Tracing electron density changes in langbeinite under pressure

Pressure is well known to dramatically alter physical properties and chemical behaviour of materials, much of which is due to the changes in chemical bonding that accompany compression. Though it is relatively easy to comprehend this correlation in the discontinuous compression regime, where phase transformations take place, understanding of the more subtle continuous compression effects is a far greater challenge, requiring insight into the finest details of electron density redistribution. In this study, a detailed examination of quantitative electron density redistribution in the mineral langbeinite was conducted at high pressure. Langbeinite is a potassium magnesium sulfate mineral with the chemical formula [K2Mg2(SO4)3], and crystallizes in the isometric tetartoidal (cubic) system. The mineral is an ore of potassium, occurs in marine evaporite deposits in association with carnallite, halite and sylvite, and gives its name to the langbeinites, a family of substances with the same cubic structure, a tetrahedral anion, and large and small cations. Single-crystal X-ray diffraction data for langbeinite have been collected at ambient pressure and at 1 GPa using a combination of in-house and synchrotron techniques. Experiments were complemented by theoretical calculations within the pressure range up to 40 GPa. On the basis of changes in structural and thermal parameters, all ions in the langbeinite structure can be grouped into `soft' (potassium cations and oxygens) and `hard' (sulfur and magnesium). This analysis emphasizes the importance of atomic basins as a convenient tool to analyse the redistribution of electron density under external stimuli such as pressure or temperature. Gradual reduction of completeness of experimental data accompanying compression did not significantly reduce the quality of structural, electronic and thermal parameters obtained in experimental quantitative charge density analysis.

SYNTHESIS AND STUDY OF GAS-SENSITIVE NANOSTRUCTURES OF THE Zn-Sn-O SYSTEM

Наностержни оксида цинка синтезированы гидротермальным методом. Проведена обработка полученных образцов в водно-спиртовом растворе станната калия и мочевины при 170°С в течение 30 и 60 минут. В результате получены наноструктуры Zn - Sn - O. Химический состав поверхности образцов ZnO и Zn - Sn - O исследован с помощью рентгеновской фотоэлектронной спектроскопии. Проанализирована их чувствительность к парам изопропилового спирта (1000 мд) при температурах 120 °С, 180°С, 250 °С. Показано перераспределение электронной плотности при формировании композитных наноструктур Zn - Sn - O, проявляющееся в химическом сдвиге пиков O1s и Zn2p. Это свидетельствует о перестроении химических связей при замещении атомов цинка оловом. Обнаружено, что чувствительность композитных структур к парам изопропилового спирта значительно превышает чувствительность ZnO во всем исследуемом температурном диапазоне. Улучшение газочувствительных свойств связано с наличием в образцах системы Zn - Sn - O поверхностных центров различного типа, принимающих участие в адсорбции и окислении изопропилового спирта. Zinc oxide nanorods were synthesized by the hydrothermal method. The obtained samples were processed in an aqueous-alcohol solution of potassium stannate and urea at 170 °C during different times. As a result, Zn - Sn - O nanostructures were obtained. The surface chemical composition of ZnO and Zn - Sn - O was studied using the X-ray photoelectron spectroscopy. Its sensitivity to vapors of isopropyl alcohol (1000 ppm) at 120 °C, 180 °C, 250 °C was analyzed. The electron density redistribution during the Zn - Sn - O composite nanostructures formation manifests itself in the chemical shift of the O1s and Zn2p peaks. It confirm the rearrangement of chemical bonds when zinc atoms are replaced by tin ones. It was found that the sensitivity of composite structures to isopropyl alcohol vapors significantly exceeds that of ZnO in the entire temperature range under study. The improvement of gas-sensitive properties is associated with the presence of various types of surface centers in the Zn - Sn - O samples that participate in the adsorption and oxidation of isopropyl alcohol.

Динамические пространственно-временные структуры на поверхности нагруженных твердых тел

A model and a method are proposed to explain the origin and main specific features of the dynamic patterns of different types, which were previously observed on the free surface of solids under load in experiments. It is shown that the pattern is formed due to the dynamic instability of the flat surface of the solid under load. This instability develops at the relaxation mechanism caused by the dynamic atomic displacements due to a change in the interatomic interaction during the electron-density redistribution.

ELECTRON PARAMETERS OF 1,1,1 – TRILUOROALKANES

On the basis of quantum-chemical calculations the properties of trifluoro-substituted hydrocarbon molecules CnH2n+1-CF3 (n ≤ 9) received from their electron density distribution were considered. The geometry optimization of ten structures was carried out. The surfaces of zero flow electron charge density gradient were specified, and the basins of atomic groups and fluorine atoms were found. The electron integral parameters (charges q(R), energies E(R) and volumes V(R)) of atomic groups in trifluoroalkane molecules were calculated and analyzed. The relationship between the length of the hydrocarbon chains and the transferability of the properties of the selected groups (CF3, CH3, CH2) was revealed, that is reflected in their transferable parameters. For the studied homologous series the qualitative group electronegativity scale was made up and inductive effect (I - effect) of fluorine containing group was considered. The attenuation of I – effect in CnH2n+1-CF3 (n ≥ 6) within molecular fragments CF3-(CH2)4 and CH3-CH2 was identified. In this regard, the appearance of the «unperturbed» CH2 group was registered at n > 6. The «standard» (or «transferable») value of the total group energy E(R) was introduced and computing of the relative group energy ΔE(R) was described. It was shown, that the reduction of the volumes of the nearest СН2 to the СF3 was caused by the electron density redistribution. The comparative analysis of the group charges q(R) in CnH2n+1-CF3 (n ≤ 9) with corresponding q(R) in monofluorine alkanes, monofluorine alkane radicals, difluorine alkanes and difluorine alkane radicals was performed. The comparison of the charges of relevant groups and fluorine-containing fragments of the fluoro-substituted nonane and their radicals was presented as graphic dependence, which provides an understanding of the attenuation of I – effect from СF3.

Ammonia-modified Co(ii) sites in zeolites: spin and electron density redistribution through the CoII–NO bond

Both considerable share of the CoIII–NO− resonance structure and large π*-backdonation (evidenced by electron density transfer channels) rationalize the huge activation of the NO ligand in [Co(ii)(NH3)n]–NO adducts.

H2O–CH4and H2S–CH4complexes: a direct comparison through molecular beam experiments and ab initio calculations

Electron density redistribution upon the formation of the water–methane complex arises from polarisation and charge transfer effects.