Hexaferrites and phase relations in the iron-rich part of the system Sr-La-Co-Fe-O

The iron rich part of the system was examined in the temperature range of 1200-1380 deg. C in air, with focus on the solid solutions of M-type hexaferrites. Samples of suitable compositions were studied by electronprobe microanalysis (EPMA). Substituted Sr-hexaferrites in the system Sr-La-Co-Fe-O do not follow the 1:1 substitution mechanism of La/Co in M-type ferrites. Due to the presence and limited Co2+-incorporation Fe3+-ions are reduced to Fe2+ within the crystal lattice to obtain charge balance. In all examined M-type ferrites divalent iron is formed, even at 1200 deg. C. The substitution principle Sr2++Fe3+↔La3++(Fe2+, Co2+) yields to the general substitution formula for the M-type hexaferrite Sr2+1-xLa3+xFe2+x-yCo2+yFe3+12-xO19 (0≤x≤1 and 0≤y≤x). In addition Sr/La-perovskiteSS (SS=solid solution), Co/Fe-spinelSS, hematite and magnetite are formed. Sr-hexaferrite exhibits at 1200 deg. C a limited solid solution with small amounts of Fe2+ (SrFe12O19↔Sr0.3La0.7Co0.5Fe2+0.2Fe11.3O19). At 1300 and 1380 deg. C a continuous solid solution series of the M-type hexaferrite is stable. SrFe12O19 and LaCo0.4Fe2+0.6Fe11O19 are the end members at 1300 deg. C. The maximum Fe2+O content is about 13 mol% in the M-type ferrite at 1380 deg. C (LaCo0.1Fe2+0.9Fe11O19). - Graphical abstract: M-type hexaferrite solid solution series Sr1-xLaxFe2+x-yCo2+yFe3+12-xO19 (0≤x≤1 and 0≤y≤0.40) at 1300 deg. C; M-type contains significant amounts of FeO even at 1200 deg. C; blue=data from electronprobe microanalyses; SF6=SrFe3+12O19; LCoFf6=LaCo0.4Fe2+0.6Fe3+11O19; S=SrO; L=La2O3; Co=CoO; F=Fe2O3; f=FeO.