The formation of nanocrystalline SrFeO3-; using mechano-synthesis and subsequent sintering: structural and Mössbauer studies

H. WIDATALLAH; A. D. AL- RAWAS; C. JOHNSON; S. H AL-HARTHI; A. . M. GISMELSEED; E. A. MOORE; S. J. STEWART

JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY

AMER SCIENTIFIC PUBLISHERS

Año: 2009 vol. 9 p. 2510 - 2517

1533-4880

The influence of mechanical milling and subsequent sintering of a 2:1 molar mixture of SrCO3 and a-Fe2O3 on the formation of SrFeO3− 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 Sr2+ in the a-Fe2O3 crystal structure thus facilitating the formation of a 2:1 nanocrystalline mixture of SrFeO3 and SrFeO2 875 by sintering the pre-milled mixture in air at 800 ºC (12 h). This temperatureThis temperature is ∼300 ºC lower than those at which SrFeO3− phases are synthesized by the conventional ceramic techniques. Pre-milling theprecursors was found to result in a smaller oxygen deficiency (delta) relative to conventional ceramic synthesis of SrFeO3−delta. Rietveld refinement of the X-ray diffraction shows the interatomic distancesin the resulting SrFeO2.875 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 SrFeO3−d nanocrystalline particles shows a significant partial reduction of Fe4+ to Fe3+ due to ambient conditions and the presence of an appreciable amount of SrCO3 as well.