The formation and structure of mechano-synthesized nanocrystalline Sr₃Fe₂O_6.4: XRD Rietveld, Mössabuer and XPS analyses

The influence of ball milling and subsequent sintering of a 3:1 molar mixture of SrCO₃ and α-Fe₂O₃ on the formation of Sr₃Fe₂O_7-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 Sr₃Fe₂O_7-δ phase that continued to increase with increasing sintering temperature until a single-phased nanocrystalline Sr₃Fe₂O_7-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 Sr₃Fe₂O_6.4 nanoparticles are superparamagnetic with blocking temperatures below 78 K. The surfaces of the Sr₃Fe₂O_6.4 nanoparticles were shown to have a complex structure and composition relative to those of their cores with traces of SrCO₃, SrO and SrFeO_3-δ being detected.