A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2FeMoO6 With an Extraordinarily High Degree of Anti-Site Disorder

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.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that [Formula: see text] ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides [Formula: see text], Li-bearing garnets ([Formula: see text]) and thiophosphates such as [Formula: see text]. In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in [Formula: see text] ([Formula: see text]) cation mixing influences F anion dynamics, in [Formula: see text] ([Formula: see text]) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor [Formula: see text] cation-cation repulsions immediately lead to a boost in [Formula: see text] diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured [Formula: see text] or [Formula: see text]. Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

F anion transport in nanocrystalline SmF3 and in mechanosynthesized, vacancy-rich Sm1—x BaxF3—x

Nanostructured materials can show considerably different properties as compared to their coarse-grained counterparts. Especially prepared by high-energy ball milling they are to be characterized by a large fraction of point defects in the bulk and structurally disordered interfacial regions. Here, we explored how the overall conductivity of SmF3 can be enhanced by mechanical treatment and to which degree aliovalent substitution is able to further enhance anion transport. For this purpose nanocrystalline (hexagonal) SmF3 was prepared by high-energy ball milling; mechanosynthesis helped us to replace Sm3+ in SmF3 by Ba2+ and to create vacancies in the F anion sublattice. We observed a remarkable increase in total (direct current) conductivity when going from nano-SmF3 to Sm1−x Ba x F3−x for x = 0.1. Electrical modulus spectroscopy was used to further characterize the corresponding increase in electrical relaxation frequencies.

Unusual cation coordination in nanostructured mullites

Nanocrystalline mullite-type bismuth-bearing complex oxides Bi2(M0.5Al0.5)4O9 (M=Fe3+, Ga3+) are prepared by high-energy ball milling of the corresponding microcrystalline counterparts. An unusual five-fold coordination of metal cations is revealed in nanostructured Bi2(M0.5Al0.5)4O9 by means of 27Al magic angle spinning nuclear magnetic resonance and 57Fe Mössbauer spectroscopies. The concentration of five-fold coordinated cations increases with decreasing crystallite size of a material at the expense of octahedrally coordinated ones. In addition to the nuclear spectroscopic methods, Rietveld analyses of the X-ray diffraction data of the as-prepared nanooxides show that the constituent tetrahedra, octahedra, and the newly formed structural units with five-fold cation coordination are strongly distorted. With decreasing crystallite size of mullites, the average volume of their octahedra increases whereas this parameter decreases for tetrahedra. The macroscopic behaviour of the non-equilibrium nanomullites is characterised by SQUID magnetometry. The Fe-containing mullites exhibit a superposition of a dominant antiferromagnetism and a weak ferromagnetism. The increase in both the remanent magnetization and the coercive field with decreasing crystallite size is attributed to the effect of spin canting. The latter is confined to the interfacial and surface regions of the nanomaterials, and arises due to both the mechanically induced deformation of constituent structural units and the formation of cation sites with the unusual five-fold coordination.

Mechanochemistry of fluoride solids: from mechanical activation to mechanically stimulated synthesis

Abstract

This lecture text is focused on the comparatively young field of mechanochemistry of fluoride solids, considering both their mechanical activation and their mechanochemical synthesis. Beside a literature survey, the mechanochemical synthesis of binary fluorides MF2, MF3, of complex fluorides MMgF4, of solid solutions MaxMb1−xF2 or M1−xLnxF2+x (Ln: Y, Eu) and of fluorine-containing coordination polymers is presented. Owing to their interesting potential applications in the field of fluoride ion conductivity or luminescence properties when doped, most of the given examples are alkaline earth metal compounds. A short historical survey, remarks on peculiarities and consequences of mechanical activation as well as the necessary technical equipment for mechanochemical reactions precede the section.

Graphic abstract

Understanding the Origin of Enhanced Li-Ion Transport in Nanocrystalline Argyrodite-Type Li6PS5I

Argyrodite-type Li₆PS₅X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation all-solid-state lithium batteries. In contrast to Li₆PS₅Br and Li₆PS₅Cl compounds showing ionic conductivities on the order of several mS cm^-1, the iodine compound Li₆PS₅I turned out to be a poor ionic conductor. This difference has been explained by anion site disorder in Li₆PS₅Br and Li₆PS₅Cl leading to facile through-going, that is, long-range ion transport. In the structurally ordered compound, Li₆PS₅I, long-range ion transport is, however, interrupted because the important intercage Li jump-diffusion pathway, enabling the ions to diffuse over long distances, is characterized by higher activation energy than that in the sibling compounds. Here, we introduced structural disorder in the iodide by soft mechanical treatment and took advantage of a high-energy planetary mill to prepare nanocrystalline Li₆PS₅I. A milling time of only 120 min turned out to be sufficient to boost ionic conductivity by 2 orders of magnitude, reaching σ_total = 0.5 × 10^-3 S cm^-1. We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and ⁷Li nuclear magnetic relaxation. X-ray powder diffraction and high-resolution ⁶Li, ³¹P MAS NMR helped characterize structural changes and the extent of disorder introduced. Changes in attempt frequency, activation entropy, and charge carrier concentration seem to be responsible for this increase.

Spatial confinement – rapid 2D F− diffusion in micro- and nanocrystalline RbSn2F5

NMR and conductivity spectroscopy reveal 2D diffusion in both microcrystalline and nanocrystalline RbSn₂F₅.

Substitutional disorder: structure and ion dynamics of the argyrodites Li6PS5Cl, Li6PS5Br and Li6PS5I

Li NMR spectroscopy reveals rapid Li ion dynamics in the poor Li ion conductor Li₆PS₅I; long-range motion is, however, only possible for Li₆PS₅Br and Li₆PS₅Cl with anion site disorder.

Synthesis of Fast Fluoride-Ion-Conductive Fluorite-Type Ba1- xSb xF2+ x (0.1 ≤ x ≤ 0.4): A Potential Solid Electrolyte for Fluoride-Ion Batteries

Toward the development of high-performance solid electrolytes for fluoride-ion batteries, fluorite-type nanostructured solid solutions of Ba_{1- x}Sb _xF_{2+ x} ( x ≤ 0.4) were synthesized by high-energy ball-milling method. Substitution of divalent Ba²⁺ by trivalent Sb³⁺ leads to an increase in interstitial fluoride-ion concentration, which enhances the ionic conductivity of the Ba_{1- x}Sb _xF_{2+ x} (0.1 ≤ x ≤ 0.4) system. Total ionic conductivities of 4.4 × 10^-4 and 3.9 × 10^-4 S cm^-1 were obtained for Ba_0.7Sb_0.3F_2.3 and Ba_0.6Sb_0.4F_2.4 compositions at 160 °C, respectively. In comparison to isostructural Ba_0.3La_0.7F_2.3, the ionic conductivity of Ba_0.7Sb_0.3F_2.3 is significantly higher, which is attributed to the presence of an electron lone pair on Sb³⁺. Introduction of such lone pairs seems to increase fluoride-ion mobility in solid solutions. In addition, Ba_0.7Sb_0.3F_2.3 was tested as a cathode material against Ce and Zn anode using La_0.9Ba_0.1F_2.9 as the electrolyte. Ba_0.3Sb_0.7F_2.3/La_0.9Ba_0.1F_2.9/Ce cell showed high discharge and charge capacities of 301 and 170 mA h g^-1, respectively, in the first cycle at 150 °C.