E. V. Selezneva

Production of complex rubidium and cesium hydrogen sulfate‒phosphates

The solubility in the CsH2PO4‒CsHSO4‒H2O system at different temperatures (25, 50, and 75°C) is studied and the phase equilibria in the Rb3H(SO4)2‒RbH2PO4‒H2O system under isothermal conditions (at 25°C) are analyzed. The temperature and concentration conditions for forming Rb2(HSO4)(H2PO4), Rb4(HSO4)3(H2PO4), Cs4(HSO4)3(H2PO4), Cs3(HSO4)2(H2PO4), Cs2(HSO4)(H2PO4), and Cs6H(HSO4)3(H2PO4)4 compounds (the latter has been obtained for the first time) are determined. The conditions for growing large single crystals of complex acid rubidium and cesium salts are found.

Structure and properties of new crystals in CsHSO4 – CsH2PO4 – H2O system

ABSTRACT The phase diagram of the CsHSO4–CsH2PO4–H2O ternary system was systematically studied, from which it became possible to synthesize and grow new compounds of mixed sulfate-phosphates of cesium. Based on experimental data, their common structural features and differences, and structural conditionality for anomalies in physical properties were analyzed.

The Influence of Cation Substitution on the Kinetics of Phase Transitions in Crystals of (K,NH4)3H(SO4)2 Solid Solutions

The structure of (K0.967(NH4)0.033)3H(SO4)2 crystals, belonging to the K3H(SO4)2–(NH4)3H(SO4)2–H2O salt system, has been investigated by X-ray structural analysis. The room-temperature characteristics of the atomic structure of these crystals are found to be as follows: sp. gr. C2/c, Z = 4, a = 14.7025(4) Å, b = 5.6859(2) Å, c = 9.7885(3) Å, and R/wR = 0.021/0.030%. The thermal and optical properties of (K,NH4)3H(SO4)2 and K3H(SO4)2 single crystals have been investigated and compared in a temperature range of 295–500 K.

Effect of cationic substitution on the double-well hydrogen-bond potential in [K1−x(NH4)x]3H(SO4)2 proton conductors: a single-crystal neutron diffraction study

The structure of the mixed crystal [K1-x(NH4)x]3H(SO4)2 as obtained from single-crystal neutron diffraction is compared with the previously reported room-temperature neutron structure of crystalline K3H(SO4)2. The two structures are very similar, as indicated by the high value of their isostructurality index (94.8%). It was found that the replacement of even a small amount (3%) of K+ with NH4+ has a significant influence on the short strong hydrogen bond connecting the two SO42- ions. Earlier optical measurements had revealed that the kinetics of the superionic transition in the solid solution [K1-x(NH4)x]3H(SO4)2 are much faster than in K3H(SO4)2; this reported difference in the kinetics of the superionic phase transition in this class of crystal is explained on the basis of the difference in strength of the hydrogen-bond interactions in the two structures.

New crystals of the CsHSO4–CsH2PO4–H2O system

Cs6H(HSO4)3(H2PO4)4 crystals, grown for the first time based on an analysis of the phase diagram of the CsHSO4–CsH2PO4–H2O ternary system, have been investigated by structural analysis using synchrotron radiation. The atomic structure of the crystals is determined and its specific features are analyzed.

New crystals of the CsHSO{sub 4}–CsH{sub 2}PO{sub 4}–H{sub 2}O system

Cs6H(HSO4)3(H2PO4)4 crystals, grown for the first time based on an analysis of the phase diagram of the CsHSO4–CsH2PO4–H2O ternary system, have been investigated by structural analysis using synchrotron radiation. The atomic structure of the crystals is determined and its specific features are analyzed.

The Changes of Thermal, Dielectric, and Optical Properties at Insertion of Small Concentrations of Ammonium to K3H(SO4)2 Crystals

The structure of (K1–x(NH4)x)3H(SO4)2 crystals with a low ammonium concentration and the behavior of their thermal, optical, and dielectric properties in a temperature range of 275–500 K have been investigated to clarify the influence of doping on the phase transition kinetics. An examination of unit-cell parameters of (K1 – x(NH4)x)3H(SO4)2 single crystals has confirmed the existence of a superprotonic phase transition at a temperature of ≈450K. The conducting properties of single-crystal and polycrystalline samples have been studied.

Structural conditionality of the physical properties of the new representatives of the family of superprotonic crystals

To reveal the effect of isomorphic substitution on the kinetics of phase transitions, single crystals of (K1–x(NH4)x)mHn(SO4)(m + n)/2 · yH2O solid solutions are grown from the K3H(SO4)2–(NH4)3H(SO4)2–H2O system. The chemical composition of the single crystals grown is determined by energy dispersive X-ray microanalysis. The thermal, optical and dielectric properties of (K1–x(NH4)x)mHn(SO4)(m + n)/2 · yH2O single crystals are studied. The structural conditionality of the physical properties of these compounds is determined on the basis of the data of precision studies of the structure.

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.

The nature of high conductivity in solid acid proton conductors

Increasing energy consumption rates stimulate global interest in the study and development of alternative energy sources. One of the most rapidly developing fields associated with alternative energy sources is hydrogen energetics. To directly produce electrical energy, special devices, i.e., fuel cells providing direct chemical to electrical energy conversion were developed. Interest in superprotonic crystals MmHn(AO4)m+n/2·yН2О (M = K, Rb, Cs, NH4; AO4 = SO4, SeO4, HPO4, HAsO4) is associated with the determination of the influence of the hydrogen subsystem on physicochemical properties and obtainment of materials for electrochemical devices. In contrast to other hydrogen-containing compounds, phase transitions in superprotonic crystals are accompanied by a radical change in properties, including the appearance of high proton conductivity at relatively low temperature (~400 K). Structural data on these crystals are indicative of the existence of different mechanisms of changes in physical properties [1]: the formation of a dynamically disordered system of hydrogen bonds as in M3H(XO4)2, crystallization water diffusion and formation of channels with partially occupied sites of atoms in (K,NH4)9H7(SO4)8·H2O [2], and the formation of a multiphase state. The transformation of bonds can stabilize the superprotonic phase and make it possible to supercool this phase practically to room temperature. Replacing ions with ammonium and the formation of additional hydrogen bonds can reduce conductivity in these crystals by four orders of magnitude or cause the occurrence of conduction already under ambient conditions. The kinetics of superprotonic phase transitions in Cs3(HSO4)2(H2PO4), Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 crystals [3] depends on their structure and composition significantly. The kinetics of the phase transitions of Cs3(HSO4)2(H2PO4) and Cs4(HSO4)3(H2PO4) change considerably owing to the statistical replacement of PO4 tetrahedra by the SO4 groups and decreasing the number of hydrogen bonds. The Cs6H(HSO4)3(H2PO4)4 (Fig.) compound is chemically stable and exhibits reproducible properties. This study was supported by the DST-RFBR collaboration (No13-02-92693, INT/RFBR/P-140) and by the Scholarship of the President of the Russian Federation (no. SP-1445.2016.1).