I. G. Vasilyeva

Two-dimensionally modulated structure of the rare-earth polysulfide GdS2−x (x = 0.18 ≃ 13/72)

The crystal structure of GdS2−x is determined by single-crystal X-ray diffraction as a 144-fold superstructure of the ZrSSi structure type. The superstructure is described as a two-dimensional, commensurately modulated structure with the superspace group P4/n(αβ½)(00)(ss) and with α = 1/4 and β = 1/3. Structure refinements within the classical approach, employing the 144-fold supercell, fail because most of the superlattice reflections have zero intensities within the experimental resolution. Within the superspace approach the absent superlattice reflections are systematically classified as higher-order satellite reflections. Accordingly, the superspace approach has been used to refine the structure model comprising the basic structure positions and the amplitudes of the modulation functions of the three crystallographically independent atoms. The quality of fit to the diffraction data and the values of the refined parameters are independent of the assumption on the true symmetry (incommensurate or a 12 × 12 × 2, I-centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group I\bar{4}, corresponding to sections of superspace given by (t1, t2) equal to [(4n − 1)/48, (4m − 3)/48] or [(4n − 3)/48, (4m − 1)/48] (n and m are integers). Analysis of the structure, employing both superspace techniques (t plots) and the supercell structure model all show that the superstructure corresponds to an ordering of vacancies and an orientational ordering of S_2^{2-} dimers within the square layers of the S2 atoms.

Rapid Heating in High-Temperature Thermomicroscopic Analysis

A new technique of rapid heating in high-temperature thermomicroscopic analysis is suggested. The apparatus is described, the metrological features of the method and its advantages and limitations are discussed. Application of the technique to studying the behaviour of refractory Nd3S4, GaP and MoS2 compounds in the temperature range from 25 to 2500°C shown.

On the nature of the phase “η-TiO2”

Conditions for a sulfate method for the synthesis of a metastable modification, which has been previously described as “η-TiO2” (Dadachov, 2006), were found and the stability region of this phase in the hydrolysis temperature–hydrolysis duration coordinates was determined. Investigation by a number of methods (X-ray powder diffraction, a differential dissolution method, thermogravimetry, IR spectroscopy, Raman spectroscopy) showed that the η-phase is not a polymorph of TiO2 but is a pseudo-polymorph of titanium dioxide hydrate. It was demonstrated that nanoparticles of the low-temperature η-phase consist of the [TiO2−x·mH2O] core, the structure of which can be described as a superstructure in relation to anatase, and an amorphous shell containing TiO2−x (trace amount), OH, HSO4 and water. The average crystallite size depends on the ratio of the constituents.

Determination of Melting Points for A2B6 Compounds

A new technique of rapid heating, recording both the melting and evaporation processes in an open system was proposed to determine the melting points of A2B6 (A=Zn, Cd) compounds. The different melting character and temperatures were detected for the equiatomic and non-stoichiometric composition of A2B6. A fit of these data to previously obtained results was considered and the reasons for their discrepancy were discussed.

Study of individual phase composition in multiphase ceramics of BiPbCaSrCuO system

Abstract The new chemical method of the phase analysis, differential dissolution was used to determine the amount and the composition of each phase in multiphase samples of the BiPbCaSrCuO system. The results obtained were in agreement with the x-ray, X (T), microprobe analysis data. The 50% Bi lack has been observed for the high Tc-phase with 2223 structure. It was shown that the most of crystalline grains of low Tc-phase with 2212 structure are smaller than 10 −5 cm in size. These facts are believed to result from the formation mechanism of superconducting phases. The later is discussed as well.

Something new and something old on the phases in the superconducting systems

Abstract Using a new chemical method for characterization of phases by elemental composition — the separate dissolution — and combining data from this method together with the results of an X-ray diffraction analysis the mechanism of formation of high-Tc phases, the possible variation of the cation composition within a certain structure,the effect of the chemical composition and the structure of the phases on the superconducting properties of the test specimen are discussed.

Chapter 209 Polysulfides

Publisher Summary The compositional classification of polysulfides is based on three groups: the higher (hyperstoichiometric) polysulfides, RS 2+x ; stoichiometric disulfides, RS 2.0 ; and lower (hypostoichiometric) polysulfides, RS 2–x . The classification covers all known compounds with compositions varying in the range from RS 2.67 to RS 1.70 . The methods suited for the preparation of the polysulfides are: (a) reaction of sulfur with metals; (b) reaction of oxides or salts with H 2 S or CS 2 in an inert gas, Ar or N 2 ; (c) reaction of sulfides with sulfur. Method (a) is based on the registration of transitions from three-phase equilibria, S 1 S 2 V, to twophase equilibria, SV, and vice versa, when the phase state change occurs as a result of incongruent vaporization of the higher polysulfide in a closed system. The compositions of the intermediate polysulfides are calculated from the initial sample composition, which is known in advance, and the vapor phase in the points where complete decomposition of one of the condensed phases has taken place. In method (b), the composition is determined by chemical analysis after quenching the sample at the end of a run. In method (c), the composition is monitored continuously by means of a balance.