Magnetic dielectric oxides: Subsolidus phase relations in the BaO:Fe2O3:TiO2 system

The BaO-Fe2O3-TiO2 ternary phase diagram has been investigated at 1250-1270 degrees C in air. X-ray diffraction studies of approximately 150 polycrystalline specimens at room temperature confirmed the existence of sixteen ternary compounds. Two of these compounds, BaFe4Ti2O11 and Ba12Fe28Ti15O84, were previously reported, four were found to be isostructural with known chemically similar compounds, and ten apparently adopt new structure-types. The crystal structures of five of the new ternary phases are briefly described. The oxidation state of iron found in this study is similar to that reported for a study of the BaO-Fe2O3-SnO2 system in air at 1200 degrees C. No indications of reduction of Fe3+ of Fe2+ were observed; however, oxidation to Fe4+ was clearly indicated in an extensive solid solution BaTi1-xFexO3-z, x = 0.06 --> 0.84, with the hexagonal BaTiO3 structure. Substantial solid solution regions were also found for the hollandite-type structure (BaxFe2xTi8-ZxO16, X = 1.07 --> 1.33), and for TiO2 dissolved in the BaFe12O19 structure (end member BaFe10.8Ti0.9O19). The complexity of the BaOFe12O3-TiO2 system is attributed to the coordinative versatility of Fe3+ in a close-packed O/Ba-O matrix and the resulting opportunity to form a wide variety of layered structures with different stacking sequences and distortions from ideal packing. (C) 1996 Academic Press, Inc.