice dynamics in the process of proton migration. In the obtained revised mechanism of proton transport, the strong displacements of the vertex oxygens play a key role in the establishing the continuous hydrogen transport and in the achieving low activation energies of proton conduction which is in contrast to the standard two-stage Grotthuss mechanism of proton transport. Consequently, any realistic model description of proton transport should inevitably involve the interactions with the sublattice of the XO4 groups.

The effect of proton interactions on the conductivity behavior in systems with superionic phases

The proton conductivity in the hydrogen-bonded systems with strong correlations between protons is investigated theoretically in the context of the two-stage transport modeling approach. The distortions of the nearest ionic groups at the formation of the hydrogen bond are shown to be the reason for the strong proton–phonon coupling and polaronic effect. We evaluate as an example the conductivity for M3H(XO4)2-class of crystals with high proton mobility in (001) conducting planes. We analyze the temperature dependencies of the conductivity and its interbond and intrabond contributions in the vicinity of the transition from the disordered superionic to the ordered state. We investigate also the main peculiarities in the frequency dependencies of the conductivity appearing due to the proton interactions and compare our results with the previous conclusions obtained for the chains of noninteracting protons.

Proton ordering model of superionic phase transition in (NH4)3H(SeO4)2 crystal

Abstract The microscopic proton ordering model is developed for the description of the phase transition sequence from superionic phase II to ferroelastic phases III and IV in the (NH4)3H(SeO4)2 crystal. Symmetry analysis of proton orderings in phases III and IV is performed using structural data. The equilibrium states of the proton subsystem are studied and the corresponding phase diagram determining the necessary conditions for the existence of different proton orderings is constructed within the frame of the mean-field approximation taking into account long-range proton interactions. Investigations of thermodynamic functions as well as the temperature dependence of proton average occupation numbers are performed numerically. The resulting picture corresponds to experimental data for the network of hydrogen bonds in these crystal phases. The results are compared with the conclusions which follow from the phenomenological approach based on the Landau expansion.

Influence of inter-chain correlations on proton ordering in MeHXO4 protonic conductors

Abstract Energy spectrum and thermodynamics of the proton subsystem of the crystal with chains of hydrogen bonds are investigated. The consideration is based on the orientational-tunneling model, which includes two types of proton transfer. The strong short-range interactions and the influence of the internal ordering field arising due to the inter-chain correlations are taken into account. It is shown that under certain conditions the uniform proton distribution along the chain becomes unstable. A possible explanation of the precursor effect observed in the CsDSO 4 crystal at temperatures below the superionic transition point is proposed.

The EPR evidence of local hydrogen bond distortion evoked by ions in crystals

An investigation of the influence of the proton subsystem on the EPR spectra of ions in is performed. The EPR superhyperfine structure from protons and their temperature dependences manifest the existence of asymmetrical configurations of protons at low temperatures. The preserving of the double-minimum character of proton potential on bonds in the near vicinity of ions is realized. It is suggested to be caused by redistribution of electron charges.

Dynamics of charge transfer along hydrogen bond

The pseudospin-electron model is formulated for the description of the correlated proton-electron charge transfer in the complex with hydrogen bonds. The energy spectrum of the model is obtained. The ground state diagram is built. The frequency dependence of the real part of conductivity is calculated. The time evolution of the proton and electron transfer along the hydrogen bond is studied. The time dependences of the mean occupancies of proton positions and electron states are obtained by solving the equations of motion for the density matrix components. The conditions, at which the motion of the proton and electron charges are mutually correlated, are considered.

The effect of ionic group rotations on proton orderings in super-ionic crystals

The various proton orderings occurring in crystals are investigated in the framework of a lattice-gas-type model with account taken of the interactions of the proton subsystem with the orientations of the ionic groups. In particular, it is shown that the orientational ordering of ions is strongly involved in the I-II-III-IV phase transitions which take place in and appears to be the basic mechanism determining the observed phase sequence. The phase diagram obtained together with the microscopic parameter values calculated for agree with the experimental data for the phase orderings.

Theory of Rochelle salt: beyond the Mitsui model

Abstract A simple four-sublattice order-disorder model is proposed for description of phase transitions and dielectric properties of the Rochelle salt crystal. The model is developed as a generalization of the semimicroscopic Mitsui model. The symmetry properties of the lattice and spatial orientations of effective dipoles connected with the asymmetric structure units in the elementary cell are taken into account. The model makes it possible to investigate the temperature and field behaviour of transverse (besides longitudinal) components of dielectric susceptibility. The effect of the transverse electric field ‖ on the phase transition points and spontaneous polarization is studied.

Mott transition in the asymmetric Hubbard model at half-filling within dynamical mean-field theory

Abstract.The asymmetric Hubbard model with hopping integrals dependent on an electron spin (particle sort) is studied using an approximate analytic method within the dynamical mean-field theory. The equations of motion for Hubbard operators followed by projecting and different-time decoupling are used for solving the single-site problem. Particle spectra are investigated at half-filling within various approximations (Hubbard-I, alloy-analogy and a generalization of the Hubbard-III approximation). At half-filling these approximations can describe only continuous gap opening in the spectrum. The approach is used to describe the system between two limit cases (the Falicov-Kimball model and the standard Hubbard model) with continuous transition where Uc is dependent on the value of hopping parameters of different particles.

Dielectric anomalies and phase transition in glycinium phosphite crystal under the influence of a transverse electric field

The dielectric properties of glycinium phosphite (GPI) crystals as a function of temperature and electric field magnitude are investigated. The electric field E is applied perpendicularly to the ferroelectric b-axis in the direction of hydrogen-bonded phosphite chains in the crystal (the c-axis). The shift of the paraelectric–ferroelectric phase transition to lower temperatures proportionally to Ec2 (where Ec is an effective field in the sample) is observed. Strong anomalies in the field dependence of the permittivity in the temperature region are revealed. It is shown that the observed jump-like changes of are caused by the phase transition from the ferro- to paraelectric phase induced by the electric field. Such a transition is connected with the rearrangement of protons on hydrogen bonds and the reversal of the corresponding dipole moments, at which the compensation of their components along the b-axis takes place. The theoretical description of the observed dielectric anomalies, given on the basis of the phenomenological Landau free energy approach, is in good agreement with the experimental data.