The iron rich part of the system was examined in the temperature range of 1200-1380 1C 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 Co 2+ -incorporation Fe 3+ -ions are reduced to Fe 2+ within the crystal lattice to obtain charge balance. In all examined M-type ferrites divalent iron is formed, even at 1200 1C. The substitution principle Sr 2+ +Fe 3+ 2La 3+ +(Fe 2+ , Co 2+ ) yields to the general substitution formula for the M-type hexaferrite Sr 2þ 1Àx La 3þ
x Fe 2þ xÀy Co 2þ y Fe 3þ 12Àx O 19 (0rxr1 and 0ryrx). In addition Sr/La-perovskite SS ( SS ¼ solid solution), Co/Fe-spinel SS , hematite and magnetite are formed. Sr-hexaferrite exhibits at 1200 1C a limited solid solution with small amounts of Fe 2+ (SrFe 12 O 19 2Sr 0.3 La 0.7 Co 0.5 Fe 2+ 0.2 Fe 11.3 O 19 ). At 1300 and 1380 1C a continuous solid solution series of the M-type hexaferrite is stable. SrFe 12 O 19 and LaCo 0.4 Fe 2+ 0.6 Fe 11 O 19 are the end members at 1300 1C. The maximum Fe 2+ O content is about 13 mol% in the M-type ferrite at 1380 1C (LaCo 0.1 Fe 2+ 0.9 Fe 11 O 19 ).