Protonic conductivity at the superionic phase transitions in the M₃H(XO₄)₂crystal group

Abstract

A microscopic model for the description of the proton subsystem dynamics in superionic crystals with hydrogen bonds is developed. Besides the inclusion of the proton-transport mechanism, the effect of the displacement of the nearest oxygens during hydrogen-bond formation is taken into account. The latter effect is the cause of the strong proton-phonon coupling that leads to the polaronic effect. Using the occupation number formalism, the virtual (in superionic phases) or ordered (in low-temperature phases) character of the hydrogen-bonded system is taken into account on the basis of the proton-ordering model. Protonic conductivity studies are carried out in the framework of the Kubo theory for the cases of superionic phases as well as low-temperature phases with different types of proton ordering (as an example the M3H(XO4)2 class of crystals is considered). The temperature dependencies of the conductivity are analysed. The activation energies for the static conductivity are determined; for this case the influence of the internal field which appears as a result of the proton ordering is investigated.

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

“…In the theoretical works 16,[18][19][20][21][22] , the two-stage Grotthuss mechanism is frequently considered within phenomenological free-particle approches which typically contain two stages: (1) translation of proton within a hydrogen bond between the nearest ionic groups XO 4 ; (2) inter-bond proton transfer related to reorientations of the ionic groups XO 4 . Howerer, the experimental studies demonstrate that the transport of protons is a highly complex process which can be characterized as a transfer of hydrogen ion in the course of the reactions of creation and breaking of HXO 4 bonds.…”

“…As a consequence of such reacting environment with dynamically redistributing electronic density, the protons in conducting materials cannot be described as almost free particles. Despite the existing theoretical studies of the lowtemperature proton ordered phases and proton conduction in M 3 H(XO 4 ) 2 by phenomenological models [18][19][20][21][22] , up to date there is no first-principle-investigations of these systems. The recent first-principle molecular dynamics studies of superprotonic phase transition in a related superionic system CsHSO 4 23 were focused on the structural transformations and reorientational motion ofHSO − 4 groups, without the studies of the proton migration mechanism.…”

“…However, the polaronic concept introduced in Ref. 21 involves a number of phenomenological parameters like polaron binding energy which have been estimated by the fitting of the calculated conductivity to the experimetally measured values. To verify the role of the dynamical distortions in the proton migration mechanism, in this work we present a detailed DFT study of the mechanism of hydrogen migration in the system Rb 3 H(SeO 4 ) 2 , which belongs to the M 3 H(XO 4 ) 2 -class.…”

Strong Short-Range Interactions in One-Dimensional Proton Conductor

“…For the first time this model was used in [11] (see also [15]) to describe the molecular complexes with hydrogen bonds. Later it was used to describe proton transport and to calculate the conductivity coefficients in superionic crystals of Me 3 H(XO 4 ) 3 (Me = Cs, Rb, NH 4 ; X = S, Se) group [16,17]. This model introduces two types of proton transport according to Grotthuss mechanisms: tunnelling on the bond (Ω 0 ) and reorientational transfer (Ω R ).…”

“…For instance, in [25] the authors have shown that GRPA results are in good agreement with the results of dynamical mean-field approximation (which is exact in the limit of infinite dimension) in the case of strong one-site electron interactions. This makes it possible to use GRPA when taking into account the strong (in our case proton) interactions to calculate Green's function (16). This will be the aim of our future investigation.…”