A. Pawłowski

Structural aspects of fast proton transport in (NH4)3H(SeO4)2 single crystals

Though it is generally accepted that fast proton transport in superionic hydrogen sulphate and selenate crystals proceeds via Grotthuss mechanism and conditions of intra- and intermolecular proton transport were discussed considerably, there was no structural data support available. We report here results of X-ray studies of (NH 4 ) 3 H(SeO 4 ) 2 single crystals in superionic phases II and I. Mean thermal amplitudes of the O(2) oxygens from neighbouring SeO 4 in the (001) plane were calculated on the basis of riding model analysis of experimentally obtained anisotropic temperature factors. It is shown that thermal excitations of SeO 4 tetrahedra in the superionic phases result in a variety of O(2)-O(2) separation length conditioning both the near barrierless proton transfer and breaking of hydrogen bonds in the (001) plane.

Electric conductivity and capacity studies of Rb3H(SeO4)2 single crystal near the high temperature phase transition

Abstract Impedance spectroscopy method was used for estimation of electric conductivity of Rb3H(SeO4)2. The crystal shows the phase transition from the low conducting to the high conducting phase accompanied by conductivity and activation energy changes as well as by abrupt capacity changes at low frequencies. The high temperature phase seems to be the superionic one.

Ferroelastic domain structure and thermal behavior of some Me3H(XO4)2 crystals in the vicinity of superprotonic phase transitions

Abstract Thermal evolution of ferroelastic domain structure and differential scanning calorimetric (DSC) investigations of Rb 3 H(SeO 4 ) 2 (TRHSe), Cs 3 H(SeO 4 ) 2 (TCHSe) and (NH 4 ) 3 H(SO 4 ) 2 (TAHS) in the vicinity of the superprotonic phase transition are presented. It was established that new domain structure appears in Rb 3 H(SeO 4 ) 2 crystals at ( T s −5.8) K, and another fine domain arises at ≈( T s −1.8) K. In Cs 3 H(SeO 4 ) 2 and (NH 4 ) 3 H(SO 4 ) 2 crystals, a very dense domain pattern appears at 1.3 and 0.6 K, respectively, below the superprotonic phase transitions. DSC measurements show anomalies at similar temperatures at which changes in the ferroelastic domain pattern were observed. The anomalous behavior of domain structure indicates that the transition to superprotonic phase is more complicated than that described in the literature.

Ferroelastic domains and ferroelastic phase transition of (NH4)4LiH3(XO4)4, X: S, Se

Abstract Dielectric and optical investigations of (NH4)4LiH3(SO4)4 and (NH4)4LiH3(SeO4)4 single crystals showed that these crystals are new ferroelastic below 233 K and 267 K respectively. X-ray studies demonstrated that above mentioned crystals are isomorphous with other representatives of Me4LiH3(XO4)4 family and undergo transition from paraelastic tetragonal (P41 or P43) to ferroelastic monoclinic (P21) phase.

Superionic phase transition in Rb4LiH3(SO4)4, Rb4LiH3(SeO4)4 and K4LiH3(SO4)4 single crystals

Abstract DTA studies of a new family of ferroelastic crystals of Me4LiH3(XO4)4 stoichiometry revealed at Ts another phase transition in the paraelastic phase, accompanied by an anomalous increase in the capacitance. Complex impedance method was used to study temperature variation of electric conductivity and its anisotropy of Rb4LiH3(SO4)4, Rb4LiH3(SeO4)4 and K4LiH3(SO4)4 single crystals. The bulk conductivity of the crystals varies from 10−6 to 10−4 (ωm)−1 in the temperature range from ∼ 400 K to temperatures below melting. The conductivity σc is ∼ one order of magnitude lower than σa ∽ σb. At Ts the activation energy Wa ∽ Wb decreases from 0.94 – 1.35 eV below Ts to 0.27 –0.28 eV at T > Ts. Comparison of the data with the crystallographic structure allowed us to classify the transition as superionic.

Pretransitional effects at the superionic phase transition of Rb3H(SeO4)2 protonic conductor

Abstract Rb 3 H(SeO 4 ) 2 single crystal undergoes a transition from low-temperature ferroelastic phase to superionic, paraelastic one at T S ≈456 K. The low-temperature monoclinic structure contains a zero-dimensional hydrogen bond network. Above T S , the reorienting HSeO 4 groups form a dynamic hydrogen bond network in the (001) plane. The presented results of proton conductivity, ferroelastic domain pattern and differential scanning calorimetry (DSC) studies in the vicinity of T S show that the transition region is stretched into ∼10 K and the transition process proceeds in steps: the changes in the ferroelastic domain pattern close to T S are accompanied by DSC anomalies. The temperature dependence of the conductivity σ [100] on heating and cooling revealed ∼2 K hysteresis of the superionic phase transition. The experimental results support the suggestion of recent papers on theoretical studies of the behaviour of protonic conductivity in M 3 H(XO 4 ) 2 crystal group in the superionic phase transition region.

Specific heat at the superionic phase transition of cs3h(seo4)2

Abstract Electric conductivity and specific heat measurements were performed in the superionic phase transition region for Cs3H(SeO4)2. Activation energy of the conductivity was found to follow the ordering enthalpy of protons. The experimental value of entropy change at Ts, ▭ SII → I = 9.4 J/mol K, is in a good agreement with the value of configurational entropy change ▭ S = 9.13 J/mol K resulting from proton disordering.

Structural relaxation in superprotonic tetraammonium dihydrogen triselenate single crystals

Abstract (NH 4 ) 4 H 2 (SeO 4 ) 3 single crystals undergo a transition to superionic phase at T s =378 K and the phase is characterized by high structural disorder. When the crystal is cooled from highly disordered superprotonic phase to T ≪ T s the order, characteristic of low-temperature phase (trimers formed by three SeO 4 tetrahedra linked with two short hydrogen bonds, lying along [021] direction), has to be recovered. The recovery process, termed by us as structural relaxation, was studied by means of impedance and Raman spectroscopy. Results of conductivity recovery measurements allow to distinguish three rates of conductivity decrease. Time variation of SeO 4 skeletal modes after cooling the crystal from the superprotonic state to temperatures T ≪ T s points to the following processes of the recovery: i) relaxation to the low-temperature molecular structure (formation of trimers is proceeded by formation and disappearance of dimers as a transitional form) and ii) relaxation to the low-temperature crystallographic structure (trimers lying along [021] direction). We observe the process of dimer formation and disappearance within ∼30 min at 303 K, whereas the formation of trimers lasts ∼1 h at the same temperature.

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

Abstract DSC and complex impedance studies of the protonic conductor (NH4)4H2(SeO4)3, which undergoes a superionic phase transition of first order at Ts = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔCp of the crystal are proportional: d(lg σ)/dT = XΔCp/RTs + const, as found for MAg4I5 crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at Ts, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH4)4H2(SeO4)3 crystal above Ts.

Raman studies of superionic phase transition in (NH4)4H2(SeO4)3

Abstract NIR FT Raman spectroscopy was used to investigate molecular groups dynamics in the vicinity of superionic phase transition in (NH4)4H2(SeO4)3. It was found that the low temperature phase transition becomes unstable about 15 K below the superionic phase transition Ts. The observed changes refer mainly to the dynamics of SeO4 and HSeO4 entities. The Raman spectra of PEO+ +(NH4)4H2(SeO4)3 polymer are presented as well.