Vladimir A. Komornikov

Superprotonic crystals Cs₃(HSO₄)₂(H₂PO₄), grown in the water-salt system CsHSO₄–CsH₂PO₄–H₂O, have been studied using neutron diffraction methods at the MOND station (National Research Centre “Kurchatov Institute”). Experimental diffraction data were obtained, structural parameters were refined, hydrogen atoms were localized with high accuracy. The presence of three types of hydrogen bonds in the crystal structure was shown: characterized by a double-minimum potential curve between SO₄ and PO₄ tetrahedra; with a symmetrical double-minimum potential curve between PO₄ tetrahedra; and, unlike the initial compounds of the system, dynamically disordered bonds between SO₄ tetrahedra already at room temperature. Using atomic force microscopy, the correlation of the growth surface structure of single crystals with the atomic structure and the hydrogen bond system has been established.

Atomic force microscopy (AFM) was used to diagnose the surface morphology and local electrical properties of xCs₄(HSO₄)₃(H₂PO₄) + (1 – x)AlPO₄ composite materials of compositions x = 0.9, and 0.8. The results of statistical analysis of the relief parameters and grain sizes of polycrystalline proton-conducting compound are presented. The local current–voltage characteristics, reflecting the effect of the composition on the sample conductivity, were obtained. Images of the samples were recorded, and heterogeneity of the spatial distribution of the surface potential and surface conductivity of the layers at the microscopic level was established. Methodical approaches for the application of AFM to the study of the surface, microstructure, and local electrical properties of composite materials based on acid salts of alkali metal are proposed.

The temperature dependence of solubility in the NiCl₂–H₂O system in the temperature range of 25–60°C has been studied. The temperature ranges of crystallization of NiCl₂⋅6H₂O and NiCl₂⋅4H₂O compounds have been determined. Single crystals of nickel chloride hexahydrate and tetrahydrate have been obtained, the size and quality of which allowed to estimate the equilibrium faceting and optical spectra. A comparative analysis of the thermal stability of the obtained crystal hydrates was carried out. It was found that the transparency band in the short-wavelength range for nickel chloride crystals is shifted to the long-wavelength range as compared to the nickel sulfate crystal.

Rb₃CoCl₄NO₃ samples applicable for studying spectral characteristics have been obtained for the first time and investigated in the wavelength range λ = 200–800 nm. Transparency windows are found at the UV–visible and visible–IR boundaries. Differential-scanning calorimetry/thermogravimetry analysis revealed that the Rb₃CoCl₄NO₃ crystal is thermally stable, melts without decomposition, and undergoes two reversible phase transitions. The structure of Rb₃CoCl₄NO₃ single crystals was studied by X-ray diffraction in the temperature range of 293–625 K.

The crystal structure of superprotonic conductor caesium hydrogen sulfate phosphate [Cs₄(HSO₄)₃(H₂PO₄)] have been analyzed using neutron diffraction methods. Additionally, its structure and surface layers have been investigated using atomic force microscopy. From the diffraction data obtained, Fourier syntheses of neutron scattering densities were calculated, and the localization of hydrogen atoms and the parameters of three types of hydrogen bonds in the crystal structure were accurately determined. Correlation of surface characteristics of samples obtained by atomic force microscopy with their crystal structure is shown.

A series of solid acid compounds, representing the large family M_mH_n(AO₄)_{(m + n)/2}·yH₂O (where M = K, Rb, Cs, NH₄; AO₄ = SO₄, SeO₄, HPO₄, HAsO₄), is characterized by high values of own proton conductivity, which arises as a result of a phase transition through the formation of a dynamically disordered hydrogen bond network. Such superprotonic phase transitions are observed, however, not for all compounds of the family and Rb₃H(SO₄)₂ is one of them. The occurrence of superprotonic phase transitions has been experimentally demonstrated in the (K_xRb_1−x)₃H(SO₄)₂ solid solutions through cation substitution. The high-temperature phases are unstable towards decomposition reaction, and their temperature range of existence is about 1–7 °C. The implementation of superprotonic transitions is discussed in terms of hydrogen bond lengths.

In this paper, the conditions of phase formation in the system CsOH–H₂SO₄–H₃PO₄–H₂O are considered for the first time. The phase formation of Cs₆(SO₄)₃(H₃PO₄)₄ at t = 50 °C has been studied extensively. The main concentration boundary conditions for this compound are considered for the first time. The solubility congruence of Cs₆(SO₄)₃(H₃PO₄)₄ is shown. Conditions and approaches for obtaining crystals by isothermal evaporation and saturated solution temperature reduction methods are considered. The results of obtaining Cs₆(SO₄)₃(H₃PO₄)₄ crystals with maximum dimensions of ~20 mm are presented.