The formation and structure of mechano-synthesized nanocrystalline Sr3Fe2O6.4: XRD Rietveld, Mössabuer and XPS analyses

• The formation of mechano-synthesized nanocrystalline Sr3Fe2O7−δ is investigated.
• Pre-milling the reactants substantially lowers the formation temperature.
• The core and surface structures were studied.
• XRD and 57Fe Mössbauer spectroscopic analyses indicate the δ-value to be 0.60.
• XPS shows a complex surface structure for the mechanosynthesized Sr3Fe2O7−δ nanoparticles.

The influence of ball milling and subsequent sintering of a 3:1 molar mixture of SrCO3 and α-Fe2O3 on the formation of Sr3Fe2O7−δ double perovskite is investigated with different analytical techniques. Milling the mixture for 110 h leads to the formation of SrCO3-α-Fe2O3 nanocomposites and the structural deformation of α-Fe2O3 via the incorporation of Sr2+ ions. Subsequent sintering of the pre-milled reactants’ mixture has led to the partial formation of an SrFeO3 perovskite-related phase in the temperature range 400–600 °C. This was followed by the progressive development of an Sr3Fe2O7−δ phase that continued to increase with increasing sintering temperature until a single-phased nanocrystalline Sr3Fe2O7−δ phase was attained at 950 °C (12 h). This temperature is ∼350 °C lower than the temperature at which the material is prepared conventionally using the ceramic method. The evolution of different structural phases during the reaction process is discussed. Rietveld refinement of the X-ray diffraction data shows a value of 0.60 for the oxygen deficiency δ, in consistency with the Fe3+/Fe4+ ratio derived from the 57Fe Mössbauer data recorded at both 300 K and 78 K. The Mössbauer data suggests that the Sr3Fe2O6.4 nanoparticles are superparamagnetic with blocking temperatures below 78 K. The surfaces of the Sr3Fe2O6.4 nanoparticles were shown to have a complex structure and composition relative to those of their cores with traces of SrCO3, SrO and SrFeO3−δ being detected.

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