Study on Factors Hindering the Single-phase Formation of Divalent-ion-stabilized W-type Ferrites

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 p_O2 = 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 SrMe_2-δFe_16+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⁺.