The formation of nanocrystalline SrFeO _3-δ using mechano-synthesis and subsequent sintering: Structural and Mössbauer studies

The influence of mechanical milling and subsequent sintering of a 2:1 molar mixture of SrCO ₃ and Cr-Fe ₂O ₃ on the formation of SrFeO _3-δ pervoskite-related nanocrystalline particles is investigated. The structural evolution during the formation process is systematically investigated using X-ray diffraction, thermal analysis, X-ray photoelectron spectroscopy and Mössbauer spectroscopy. Premilling the mixture in air for 120 h leads to the incorporation of Sr ²⁺ in the α-Fe ₂O ₃ crystal structure thus facilitating the formation of a 2:1 nanocrystalline mixture of SrFeO ₃ and SrFeO _2.875 by sintering the pre-milled mixture in air at 800°C (12 h). This temperature is ∼300°C lower than those at which SrFeO _3-δ phases are synthesized by the conventional ceramic techniques. Pre-milling the precursors was found to result in a smaller oxygen deficiency (δ) relative to conventional ceramic synthesis of SrFeO _3-δ. Rietveld refinement of the X-ray diffraction shows the interatomic distances in the resulting SrFeO _2.875 nanocrystalline phase to be slightly different from those of the conventionally prepared bulk leading, in turn, to a crystal structure with tilted polyhedral cationic sites. This structural distortion is related to both small-size and surface effects in the nanoparticles that have no counterparts in the corresponding bulk material. The surface structure of the attained SrFeO _3-δ nanocrystalline particles shows a significant partial reduction of Fe ⁴⁺ to Fe ³⁺ due to ambient conditions and the presence of an appreciable amount of SrCO ₃ as well.