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

Abstract

<jats:p>Strontium ferromolybdate, Sr<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub>, 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 <jats:italic>via</jats:italic> a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub> can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO<jats:sub>3</jats:sub>) 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<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub> 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, <jats:sup>57</jats:sup>Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe<jats:sup>0</jats:sup> → Fe<jats:sup>3+</jats:sup>) with simultaneous reduction of Mo cations (Mo<jats:sup>6+</jats:sup> → Mo<jats:sup>5+</jats:sup>), occuring during the mechanosynthesis of Sr<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub>, 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<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub>, 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<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub> is characterized by both, the strongly distorted geometry of the constituent FeO<jats:sub>6</jats:sub> octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder <jats:italic>ASD</jats:italic> = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe<jats:sup>3+</jats:sup> and Mo<jats:sup>5+</jats:sup> cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr<jats:sub>2</jats:sub>FeMoO<jats:sub>6</jats:sub> nanoparticles exhibit superparamagnetism with the blocking temperature <jats:italic>T</jats:italic><jats:sub>B</jats:sub> = 240 K and the deteriorated effective magnetic moment <jats:italic>μ</jats:italic> = 0.055 <jats:italic>μ</jats:italic><jats:sub>B</jats:sub> per formula unit.</jats:p>