Abstract
Me-substituted W-type ferrites (AMe2Fe16O27 with A = Sr, Ba, …, and Me = Co, Ni, Zn, Mg, …) can be obtained by standard solid state reaction but always contain secondary phases. Phase stability of SrMe2Fe16O27 with Me = Co, Ni, Zn, and Mg sintered in various oxygen pressures of pO2 = 0.2, 1, 10 and 387 atm were investigated using X-ray diffraction analysis, wavelength-dispersive X-ray (WDX) analysis, and transmission electron microscopy (TEM). WDX analysis revealed that the W-type ferrites are described as SrMe2-δFe16+δO27 because Fe3+ is partially reduced to Fe2+ even when synthesis is initiated from SrMe2Fe16O27. Increasing the oxygen pressure suppresses the reduction of Fe3+ and the formation of the secondary phases. In addition, TEM analysis shows that the SrCo2Fe16O27 single crystal is free of stacking faults. We conclude that the single-phase formation of the Me-substituted W-type ferrites is hampered by the discrepancy between initial and actual chemical composition caused by the appearance of Fe2+.
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