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Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMoO6 can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO3) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr2FeMoO6 phase and the far-from-equilibrium structural state of the reaction product are systematically monitored with XRD and a variety of spectroscopic techniques including Raman spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe0 → Fe3+) with simultaneous reduction of Mo cations (Mo6+ → Mo5+), occuring during the mechanosynthesis of Sr2FeMoO6, is attributed to the mechanically triggered formation of tiny metallic iron nanoparticles in superparamagnetic state with a large reaction surface and a high oxidation affinity, whose steady presence in the reaction mixture of the milled educts initiates/promotes the swift redox reaction. High-resolution transmission electron microscopy observations reveal that the mechanosynthesized Sr2FeMoO6, even after its moderate thermal treatment at 923 K for 30 min in air, exhibits the nanostructured nature with the average particle size of 21(4) nm. At the short-range scale, the nanostructure of the as-prepared Sr2FeMoO6 is characterized by both, the strongly distorted geometry of the constituent FeO6 octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder ASD = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe3+ and Mo5+ cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr2FeMoO6 nanoparticles exhibit superparamagnetism with the blocking temperature T B = 240 K and the deteriorated effective magnetic moment μ = 0.055 μ B per formula unit.

Mechanochemical technology is developing rapidly, judging by the scientific information in both basic and applied studies. However, many issues and points of view remain to be discussed. This review presents some new key issues for the optimization of mechanochemical processes in terms of theoretical and practical aspects. Emphasis is placed on powder technology aspects, which are not always discussed compared to functional or microscopic viewpoints. The transfer of chemical species across the interparticle interface between dissimilar species during the mechanosynthesis of nanocomposites offers many new opportunities. Since almost all material transport is preceded by charge transfer, its driving force has been sought using terminology beyond the well-established electrochemical terms. In particular, the valence state of the cationic species involved is of importance. The role of organic compounds throughout the process is emphasized, regardless of their survival in the final product. The similarity with pharmaceutical phenomena is pointed out, although its mentality is very different from that of the synthesis of nanocomposites. The rational amorphization and stabilization of molecular dispersion states with the participation of excipients are discussed. The effects of liquids, either added or formed by mechanochemical auto-liquefaction, are presented with reference to the comparison between wet and dry grinding. The mechanisms of the apparent stabilization of the mechanically activated states of the products are elucidated to investigate the practical applicability of these mechanochemically synthesized products. Finally, the most important aspects for the optimization of the mechanochemical processes of functional nanocomposites are listed. [ABSTRACT FROM AUTHOR]

The increasing demand for batteries forced the development of energy storage systems that rely on materials consisting of abundant elements in the Earth's crust. Switching from Li+ to K+ as the main ionic charge carrier needs highly conducting potassium-bearing electrolytes to realize K+ ion batteries (PIBs). The knowledge gained from the design of Li-ion batteries (LIBs) and Na-ion batteries (NIBs) may conceptually inspire also the establishment of PIBs. Considering, for instance, the hexatitanates Na2Ti6O13, Li2Ti6O13, and H2Ti6O13, which were previously investigated as components for LIBs and NIBs, here we investigated ion dynamics in the K-analog K2Ti6O13. Ionic transport in polycrystalline samples of K2Ti6O13 was studied in a moisture-free atmosphere by broadband impedance spectroscopy in a temperature range from 20 °C to 450 °C. The current study aims at establishing a correlation between structural features of K2Ti6O13 and long-range ionic transport. As expected for K+ transport in K2Ti6O13 with its geometrically obstructed structure, the overall activation energy of ion transport in the ternary oxide takes a rather high value of 0.97(2) eV. Almost the same result (0.95(3) eV) is obtained for the migration activation energy, which we extracted from the analysis of crossover frequencies of the corresponding conductivity isotherms. By comparing our results with those of Na2Ti6O13 (0.82 eV), Li2Ti6O13 (0.65 eV), and H2Ti6O13, we clearly see how the size of the mobile cation correlates with both specific conductivities and activation energies. This comparison points to K+ being the main charge carrier in K2Ti6O13. It also helps in laying the foundations to derive the relevant structure-property relationships in this class of materials. [ABSTRACT FROM AUTHOR]