Abstract
Formation of magnetoplumbite(MP)-type ferrite from β-alumina type ferrite has been investigated in terms of the structural similarity of both ferrites, which have the Mirror plane separating spinel blocks. Ba2+- β-ferrite (0.20 K2O.1.45 BaO.11 Fe2O3) and the mixed phase of Sr2+-β-ferrite and MP-type Sr-ferrite (0.18 K2O.1.33 SrO.11 Fe2O3) were obtained by immersing K+-β-ferrite powder into Ba(NO3)2 and Sr(NO3)2 melts, respectively, at 650°C. A large portions of these divalent β-ferrites transformed into MP-type ferrite at about 800°C without rearrangement of atoms in the spinel blocks. On the other hand, β"-alumina type Ba2+-β"-ferrite (0.02 K 2O.1.10 B aO5.5 Fe2O3), in which the mirror plane is not in the st ructure, transformed into disordered intermediate phase at 700--800°C, and then formed MP-type ferrite at 1000°C. Therefore, the transformation of β structure into MP structure seems to be topotactic reaction based on the structural similarity of β and MP-type structures. This was confirmed using a single crystal; that is, the (001) plane of Ba2+-β-ferrite crystal turned into the (001) plane of MP-type Ba-ferrite. However, the single phase formed at higher temperature of 1000°C. Furthermore, the saturation magnetizations of MP-type ferrite obtained from Ba2+- and Sr2+- β-ferrites were respectively lower than those of normal MP-type ferrite. These would be associated with the residual K+ ions in these ferrites.
