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

Abstract

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 TB = 240 K and the deteriorated effective magnetic moment μ = 0.055 μB per formula unit.

The Optimization of Mechanochemical Processes toward Functional Nanocomposite Materials

“…A further step towards the control of ASD is also being studied intensively [75]. An explicit example of ASD is explained in Figure 3 for Sr 2 FeMoO 6 (SFMO) from our recent study [20].…”

“…57 Fe Mössbauer spectra at 13K of 240 min milled mixture; as-milled (left) and with subsequent heating at 923 K for 30 min in air (right). Reprinted with permission from [20].…”

“…Materials with even more complicated structures, such as double perovskites [12][13][14] or metal organic frameworks (MOFs) [15][16][17][18][19], have been also mechanosynthesized. Very fine structural details, such as controlling the anti-site disorder in double perovskites, have also been challenged [20,21]. Increasing interest in downstream mechanochemistry toward industrial application is in line with greener chemical processes, and hence with the UN-led SDGs [22][23][24].…”

Mechanochemical synthesis of nanostructured and composite oxide ceramics: From mechanisms to tailored properties

“…Additionally, mechanochemical methods operate under milder conditions, enabling reactions that typically require high temperature and pressure to proceed smoothly at lower temperatures. 22,27,28 Moreover, traditional liquid and gas-phase methods often result in severe particle agglomeration due to the absence of a solid physical barrier during particle growth, affecting productivity. 24 Mechanochemical methods counter this by allowing for controlled grinding parameters, reducing agglomeration and producing more uniformly sized particles.…”

“…This allows for the synthesis of high‐performance new materials while avoiding the use of organic solvents and reducing environmental impact. Additionally, mechanochemical methods operate under milder conditions, enabling reactions that typically require high temperature and pressure to proceed smoothly at lower temperatures 22,27,28 . Moreover, traditional liquid and gas‐phase methods often result in severe particle agglomeration due to the absence of a solid physical barrier during particle growth, affecting productivity 24 .…”