Proton Transport Mechanism and Pathways in the Superprotonic Phase of M3H(AO4)2 Solid Acids from Ab Initio Molecular Dynamics Simulations

The proton transport mechanism in superprotonic phases of solid acids is a subject of experimental and theoretical studies for a number of years. Despite this, details of the mechanism still...

New superprotonic crystals with dynamically disordered hydrogen bonds: cation replacements as the alternative to temperature increase

Investigations of new single crystals grown in the K3H(SO4)2–(NH4)3H(SO4)2–H2O system from solutions with different K:NH4 concentration ratios have been carried out. Based on the X-ray diffraction data, the atomic structure of the crystals was determined at room temperature taking H atoms into account. It has been determined that [K0.43(NH4)0.57]3H(SO4)2 crystals are trigonal at ambient conditions such as the superprotonic phase of (NH4)3H(SO4)2 at high temperature. A distribution of the K and N atoms in the crystal was modelled on the basis of the refined occupancies of K/N positions. Studies of dielectric properties over the temperature range 223–353 K revealed high values of conductivity of the crystals comparable with the conductivity of known superprotonic compounds at high temperatures, and an anomaly corresponding to a transition to the phase with low conductivity upon cooling.

THERMAL AND ELECTRICAL BEHAVIOUR OF THE SUPERPROTONIC CONDUCTOR PHASE IN Rb2(HSeO4)1.5(H2AsO4)0.5

Crystals of a new compound with a superprotonic phase transition Rb2(HSeO4)1.5(H2AsO4)0.5 (noted RbHSeAs), were synthesized by slow evaporation of an aqueous solution at room temperature. The differential scanning calorimetric analyses showed two endothermic peaks at 465 K and 566 K. The last peak corresponds to the decomposition of the material. The first transition was characterized by several techniques (impedance spectroscopy, complex modulus, Raman and X-ray diffraction powder depending on temperature). ac impedance measurements revealed that, upon heating, the compound undergoes at ~ 443 K a sharp increase in conductivity from a low temperature protonic phase to a superprotonic conductivity phase. The activation energies calculated from the modulus (Ef) and impedance (E) spectra respectively are approximately equal, suggesting that the transport properties in this material above and below the superprotonic phase transition (443 K) are probably due to an H+ protons hopping mechanism.

New representatives of solid acid conductors: CsmHn((HSO4),(H2PO4))m+n

Crystals - superprotonics are extensively studied with the goal of elucidating the influence of the hydrogen subsystem on the physicochemical properties and designing new functional materials. As opposed to other hydrogen-containing compounds, phase transitions in these crystals are accompanied by a hydrogen-bond network rearrangement, resulting in radical changes of their properties, in particular, in the appearance of proton conductivity about 10–1 Ω–1 cm–1. These crystals are unique in the class of proton conductors, since the superprotonic conductivity is related to the structural features of these compounds rather than to the presence of doping additives. The occurrence of high superprotonic conductivity in the Me3H(XO4)2 (Me = K, Rb, Cs, NH4; X = S, Se, P, As) crystals is associated with the formation of a qualitatively new and dynamically disordered hydrogen-bond system [1]. In K9H7(SO4)8·N2O crystals, the only known representative of the Me9H7(XO4)8·xN2O family, the occurrence of high conductivity is associated with the outward diffusion of water molecules, the hydrogen-bond network rearrangement, and the formation of channels for the possible motion of K+ ions [2]. The hydrogen-bond rearrangement and the hindered back diffusion of water to the crystal bulk stabilize the high-temperature crystal structure and ensure its supercooling to low temperatures. The new crystals of Cs3(HSO4)2(H2PO4), Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 were grown up in the CsHSO4–CsH2PO4-H2O system - enough big, with good optic quality [3]. The thermal and optical properties of crystals as well as their conductivity have been investigated in the temperature range 295 – 445 K. It was observed superprotonic phase transitions at 409, 411 and 365 K correspondingly. The distinction in the properties of Cs3(HSO4)2(H2PO4) and Cs4(HSO4)3(H2PO4) (sp. gr. C2/c at 295 K) is related to differences in nets of hydrogen bonds formed between different-occupied XO4 tetrahedra. Cs6H(HSO4)3(H2PO4)4 srystals (sp. gr. I-43d at 295 K) have the net of hydrogen bonds which is completely different. After cooling the high-temperature superprotonic phase preserves long enough without essential decrease in conductivity. This study was supported by the Russian Foundation for Basic Research (projects 13-03-12216 and 13-02-92693).