The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the Fe-57 Mossbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe3+ cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.DFG/SPP/1415APVV/0528-11VEGA/2/0097/1

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

Spin-state transition of iron in (Ba 0:5 Sr 0:5 ÞðFe 0:8 Zn 0:2 ÞO 3Àd perovskite

a b s t r a c t The redox behavior of iron during heating of a high-performance perovskite for ceramic oxygen separation membranes was studied by combined electron energy-loss (EELS, esp. ELNES) and Mössbauer spectroscopical in situ methods. At room temperature, the iron in ðBa 0:5 Sr 0:5 Þ ðFe 0:8 Zn 0:2 ÞO 3Àd (BSFZ) is in a mixed valence state of 75% Fe 4þ in the high-spin state and 25% Fe 3þ predominantly in the low-spin state. When heated to 900 3 C, a slight reduction of iron is observed that increases the quantity of Fe 3þ species. However, the dominant occurrence is a gradual transition in the spin-state of trivalent iron from a mixed low-spin/high-spin to a pure high-spin configuration. In addition, a remarkable amount of hybridization is found in the Fe-O bonds that are highly polar rather than purely ionic. The coupled valence/spin-state transition correlates with anomalies in thermogravimetry and thermal expansion behavior observed by X-ray diffraction and dilatometry, respectively. Since the effective cationic radii depend not only on the valence but also on the spin-state, both have to be considered when estimating under which conditions a cubic perovskite will tolerate specific cations. It is concluded that an excellent phase stability of perovskite-based membrane materials demands a tailoring, which enables pure high-spin states under operational conditions, even if mixed valence states are present. The low spin-state transition temperature of BSFZ provides that all iron species are in a pure high-spin configuration already above ca. 500 3 C making this ceramic highly attractive for intermediate temperature applications (5002800 3 C)