Structure and ion dynamics of mechanosynthesized oxides and fluorides

In many cases, limitations in conventional synthesis routes hamper the accessibility to materials with properties that have been predicted by theory. For instance, metastable compounds with local non-equilibrium structures can hardly be accessed by solid-state preparation techniques often requiring high synthesis temperatures. Also other ways of preparation lead to the thermodynamically stable rather than metastable products. Fortunately, such hurdles can be overcome by mechanochemical synthesis. Mechanical treatment of two or three starting materials in high-energy ball mills enables the synthesis of not only new, metastable compounds but also of nanocrystalline materials with unusual or enhanced properties such as ion transport. In this short review we report about local structures and ion transport of oxides and fluorides mechanochemically prepared by high-energy ball-milling

X-ray diffraction and thermoanalytical datasets of precursors of the Gd6_{6}UO12−δ_{12-δ} phase processed by combined mechanochemical−thermal routes

The datasets presented here are related to the research paper entitled “Disordered Gd$_{6}$UO$_{12-δ}$ with the cation antisite defects prepared by a combined mechanochemical−thermal method”. The datasets complement the findings on the effect of the combined mechanochemical−thermal processing of the stoichiometric mixture of solid precursors (3Gd$_{2}$O$_{3}$ + UO$_{2}$) on the formation of Gd$_{6}$UO$_{12-δ}$ phase. In this article, we provide (i) X-ray diffraction (XRD) data of the 3Gd$_{2}$O$_{3}$ + UO$_{2}$ mixture milled for 12 h, (ii) the refined XRD data of the non-milled 3Gd$_{2}$O$_{3}$ + UO$_{2}$ mixture after annealing at 1282 °C for 3 h in air, and (iii) the thermogravimetric and differential thermal analysis (TG-DTA) data for non-milled and mechanically preactivated 3Gd$_{2}$O$_{3}$ + UO$_{2}$ mixture measured in air at a heat rate of 10 K/min

Mechanosynthesis of nanocrystalline fayalite, Fe 2SiO 4

Nanostructured fayalite (α-Fe 2SiO 4) with a large volume fraction of interfaces is synthesized for the first time via single-step mechanosynthesis, starting from a 2α-Fe 2O 3 + 2Fe + 3SiO 2 mixture. The nonequilibrium state of the as-prepared silicate is characterized by the presence of deformed polyhedra in the interface/surface regions of nanoparticles. © 2012 The Royal Society of Chemistry

Suppression of the Cycloidal Spin Arrangement in BiFeO₃ Caused by the Mechanically Induced Structural Distortion and Its Effect on Magnetism

Bismuth ferrite (BiFeO₃) particles are prepared by a combined mechanochemical−thermal processing of a Bi₂O₃ + α-Fe₂O₃ mixture. Structural, magnetic, hyperfine, morphological and chemical properties of the as-prepared BiFeO₃ are studied using X-ray diffraction (Rietveld refinement), ⁵⁷Fe Mössbauer spectroscopy, SQUID magnetometry, electron microscopy and energy dispersive X-ray spectroscopy. It is revealed that the structure of the ferrite exhibits the long-range distortion (significantly tilted FeO₆ octahedra) and the short-range disorder (deformed FeO₆ octahedra). Consequently, these structural features result in the suppression of a space modulated cycloidal spin arrangement in the material. The latter manifests itself by the appearance of only single spectral component in the ⁵⁷Fe Mössbauer spectrum of BiFeO₃. The macroscopic magnetic behavior of the material is interpreted as a superposition of ferromagnetic and antiferromagnetic contributions with a large coercive field and remanent magnetization. Taking into account the average particle size of the as-prepared BiFeO₃ particles (∼98 nm), exceeding the typical period length of cycloid (∼62 nm), both the suppression of the spiral spin structure in the material and its partly ferromagnetic behavior are attributed to the crystal lattice distortion caused by mechanical stress during the preparation procedure

A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2_{2}FeMoO6_{6} With an Extraordinarily High Degree of Anti-Site Disorder

Strontium ferromolybdate, Sr(2)FeMoO(6), 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 Sr(2)FeMoO(6) can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO(3)) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr(2)FeMoO(6) 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, (57)Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe(0) → Fe(3+)) with simultaneous reduction of Mo cations (Mo(6+) → Mo(5+)), occuring during the mechanosynthesis of Sr(2)FeMoO(6), 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 Sr(2)FeMoO(6), 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 Sr(2)FeMoO(6) is characterized by both, the strongly distorted geometry of the constituent FeO(6) 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 Fe(3+) and Mo(5+) cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr(2)FeMoO(6) 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

A local distortion of polyhedra in the novel chromium doped mullite-type bismuth ferrite Bi-2(CrxFe1-x)(4)O-9

Chromium doped mullite-type ferrite, Bi2(Cr0.1Fe0.9)4O9, is synthesized for the first time by a combined mechanochemical/thermal route. The microstructure of the as-prepared material on the long-range and local atomic scales, revealed by X-ray diffraction and 57Fe Mössbauer spectroscopy, respectively, is found to be quite different from that reported previously. It is shown that the presence of Cr3+ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. Quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters.Web of Science16149048

Transfer and State Changes of Fluorine at Polytetrafluoroethylene/Titania Boundaries by Mechanical Stressing and Thermal Annealing

Fluorine in polytetrafluoroethylene (PTFE) changes its states and transfers to titania by comilling and annealing of a titania-PTFE mixture. XPS, F-19 MAS NMR, FT-IR. Raman spectra, TEM and EDX analyses consistently indicated the oxidative decomposition of PTFE, inducing partial fluorination of titania. Incorporation of fluorine into titania was unambiguously confirmed by the appearance of a new XPS F1s peak at similar to 685 eV, similar to those observed from TiOF2. The intensity of the new F1s peak increased with comilling time and became a sole peak after milling for 3 h. At the same time, F-19 MAS NMR lines specific to PTFE disappeared. From these observations, we concluded that extensive decomposition of PTFE with simultaneous change in the chemical states of fluorine took place with the aid of titania. Upon annealing the comilled mixture in air, the amount of fluorine in the mixture decreased with increasing temperature, while the remaining fluorine migrated into the titania lattice, as confirmed by the decrease in the chemical shift of the F-19 MAS NMR lines and the decrease in the rutile (211) and (220) interplanar distances. These findings allow us to devise how to control the amount, states, and spatial distribution of the fluorine incorporated into titania