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

Abstract

The iron rich part of the system was examined in the temperature range of 1200–1380 °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 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 °C. The substitution principle Sr 2++Fe 3+↔La 3++(Fe 2+, Co 2+) yields to the general substitution formula for the M-type hexaferrite Sr 2 + 1 - x La 3 + x Fe 2 + x - y Co 2 + y Fe 3 + 12 - x O 19 (0≤ x≤1 and 0≤ y≤ x). In addition Sr/La-perovskite SS ( SS=solid solution), Co/Fe-spinel SS, hematite and magnetite are formed. Sr-hexaferrite exhibits at 1200 °C a limited solid solution with small amounts of Fe 2+ (SrFe 12O 19↔Sr 0.3La 0.7Co 0.5Fe 2+ 0.2Fe 11.3O 19). At 1300 and 1380 °C a continuous solid solution series of the M-type hexaferrite is stable. SrFe 12O 19 and LaCo 0.4Fe 2+ 0.6Fe 11O 19 are the end members at 1300 °C. The maximum Fe 2+O content is about 13 mol% in the M-type ferrite at 1380 °C (LaCo 0.1Fe 2+ 0.9Fe 11O 19).