Shinji NAKAI, Takeshi Waki, Yoshikazu Tabata, Hiroto Ohta, Masaki Kato, Christian Teichmann, Jörg Töpfer, Hiroyuki Nakamura
W-type ferrite (AT2+2Fe3+16O27, A = Sr, Ba, etc., T2+ = Fe2+, Co2+, Zn2+, etc.) is a type of hexagonal ferrite with different magnetic properties depending on T2+. It is a promising functional material for various applications. However, its phase stability, particularly when T2+ is partially or fully a stable divalent cation such as Co2+, is not well understood from an equilibrium perspective. We performed thermogravimetry (TG) measurements, compositional analyses, as well as high oxygen pressure (PO2) synthesis, to estimate the equilibrium PO2 of the reduction reaction of the W-type ferrite solid-solution system, SrCoxFe18−xO27. Furthermore, we thermodynamically evaluated the behavior of the equilibrium PO2 versus Co concentration in the W phase. Our results show that the equilibrium PO2 of the solid-solution W, SrCoxFe18−xO27, diverges as the composition approaches SrCo2Fe16O27 because the configuration entropy change associated with Fe2+ formation in the W phase diverges as Fe2+ vanishes.