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.

Research subject. Crystals of hydrogen-containing compounds belonging to the superprotonic family. Aim. To obtain knowledge about regular relations between composition, atomic structure, real structure and physical properties of materials, with the purpose of elucidating processes occurring in condensed state and forming the basis for modification of known or obtaining new compounds. Materials and methods. Experimental data were obtained using a set of complementary physical methods, including structural analysis using X-rays, synchrotron radiation and neutrons, optical microscopy, and atomic force microscopy. Results. Experimental data on the atomic structure, real structure, and physical properties of superprotonic crystals, including systems of hydrogen bonds and their changes, were obtained. Conclusions. The physical properties of superprotonic crystals are significantly affected by hydrogen bonding systems and their changes, primarily by the formation of dynamically disordered hydrogen bonds with energetically equivalent positions of hydrogen atoms. When carrying out diagnostics of crystalline samples, account should be taken of their real structure, including the structure of surface layers and the presence of crystallization water. These factors may affect the measured physical parameters, the boundaries of existence of phases, the formation of a multiphase state under variations in temperature.

The dynamics of the phase transition (PT) in triglycine sulfate crystals, either nominally pure or doped in different ways (upon homogeneous and profiled impurity introduction into the crystal) with chrome impurity (Сr 3+ ), has been studied by analyzing the dielectric spectra measured in the ranges of frequencies 1–10 7 Hz and temperatures 23–60°C upon heating and cooling. It is shown that the introduction of impurity shifts the PT temperature range both upon heating and cooling. The PT temperature dynamics is found to depend both on the presence of impurity and on the way of its introduction into the crystal. The dependence of the width of the PT temperature range on the frequency at which the dielectric spectra were measured was shown to increase directly proportion to frequency.

Single crystals of [K1-x(NH4)x]3H(SO4)2 (x ≥ 0.57) grown in the K3H(SO4)2-(NH4)3H(SO4)2-H2O water-salt system are studied. The atomic structure including H atoms was determined at room temperature using X-ray structural analysis. [K1-x(NH4)x]3H(SO4)2 (x ≥ 0.57) crystals have trigonal symmetry and disordered hydrogen-bond networks at ambient conditions similar to the high-temperature phases of K3H(SO4)2, (NH4)3H(SO4)2 and other superprotonic compounds M3H(AO4)2. Impedance measurements performed on single crystals show high values of conductivity characteristic for superprotonic phases. Using the methods of impedance spectroscopy and atomic force microscopy, a significant anisotropy of the conductivity of crystals has been detected. It was also shown that there is a qualitative correlation of bulk and local conductivity measured for samples of the same composition and orientation at room temperature, which is due to the peculiarities of their crystal structure.

The results of comprehensive studies of structural and phase transformations in (K 0.43 (NH 4 ) 0.57 ) 3 H(SO 4 ) 2 superprotonic crystals under the influence of atmospheric moisture are presented. The real structure, composition, and thickness of the modified surface layers have been analyzed using scanning electron microscopy and X-ray microscopy. The local characteristics of nanostructures, formed on the freshly cleaved (001) crystal surface subjected to the electrostatic effect, have been investigated by conductive atomic force microscopy. A correlation has been established between the time changes in the structure, composition, and magnitude of the electrostatic potential of the crystal surface. The results are considered in the context of evaluation of the chemical stability of the samples and searching for the ways to optimize the compositions and functional properties of superprotonic compounds.

Surface properties of superprotonic (K1-x(NH4)(x))(3)H(SO4)(2) (x >= 0.57) single crystals and their evolution under humidity were studied by optical polarization microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDXS). Traditional method of atomic force microscopy (AFM) and sophisticated methods such as Kelvin probe force microscopy (KPFM) and scanning capacitance force microscopy (SCFM) were also used. Fresh and aged cleavage surfaces perpendicular and parallel to the c axis of the trigonal superprotonic phase were examined. The effect of air humidity on the surface morphology, surface conductivity, surface potential and surface capacitance was studied. The influence of material composition and surface orientation was considered. During ageing for 430 h, the (001) sample surfaces, initially stepped, were smoothened, surface electric potential changed from negative greater than a hundred of millivolts in absolute value to positive of about 80 mV, and a modified layer containing new crystal phases was formed.

Single crystals of (K1‒x(NH4)x)3H(SO4)2 (x ≥ 0.57) solid solutions have been studied by electrical atomic force microscopy. At 296 K, the local current–voltage characteristics of trigonal superprotonic phases were measured for the samples with different concentrations of K and NH4 cations as a function of the c axis orientation, and anisotropy of conductivity was established. The degree of the dependence of the conduction properties on the composition is determined. The temporal changes in the morphology, potential, and surface capacitance derivatives of the (001) (K0.43(NH4)0.57)3H(SO4)2 surface were traced and evaluated when the samples were held in air under conditions of constant humidity and temperature.

A technique for studying the local elastic properties of inhomogeneous ferroelectrics has been proposed by the example of growth striations in triglycine sulfate crystals, layer-by-layer doped with chromium and L-α-alanine impurities. The data on the width and impurity compositions of stripes were obtained by X-ray fluorescence analysis and X-ray topography. The positions of the boundaries of the stripes emerging at the surface and the domain structure morphology were determined by the methods of correlation electrical atomic-force microscopy. The force curves were measured and Young’s moduli were calculated for impurity-free and doped crystal stripes using contact atomic-force spectroscopy. The introduction of impurity decreases Young’s modulus: the difference was found to be 20–25 and 12–14% for chromium and L-α-alanine, respectively.

Superprotonic (NH4)3H(SeO4)2 crystals in the ferroelastic phase are studied via optical polarization microscopy and atomic force microscopy to obtain data on their true structure. Twinned structures on cleavages parallel and perpendicular to the (001) plane of the high-temperature paraelastic phase and their correlation with the atomic structure of crystals are considered.

Surface of (NH4)(3)H(SeO4)(2) crystals and its evolution in the course of the transition from the ferroelastic to the superpmtonic phase were studied by optical polarization microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDXS). The morphology of fresh cleavage surfaces was examined. The composition and local electrical properties of the surface crystal layers were investigated. Local measurements of current-voltage curves and piezoresponse hysteresis loops were carried out. A transition to the phase with a superpmtonic conductivity was proved to exist at T approximate to 308 K. A thin crystal layer of varying composition was found to form owing to topochemical reactions. Phase transformations and solid-state reactions at the crystal surface were analyzed based on the data obtained.

The (NH4)3H(SeO4)2 crystals in the temperature range of 296–343 K and the evolution of their surface at phase transitions have been studied by atomic force microscopy. Data on morphology and local electrical characteristics of crystal surface are obtained. Local current–voltage characteristics are measured, and the existence of structural phase transition to the phase with superprotonic conductivity at T ≈ 308 K is confirmed. A layer of non-conducting phase of variable composition is shown to appear on the crystal surface at the phase transitions.

Dependencies of the complex permittivity of chromium-doped ferroelectric triglycine sulfate crystals (TGS + Cr) have been measured using dielectric spectroscopy method in a frequency range of 1–107 Hz and a temperature range of 23–60°C upon heating and cooling. The dielectric spectra are analyzed by considering the temperature‒frequency dependences of the real (ε') and imaginary (ε") parts of permittivity in the immediate vicinity of the phase transition using the dynamic conductance model and taking into account resonance processes. Good agreement between calculated and experimental data is obtained for the dielectric response of TGS + Cr crystal near the Curie temperature.

An investigation of new superprotonic crystals—namely, acid salts of potassium–ammonium sulfate (K1 – x(NH4)x)3H(SO4)2, x ≥ 0.57—is carried out. The surface morphology, domain structure, and conductivity of the samples are studied using atomic force microscopy. The stability and degradation of the surface of superprotonic crystals is first studied at the nanoscale. The data of piezoelectric response microscopy make it possible to establish that the crystal of (K0.43(NH4)0.57)3H(SO4)2 transits from the paraelectric phase to the ferroelectric phase when the temperature is decreased from 296 K to 282 K.

Complex studies of new superprotonic crystals - acidic salts of potassium – ammonium sulfate (K1-x(NH4)x)3H(SO4)2, x ≥ 0.57 were carried out. Data on surface morphology, domain structure and conductivity of samples were obtained using atomic force microscopy. For the first time, the stability and degradation of the surface of superprotonic crystals were studied at the nanoscale. Based on piezoelectric response microscopy data, it was found that when the temperature decreased from 296 to 282 K, the (K0.43(NH4)0.57)3H(SO4)2 crystal transits from the paraelectric phase to the ferroelectric one.