Characterization of Ternary Compounds in the BaO:Fe2O3:TiO2System: Ba6Fe45Ti17O106and BaFe11Ti3O23

Single crystals of Ba6Fe45Ti17O106and BaFe11Ti3O23were obtained as major and minor coproducts, respectively, by slow-cooling an off-stoichiometric BaO:Fe2O3:TiO2melt. The former compound exhibits variable stoichiometry, Ba6Fe48-xTi14-xO106, with the Fe:Ti ratio dependent upon the partial pressure of oxygen. The value ofxcorresponds to the equivalents of reduction that occur to maintain electroneutrality as the Ti-content increases. When prepared in air, this phase occurs atx=3 with the stoichiometry Ba6Fe45Ti17O106, while in 100% oxygen thex-value approaches zero with the resulting stoichiometry Ba6Fe48Ti14O106 (all Fe3+and Ti4+). The structures of Ba6Fe45Ti17O106and BaFe11Ti3O23were solved using single-crystal X-ray diffraction methods. Ba6Fe45Ti17O106was prepared in polycrystalline form, and further structural details, including accurate Fe/Ti occupancy factors, were determined by a combined refinement using neutron and synchrotron powder diffraction data. (Ba6Fe45Ti17O106: Space groupC2/m(No. 12);a=19.390(1) ?,b=20.260(1) ?,c=10.076(1) ?,beta=105.27(1)°;V=3818.5(3)?3;Z=2;rho calc=5.08 g/cm3. Ba6Fe11Ti3O23= Space groupC2/c (No. 15);a=19.561(1) ?,b=8.6614(7) ?,c=10.120(1) ?,beta=105.62(1)°;V= 1651.1(3) ?3;Z=4;rho calc=5.08 g/cm3.) Both compounds adopt eight-layer close-packed structures built from alternatingccpandhcp[O, (Ba, O)] layers stacked along thea-direction with a (ch)4repeat sequence. Both structures feature octahedral sites occupied by a mixture of Fe and Ti as well as tetrahedral sites occupied by Fe3+the structural formulas areXIIBa6IVFe6VI(Fe39Ti17)O106andXIIBaIVFe2VI(Fe9Ti3)O23. Both compounds are partially reduced; the former contains 3 moles of Fe2+(or Ti3+) per formula unit, and the latter contains 1mole. The formation of Fe2+is considered more likely than Ti3+, but could not be experimentally confirmed. BaFe11Ti3O23is apparently metastable in air when cooled from above the solidus and could not be prepared as a polycrystalline sample. Indexed experimental X-ray powder diffraction data for Ba6Fe45Ti17O106are given. Polycrystalline samples of this compound were used to measure its magnetic and electrical properties. The magnetic behavior of Ba6Fe45Ti17O106above room temperature up to 1073 K was found to obey the Curie-Weiss law, which indicated a small effective magnetic moment (34mu Bper mole Ba6Fe45Ti17O106) and a large negative temperature intercept (-806 K). Electrical resistivity measurements between room temperature and 120 K revealed nonmetallic behavior with an activation energy on the order of 0.17eV. At 347 MHz under ambient conditions, Ba6Fe45Ti17O106exhibited a relative permittivity of 24 and a dielectric loss tangent of 0.10.