Nonstoichiometry of refractory inorganic compounds with a volatile component determined by new methods of physicochemical analysis. Review

The nonstoichiometry of refractory compounds with a volatile component is based on solid-phase and heterophase processes. At the same time, measurements of the parameters of these high-temperature phase transformations are often inaccurate. The inaccuracy results from the instability of the equipment as well as the behavior of themselves samples. To overcome these limitations, we developed three new methods of physicochemical analysis, which were then used in a comprehensive approach to the study of nonstoichiometry and the chemical composition of defective phases at the macro and micro levels. We refer to are high-speed thermal analysis, static tensimetric membrane technique, and stoichiographic differential dissolution method. The methods were used to measure temperatures up to 2400 °С, pressure up to 10 atm, and the degree of nonstoichiometry up to 10–4 mol. %. The review demonstrates the effectiveness of the proposed methodology applied to refractory compounds LnS, Ln2S3 (Ln = РЗМ), Yb(Ln)14MnSb11, ZrGeO4, Zr3GeO8, MgO, and Mg(Ru)O, as well as highly volatile REM polychalcogenides and ZnMo(W)O4, presented in the form of powders, large crystals, ceramics, and films. For each of the studied substances, fundamental knowledge was gained regarding their spatial-temporal evolution responsible for the occurrence and the degree of nonstoichiometry. The data was obtained by studying Т-х and р-Т diagrams and by using of the stoichiographic method to determine the phase purity, microstructural inclusions, and spatial chemical inhomogeneity of individual phases at a micro level. The obtained new quantitative thermodynamic and physicochemical data regarding the nonstoichiometry of the studied compounds was used as a basis for the choice of the composition and the design of the crystallization process, sintering and chemical deposition of thin films to realize a directed synthesis of materials with the desired properties. The review was initiated by Professor Magomed Babanly, DSc in Chemistry, Associate Member of the Russian Academy of Sciences, and editor of the special issue of the journal Condensed Matter and Interphases dedicated to physicochemical analysis in material science.